Patent Publication Number: US-2023153225-A1

Title: Pull request risk prediction for bug-introducing changes

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to the field of software development, and more particularly to pull request risk prediction for bug-introducing changes. 
     Currently, many industries are trending toward cognitive models enabled by big data platforms and machine learning models. Cognitive models, also referred to as cognitive entities, are designed to remember the past, interact with humans, continuously learn, and continuously refine responses for the future with increasing levels of prediction. Machine learning explores the study and construction of algorithms that can learn from and make predictions on data. Such algorithms operate by building a model from example inputs in order to make data-driven predictions or decisions expressed as outputs, rather than following strictly static program instructions. Within the field of data analytics, machine learning is a method used to devise complex models and algorithms that lend themselves to prediction. These analytical models allow researchers, data scientists, engineers, and analysts to produce reliable, repeatable decisions and results and to uncover hidden insights through learning from historical relationships and trends in the data. 
     In software development, a distributed version control system (also known as distributed revision control) is a form of version control in which the complete codebase, including its full history, is mirrored on each developer&#39;s computer, enabling automatic management branching and merging. When a bug is discovered or an enhancement defined, an “issue” (more generically, a “ticket”) is created. Once a developer starts to work on that ticket, the developer creates a “branch,” which, in effect, takes a snapshot of the current state of the codebase, and then tracks a series of “commits” where a developer “checks in” a change to the code. Once the changes are complete, the developer creates a “pull request” (PR) that constitutes a formal request to merge the changes in this branch back to the main codebase. Typically, the pull request must be reviewed by one or more other developers to validate the testing, correctness, etc. If a reviewer has questions, the questions are included in the review, which is attached to the pull request. Once all questions are answered, the pull request is approved, and merged into its parent. 
     After a PR has been merged, if a bug is discovered, the standard procedure is to fix the bug and not spend additional time updating the change history to tag the code change that caused the problem. Therefore, it may be difficult to train a supervised machine learning model from the history of problematic changes, since a labelled training set does not exist. In order to improve release frequency and deployment success, the feedback cycle between release and reflection must be shortened. One way to accelerate release frequency is to quickly and proactively identify whether incorporating file changes into production may introduce a risk of a disruption, for example, a bug. A large portion of incidents may be attributed to source code changes, configuration changes, environment changes, outdated security changes, etc. Even changes that are closed as successful can lead to incidents. 
     SUMMARY 
     A first aspect of the present invention discloses a computer-implemented method including a computer retrieving one or more historic pull requests, where each of the one or more historic pull requests includes one or more files. A computer determines a unique file linking for each of one or more files included in the one or more historic pull requests. A computer generates a file risk dataset based on the unique file linking. A computer performs chronological partitioning on the file risk dataset. A computer determines one or more bug-introducing changes in the file risk dataset based on the chronological partitioning. A computer computes a collaborative file association between two or more of the one or more files in the file risk dataset. A computer labels each of the one or more files in the file risk dataset with an associated risk of introducing a bug. A computer generates a labelled file risk inducing ground truth dataset based on the labelled one or more files in the file risk dataset. A computer inputs the labelled file risk inducing ground truth dataset to a file risk prediction model to generate a file risk assessment. A computer extracts one or more pull request features from the one or more historic pull requests. A computer generates a pull request risk prediction model based on the file risk assessment and the extracted one or more pull request features. The present invention has the advantage of using a pull request risk prediction model to predict the riskiness of merging a pull request by identifying risky changes early in a continuous integration/continuous deployment (CI/CD) environment, such as at the pull request merging level, which can significantly reduce the total number of incidents. 
     A second aspect of the present invention discloses a computer program product including one or more computer readable storage media and program instructions collectively stored on the one or more computer readable storage media. The stored program instructions include program instructions to retrieve one or more historic pull requests, where each of the one or more historic pull requests includes one or more files. The stored program instructions include program instructions to determine a unique file linking for each of one or more files included in the one or more historic pull requests. The stored program instructions include program instructions to generate a file risk dataset based on the unique file linking. The stored program instructions include program instructions to perform chronological partitioning on the file risk dataset. The stored program instructions include program instructions to determine one or more bug-introducing changes in the file risk dataset based on the chronological partitioning. The stored program instructions include program instructions to compute a collaborative file association between two or more of the one or more files in the file risk dataset. The stored program instructions include program instructions to label each of the one or more files in the file risk dataset with an associated risk of introducing a bug. The stored program instructions include program instructions to generate a labelled file risk inducing ground truth dataset based on the labelled one or more files in the file risk dataset. The stored program instructions include program instructions to input the labelled file risk inducing ground truth dataset to a file risk prediction model to generate a file risk assessment. The stored program instructions include program instructions to extract one or more pull request features from the one or more historic pull requests. The stored program instructions include program instructions to generate a pull request risk prediction model based on the file risk assessment and the extracted one or more pull request features. 
     A third aspect of the present invention discloses a computer system including one or more computer processors and one or more computer readable storage media, where program instructions are collectively stored on the one or more computer readable storage media. The stored program instructions include program instructions to retrieve one or more historic pull requests, where each of the one or more historic pull requests includes one or more files. The stored program instructions include program instructions to determine a unique file linking for each of one or more files included in the one or more historic pull requests. The stored program instructions include program instructions to generate a file risk dataset based on the unique file linking. The stored program instructions include program instructions to perform chronological partitioning on the file risk dataset. The stored program instructions include program instructions to determine one or more bug-introducing changes in the file risk dataset based on the chronological partitioning. The stored program instructions include program instructions to compute a collaborative file association between two or more of the one or more files in the file risk dataset. The stored program instructions include program instructions to label each of the one or more files in the file risk dataset with an associated risk of introducing a bug. The stored program instructions include program instructions to generate a labelled file risk inducing ground truth dataset based on the labelled one or more files in the file risk dataset. The stored program instructions include program instructions to input the labelled file risk inducing ground truth dataset to a file risk prediction model to generate a file risk assessment. The stored program instructions include program instructions to extract one or more pull request features from the one or more historic pull requests. The stored program instructions include program instructions to generate a pull request risk prediction model based on the file risk assessment and the extracted one or more pull request features. 
     In another aspect, the present invention discloses a method for chronological partitioning on the file risk dataset including one or more computer processors determining one or more files within the file risk dataset that are associated with commits that were bug-fixing commits. One or more computer processors identify one or more commits introduced prior to the bug-fixing commits as bug-introducing commits. One or more computer processors identify one or more files associated with the bug-introducing commits. An advantage of using chronological partitioning (determining which commits were made before a bug-fixing commit) to identify a bug-introducing pull request is that the process can take place much earlier in the development cycle, as opposed to many change-induced incident identification methods that rely on natural language techniques to match text between the change and incident tickets, which are used later at the software deployment stage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a functional block diagram illustrating a distributed data processing environment, in accordance with an embodiment of the present invention; 
         FIG.  2    is a flowchart depicting operational steps of a pull request risk prediction program, on a server computer within the distributed data processing environment of  FIG.  1   , for creating a pull request risk prediction model to predict the risk of bug-introducing changes, in accordance with an embodiment of the present invention; 
         FIG.  3    is a flowchart depicting operational steps of the pull request risk prediction program, on the server computer within the distributed data processing environment of  FIG.  1   , for using the pull request risk prediction model, in accordance with an embodiment of the present invention; and 
         FIG.  4    depicts a block diagram of components of the server computer executing the pull request risk prediction program within the distributed data processing environment of  FIG.  1   , in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention recognize that proactively assessing risk associated with source code changes and providing risk explanation and risk mitigation actions can be a valuable input for site reliability engineers before deploying the changes as it can help prevent potential incidents and failures, thus minimizing the overall operational risk. Embodiments of the present invention also recognize that efficiency may be gained by automatically detecting risky changes at the time of a pull request by reducing the number of incidents and, therefore, accelerating release velocity and freeing software developers to continue writing code. In addition, embodiments of the present invention recognize that identifying a bug-introducing change is more difficult than identifying a bug-fixing change because a bug-fixing change explicitly specifies that it is addressing an identified bug, while a bug-introducing change does not. Further, embodiments of the present invention recognize that creating a labelled dataset for identifying bug-introducing changes is critical for creating a pull request risk prediction model. Implementation of embodiments of the invention may take a variety of forms, and exemplary implementation details are discussed subsequently with reference to the Figures. 
       FIG.  1    is a functional block diagram illustrating a distributed data processing environment, generally designated  100 , in accordance with one embodiment of the present invention. The term “distributed” as used herein describes a computer system that includes multiple, physically distinct devices that operate together as a single computer system.  FIG.  1    provides only an illustration of one implementation and does not imply any limitations with regard to the environments in which different embodiments may be implemented. Many modifications to the depicted environment may be made by those skilled in the art without departing from the scope of the invention as recited by the claims. 
     Distributed data processing environment  100  includes server computer  104  and client computing device  116 , interconnected over network  102 . Network  102  can be, for example, a telecommunications network, a local area network (LAN), a wide area network (WAN), such as the Internet, or a combination of the three, and can include wired, wireless, or fiber optic connections. Network  102  can include one or more wired and/or wireless networks capable of receiving and transmitting data, voice, and/or video signals, including multimedia signals that include voice, data, and video information. In general, network  102  can be any combination of connections and protocols that will support communications between server computer  104 , client computing device  116 , and other computing devices (not shown) within distributed data processing environment  100 . 
     Server computer  104  can be a standalone computing device, a management server, a web server, a mobile computing device, or any other electronic device or computing system capable of receiving, sending, and processing data. In other embodiments, server computer  104  can represent a server computing system utilizing multiple computers as a server system, such as in a cloud computing environment. In another embodiment, server computer  104  can be a laptop computer, a tablet computer, a netbook computer, a personal computer (PC), a desktop computer, a personal digital assistant (PDA), a smart phone, an edge device, a containerized workload, or any programmable electronic device capable of communicating with client computing device  116  and other computing devices (not shown) within distributed data processing environment  100  via network  102 . In another embodiment, server computer  104  represents a computing system utilizing clustered computers and components (e.g., database server computers, application server computers, etc.) that act as a single pool of seamless resources when accessed within distributed data processing environment  100 . Server computer  104  includes pull request risk prediction program  106  and code management program  112 . Server computer  104  may include internal and external hardware components, as depicted and described in further detail with respect to  FIG.  4   . 
     Pull request risk prediction program  106  predicts the risk of a pull request introducing a bug into one or more files of source code by generating a pull request risk prediction model based on historical pull requests and then using the pull request risk prediction model on a newly created pull request. In the depicted embodiment, pull request risk prediction program  106  is a standalone program. In another embodiment, pull request risk prediction program  106  is integrated as a component of code management program  112 . To generate the pull request risk prediction model, pull request risk prediction program  106  retrieves historic pull requests and determines unique file linking among the files in the historic pull requests. Based on the determined unique file linking, pull request risk prediction program  106  generates a file risk dataset. Pull request risk prediction program  106  performs chronological partitioning on the file risk dataset to determine bug-introducing changes. Pull request risk prediction program  106  computes a collaborative file association and labels each file with an associated risk. Pull request risk prediction program  106  generates a labelled file risk inducing ground truth dataset that includes the computed collaborative file association. Pull request risk prediction program  106  inputs the file risk inducing ground truth dataset to a file risk prediction model that extracts file features from the file risk inducing ground truth dataset and computes file togetherness score features. Pull request risk prediction program  106  applies a model to the extracted and computed features to generate a file risk assessment. Pull request risk prediction program  106  stores the file risk assessment. Pull request risk prediction program  106  extracts pull request features from historic pull requests. Pull request risk prediction program  106  combines the file risk assessment with the extracted pull request features to generate pull request risk prediction model  110 . Pull request risk prediction program  106  stores pull request risk prediction model  110 . Pull request risk prediction program  106  includes file risk prediction model  108 . 
     To utilize pull request risk prediction model  110 , pull request risk prediction program  106  receives a pull request. Pull request risk prediction program  106  determines unique file linking between the pull request files and generates a file risk dataset. Pull request risk prediction program  106  extracts file features from the file risk dataset and computes associated togetherness score features. Pull request risk prediction program  106  applies file risk prediction model  108  to the extracted and computed features to generate a file risk assessment. Pull request risk prediction program  106  extracts pull request features from the pull request. Pull request risk prediction program  106  applies pull request risk prediction model  110  to the combination of the file risk assessment and the extracted pull request features. Pull request risk prediction program  106  labels the pull request with the associated risk and stores the labelled pull request. Pull request risk prediction program  106  is depicted and described in further detail with respect to  FIG.  2    and  FIG.  3   . 
     File risk prediction model  108  extracts file features from one or more files included in a commit that are labelled as risky and applies a statistical model to the extracted features to output a file risk assessment. File features may include, but are not limited to, a number of commits that include the file, a churn value, e.g., a number of lines added and/or subtracted, a number of changesets, and a togetherness score. In an embodiment, file risk prediction model  108  is a weak supervised machine learning model, i.e., where noisy, limited, or imprecise sources are used to provide supervision signal for labeling large amounts of training data in a supervised learning setting. In the depicted embodiment, file risk prediction model  108  is a component of pull request risk prediction program  106 . In another embodiment, file risk prediction model may be a component of code management program  112 . In a further embodiment, file risk prediction model  108  may reside elsewhere within distributed data processing environment  100  provided pull request risk prediction program  106  has access to file risk prediction model  108 . 
     Code management program  112  is one of a plurality of distributed version control systems for managing changes to software during software development, as would be recognized by a person of skill in the art. In the depicted embodiment, code management program  112  resides on server computer  104 . In another embodiment, code management program  112  may reside elsewhere within distributed data processing environment  100  provided that code management program  112  has access to pull request risk prediction program  106 . Code management program  112  includes pull request database  114 . 
     Pull request database  114  stores information used by and generated by pull request risk prediction program  106  and code management program  112 . In the depicted embodiment, pull request database  114  resides on server computer  104 . In another embodiment, pull request database  114  may reside elsewhere within distributed data processing environment  100 , provided that pull request risk prediction program  106  and code management program  112  have access to pull request database  114 , via network  102 . A database is an organized collection of data. Pull request database  114  can be implemented with any type of storage device capable of storing data and configuration files that can be accessed and utilized by pull request risk prediction program  106  and code management program  112 , such as a database server, a hard disk drive, or a flash memory. Pull request database  114  stores data files associated with one or more software development projects. Pull request database  114  also stores pull requests and commits submitted by the user of client computing device  116 . In addition, pull request database  114  stores data used by pull request risk prediction program  106  to generate pull request risk prediction model  110 , such as historical pull request, unique file linking, one or more file risk datasets, one or more togetherness scores, a labelled file risk inducing ground truth dataset, file features, a file risk assessment, one or more pre-defined risk levels, and pull request features. Further, pull request database  114  stores the results of the application of pull request risk prediction model  110 , such as a labelled pull request. Pull request database  114  includes pull request risk prediction model  110 . 
     Pull request risk prediction model  110  is generated by pull request risk prediction program  106 , as will be discussed with respect to  FIG.  2   . Pull request risk prediction model  110  combines the file risk assessment created by file risk prediction model  108  with features extracted from a pull request to predict the risk of a bug-introducing change included in the pull request. An advantage of using a pull request risk prediction model to predict the riskiness of merging a pull request is that identifying risky changes early in a continuous integration/continuous deployment (CI/CD) environment, such as at the pull request merging level, can significantly reduce the total number of incidents. 
     Client computing device  116  can be one or more of a laptop computer, a tablet computer, a smart phone, smart watch, a smart speaker, or any programmable electronic device capable of communicating with various components and devices within distributed data processing environment  100 , via network  102 . Client computing device  116  may be a wearable computer. Wearable computers are miniature electronic devices that may be worn by the bearer under, with, or on top of clothing, as well as in or connected to glasses, hats, or other accessories. Wearable computers are especially useful for applications that require more complex computational support than merely hardware coded logics. In one embodiment, the wearable computer may be in the form of a head mounted display. The head mounted display may take the form-factor of a pair of glasses. In an embodiment, the wearable computer may be in the form of a smart watch. In an embodiment, client computing device  116  may be integrated into a vehicle of the user. For example, client computing device  116  may include a heads-up display in the windshield of the vehicle. In general, client computing device  116  represents one or more programmable electronic devices or combination of programmable electronic devices capable of executing machine readable program instructions and communicating with other computing devices (not shown) within distributed data processing environment  100  via a network, such as network  102 . Client computing device  116  includes an instance of user interface  118 . 
     User interface  118  provides an interface between code management program  112  on server computer  104  and a user of client computing device  116 . In one embodiment, user interface  118  is mobile application software. Mobile application software, or an “app,” is a computer program designed to run on smart phones, tablet computers and other mobile devices. In one embodiment, user interface  118  may be a graphical user interface (GUI) or a web user interface (WUI) and can display text, documents, web browser windows, user options, application interfaces, and instructions for operation, and include the information (such as graphic, text, and sound) that a program presents to a user and the control sequences the user employs to control the program. User interface  118  enables a user of client computing device  116  to interact with code management program  112 , for example, to create a pull request, as well as utilizing a plurality of other functions of code management program  112 . 
       FIG.  2    is a flowchart depicting operational steps of pull request risk prediction program  106 , on server computer  104  within distributed data processing environment  100  of  FIG.  1   , for creating pull request risk prediction model  110  to predict the risk of bug-introducing changes, in accordance with an embodiment of the present invention. 
     Pull request risk prediction program  106  retrieves historic pull requests (step  202 ). In an embodiment, pull request risk prediction program  106  retrieves one or more historic pull requests from pull request database  114 . A pull request is a formal request to merge file changes to a code repository, such as pull request database  114 . Generally, a pull request contains one or more commits made over a period of time, and each commit consists of one or more changes, where each change is made to one or more files. A file can belong to multiple pull requests and/or commits. Typically, a pull request message and/or a commit message reflects an action taken in the pull request. In an embodiment, the historic pull requests are from one or more previous software development projects. In another embodiment, the historic pull requests are previous pull requests from the current software development project. 
     Pull request risk prediction program  106  determines unique file linking (step  204 ). In an embodiment, pull request risk prediction program  106  determines which files among the files in the historic pull requests are associated, or “linked,” with one or more bug-introducing commits and removes any duplicate files such that pull request risk prediction program  106  identifies a unique list of linked files. 
     Pull request risk prediction program  106  generates a file risk dataset (step  206 ). Based on the determined unique file linking, i.e., the unique list of linked files, pull request risk prediction program  106  generates a file risk dataset. The file risk dataset is the set of files from each of the historic pull requests that pull request risk prediction program  106  identified by the unique file linking. In an embodiment, the files in the file risk dataset include both bug-fixing data and bug-introducing data. 
     Pull request risk prediction program  106  performs chronological partitioning on the file risk dataset to determine bug-introducing changes (step  208 ). Typically, a bug-introducing change occurs chronologically prior to a bug-fixing change. Often, the same set of files are associated with the bug-fixing change and the bug-introducing change. Chronological partitioning isolates the set of changes over a file before the bug-fixing change occurred, therefore, the subset contains only those candidate commits that include bug-introducing changes. Pull request risk prediction program  106  performs this chronological partitioning to construct a dataset, i.e., a corpus, for use in training pull request risk prediction model  110 . In an embodiment, pull request risk prediction program  106  analyzes the files within the file risk dataset to determine which of the files are associated with commits that were bug-fixing commits, i.e., commits that were introduced to fix a detected bug in the software. Pull request risk prediction program  106  then identifies the commits introduced just prior to the bug-fixing commits as bug-introducing commits and identifies the files associated with the bug-introducing commits. 
     Pull request risk prediction program  106  computes collaborative file association (step  210 ). Collaborative file association helps to associate a file related to a set of bug-introducing files that are often committed together. In an embodiment, pull request risk prediction program  106  mines the historic commits and associated files that were identified as risk inducing, i.e., bug-introducing, as a result of the chronological partitioning performed in step  208 . In an embodiment, the pairs of files are referred to as itemsets, i.e., sets of two files that were changed together or are closely associated with each other. In an embodiment, based on the computation of the collaborative file association, pull request risk prediction program  106  builds a lattice of related files consisting of sets of related files. 
     An advantage of using chronological partitioning (determining which commits were made before a bug-fixing commit) and collaborative file association (determining which files are typically committed together) to identify a bug-introducing pull request is that the process can take place much earlier in the development cycle as opposed to many change-induced incident identification methods that rely on natural language techniques to match text between the change and incident tickets, which are used later at the software deployment stage. 
     Pull request risk prediction program  106  labels each file with an associated risk (step  212 ). In an embodiment, based on the chronological partitioning and the collaborative file association, pull request risk prediction program  106  labels each file in the file risk dataset with a pre-defined risk. In an embodiment, risk levels are pre-defined by a system administrator and stored in pull request database  114 . In an embodiment, the pre-defined risk levels are a binary set, such as 1 and 0, yes and no, or risky and not risky. In another embodiment, the pre-defined risk levels are a numerical value. For example, pull request risk prediction program  106  may assign a risk value of a number between 1 and 10. In another embodiment, the pre-defined risk levels are text in natural language. For example, pull request risk prediction program  106  may assign a risk value of risky, very risky, or extremely risky. 
     Pull request risk prediction program  106  generates a labelled file risk inducing ground truth dataset (step  214 ). In an embodiment, pull request risk prediction program  106  collates the labelled files into a file risk inducing ground truth dataset. In an embodiment, pull request risk prediction program  106  stores the labelled file risk inducing ground truth dataset in pull request database  114 . In an embodiment, the file risk inducing ground truth dataset contains one or more risky files; a commit that contains a file from the file risk inducing ground truth dataset is a risky commit, and a pull request that contains a risky commit is a risky pull request. An advantage of generating a labelled file risk inducing ground truth dataset is that it provides labelled data for the supervised training of pull request risk prediction model  110 . In an embodiment, the labelled file risk inducing ground truth dataset includes only bug-introducing data and not bug-fixing data. 
     Pull request risk prediction program  106  inputs the file risk inducing ground truth dataset to file risk prediction model  108  (step  216 ). In an embodiment, pull request risk prediction program  106  uses the result of step  214 , i.e., the labelled file risk inducing ground truth dataset, as input to file risk prediction model  108 . 
     Pull request risk prediction program  106  extracts file features from the labelled file risk inducing ground truth dataset (step  218 ). In an embodiment, pull request risk prediction program  106  extracts and/or computes file features, such as a number of commits that include the file, a churn value, e.g., a number of lines added and/or subtracted, and a number of changesets, from each of the files included in the labelled file risk inducing ground truth dataset. In an embodiment, pull request risk prediction program  106  instructs file risk prediction model  108  to extract the file features from the labelled file risk inducing ground truth dataset. 
     Pull request risk prediction program  106  computes file togetherness score features (step  220 ). In an embodiment, pull request risk prediction program  106  computes a togetherness score feature for each file included in the labelled file risk inducing ground truth dataset. The togetherness score captures the probability that files in the dataset are committed together. For example, if file 1 and file 3 occur together in all the commits, then the file association between them is 1. In another example, if file 1 and file 2 occur together in 1 of 3 commits, then the file association between them is 0.34. In addition, the togetherness score is dynamic, as it depends on the set of files included in a commit. In an embodiment, pull request risk prediction program  106  uses the mined pairs, i.e., itemsets, identified during collaborative file association, as discussed with respect to step  210 , to compute the togetherness score value. In an embodiment, pull request risk prediction program  106  instructs file risk prediction model  108  to compute file togetherness score features. 
     Pull request risk prediction program  106  applies a model to the extracted and computed features (step  222 ). In an embodiment, pull request risk prediction program  106  applies a statistical model to the extracted and computed file features to generate a file risk assessment. For example, pull request risk prediction program  106  may apply a logistic regression model to predict a risk score. In another example, pull request risk prediction program  106  may use a classification model. In another embodiment, pull request risk prediction program  106  applies a machine learning model to the extracted and computed file features to generate the file risk assessment. In an embodiment, pull request risk prediction program  106  instructs file risk prediction model  108  to generate a file risk assessment based on the extracted and computed features. 
     Pull request risk prediction program  106  stores the resulting file risk assessment (step  224 ). In an embodiment, pull request risk prediction program  106  stores the file risk assessment that resulted from applying the model to the extracted and computed file features in pull request database  114 . 
     Pull request risk prediction program  106  extracts pull request features from historic pull requests (step  226 ). In an embodiment, pull request risk prediction program  106  extracts features of historic pull requests stored in pull request database  114 . Pull request features, as would be recognized by a person of skill in the art, may include, but are not limited to, a pull request description, a pull request short description, a number of participants in the pull request, a notation of whether one or more reviewers are assigned, a percentage of reviewers that approved, a number of conversations, a duration of the pull request (i.e., a time between creation and closure of the pull request), a number of problematic bug-introducing commits as a fraction or percentage of the total commits, a number of additions, a number of deletions, a unit test score, an integration test score, a code coverage score, a yapf score, and a lynting score. In an embodiment, pull request risk prediction program  106  uses one or more natural language processing (NLP) techniques to extract the features. 
     Pull request risk prediction program  106  combines the file risk assessment with the extracted pull request features (step  228 ). In an embodiment, pull request risk prediction program  106  combines the file risk assessment, as discussed with respect to step  222  and the extracted pull request features, as discussed with respect to step  226 , to generate a labelled training set and train pull request risk prediction model  110 . 
     Pull request risk prediction program  106  stores resulting pull request risk prediction model  110  (step  230 ). In an embodiment, pull request risk prediction program  106  stores pull request risk prediction model  110  as a file such that pull request risk prediction program  106  can apply the model to new pull requests as they are received. In the embodiment depicted in  FIG.  1   , pull request risk prediction program  106  stores pull request risk prediction model  110  in pull request database  114 . 
       FIG.  3    is a flowchart depicting operational steps of pull request risk prediction program  106 , on server computer  104  within distributed data processing environment  100  of  FIG.  1   , for using pull request risk prediction model  110 , in accordance with an embodiment of the present invention. 
     Pull request risk prediction program  106  receives a pull request (step  302 ). In an embodiment, a user of client computing device  116  generates a new pull request within code management program  112 , via user interface  118 , and pull request risk prediction program  106  receives the pull request. In an embodiment, code management program  112  transmits the new pull request to pull request risk prediction program  106 . 
     Pull request risk prediction program  106  determines unique file linking (step  304 ). In an embodiment, pull request risk prediction program  106  analyzes the files included in the new pull request to determine which files are linked with one or more bug-introducing commits and removes any duplicate files such that pull request risk prediction program  106  identifies a unique list of linked files, as discussed with respect to step  204  of  FIG.  2   . 
     Pull request risk prediction program  106  generates a pull request file risk dataset (step  306 ). In an embodiment, pull request risk prediction program  106  generates a pull request file risk dataset associated with the new pull request. The pull request file risk dataset is the set of files included in the new pull request that pull request risk prediction program  106  identified by the unique file linking, as discussed with respect to step  206  of  FIG.  2   . 
     Pull request risk prediction program  106  extracts file features from the pull request file risk dataset (step  308 ). In an embodiment, pull request risk prediction program  106  extracts and/or computes file features, such as a number of commits that include the file, a churn value, e.g., a number of lines added and/or subtracted, and a number of changesets, from each of the files included in the file risk dataset, as discussed with respect to step  218  of  FIG.  2   . 
     Pull request risk prediction program  106  computes togetherness score features of the file risk dataset (step  310 ). In an embodiment, pull request risk prediction program  106  computes a togetherness score feature for each file included in the file risk dataset, as discussed with respect to step  220  of  FIG.  2   . 
     Pull request risk prediction program  106  applies a model to the extracted and computed features (step  312 ). In an embodiment, pull request risk prediction program  106  applies a model to the extracted and computed file features to generate a file risk assessment, as discussed with respect to step  222  of  FIG.  2   . In an embodiment, pull request risk prediction program  106  applies a statistical model to the extracted and computed file features to generate a file risk assessment. In another embodiment, pull request risk prediction program  106  applies a machine learning model to the extracted and computed file features to generate the file risk assessment. 
     Pull request risk prediction program  106  extracts pull request features from the pull request (step  314 ). In an embodiment, pull request risk prediction program  106  extracts features of the new pull request, as discussed with respect to step  226  of  FIG.  2   . 
     Pull request risk prediction program  106  applies pull request risk prediction model  110  (step  316 ). In an embodiment, pull request risk prediction program  106  applies pull request risk prediction model  110  to the file risk assessment and the extracted and computed pull request features to determine a risk associated with the new pull request. 
     Pull request risk prediction program  106  labels the pull request with the associated risk (step  318 ). In an embodiment, pull request risk prediction program  106  annotates the new pull request with the risk prediction determined by pull request risk prediction model  110 . 
     Pull request risk prediction program  106  stores the labelled pull request (step  320 ). In an embodiment, pull request risk prediction program  106  stores the new pull request and associated risk prediction in pull request database  114 . In an embodiment, pull request risk prediction program  106  sends a notification to the user of client computing device  116 , via user interface  118 , that describes the risk prediction for the new pull request. 
       FIG.  4    depicts a block diagram of components of server computer  104  within distributed data processing environment  100  of  FIG.  1   , in accordance with an embodiment of the present invention. It should be appreciated that  FIG.  4    provides only an illustration of one implementation and does not imply any limitations with regard to the environments in which different embodiments can be implemented. Many modifications to the depicted environment can be made. 
     Server computer  104  can include processor(s)  404 , cache  414 , memory  406 , persistent storage  408 , communications unit  410 , input/output (I/O) interface(s)  412  and communications fabric  402 . Communications fabric  402  provides communications between cache  414 , memory  406 , persistent storage  408 , communications unit  410 , and input/output (I/O) interface(s)  412 . Communications fabric  402  can be implemented with any architecture designed for passing data and/or control information between processors (such as microprocessors, communications and network processors, etc.), system memory, peripheral devices, and any other hardware components within a system. For example, communications fabric  402  can be implemented with one or more buses. 
     Memory  406  and persistent storage  408  are computer readable storage media. In this embodiment, memory  406  includes random access memory (RAM). In general, memory  406  can include any suitable volatile or non-volatile computer readable storage media. Cache  414  is a fast memory that enhances the performance of processor(s)  404  by holding recently accessed data, and data near recently accessed data, from memory  406 . 
     Program instructions and data used to practice embodiments of the present invention, e.g., pull request risk prediction program  106  and code management program  112 , are stored in persistent storage  408  for execution and/or access by one or more of the respective processor(s)  404  of server computer  104  via cache  414 . In this embodiment, persistent storage  408  includes a magnetic hard disk drive. Alternatively, or in addition to a magnetic hard disk drive, persistent storage  408  can include a solid-state hard drive, a semiconductor storage device, a read-only memory (ROM), an erasable programmable read-only memory (EPROM), a flash memory, or any other computer readable storage media that is capable of storing program instructions or digital information. 
     The media used by persistent storage  408  may also be removable. For example, a removable hard drive may be used for persistent storage  408 . Other examples include optical and magnetic disks, thumb drives, and smart cards that are inserted into a drive for transfer onto another computer readable storage medium that is also part of persistent storage  408 . 
     Communications unit  410 , in these examples, provides for communications with other data processing systems or devices, including resources of client computing device  116 . In these examples, communications unit  410  includes one or more network interface cards. Communications unit  410  may provide communications through the use of either or both physical and wireless communications links. Pull request risk prediction program  106 , code management program  112 , and other programs and data used for implementation of the present invention, may be downloaded to persistent storage  408  of server computer  104  through communications unit  410 . 
     I/O interface(s)  412  allows for input and output of data with other devices that may be connected to server computer  104 . For example, I/O interface(s)  412  may provide a connection to external device(s)  416  such as a keyboard, a keypad, a touch screen, a microphone, a digital camera, and/or some other suitable input device. External device(s)  416  can also include portable computer readable storage media such as, for example, thumb drives, portable optical or magnetic disks, and memory cards. Software and data used to practice embodiments of the present invention, e.g., pull request risk prediction program  106  and code management program  112  on server computer  104 , can be stored on such portable computer readable storage media and can be loaded onto persistent storage  408  via I/O interface(s)  412 . I/O interface(s)  412  also connect to display  418 . 
     Display  418  provides a mechanism to display data to a user and may be, for example, a computer monitor. Display  418  can also function as a touch screen, such as a display of a tablet computer. 
     The programs described herein are identified based upon the application for which they are implemented in a specific embodiment of the invention. However, it should be appreciated that any particular program nomenclature herein is used merely for convenience, and thus the invention should not be limited to use solely in any specific application identified and/or implied by such nomenclature. 
     The present invention may be a system, a method, and/or a computer program product. The computer program product may include a computer readable storage medium (or media) having computer readable program instructions thereon for causing a processor to carry out aspects of the present invention. 
     The computer readable storage medium can be any tangible device that can retain and store instructions for use by an instruction execution device. The computer readable storage medium may be, for example, but is not limited to, an electronic storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination of the foregoing. A non-exhaustive list of more specific examples of the computer readable storage medium includes the following: a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), a static random access memory (SRAM), a portable compact disc read-only memory (CD-ROM), a digital versatile disk (DVD), a memory stick, a floppy disk, a mechanically encoded device such as punch-cards or raised structures in a groove having instructions recorded thereon, and any suitable combination of the foregoing. A computer readable storage medium, as used herein, is not to be construed as being transitory signals per se, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through a waveguide or other transmission media (e.g., light pulses passing through a fiber-optic cable), or electrical signals transmitted through a wire. 
     Computer readable program instructions described herein can be downloaded to respective computing/processing devices from a computer readable storage medium or to an external computer or external storage device via a network, for example, the Internet, a local area network, a wide area network and/or a wireless network. The network may comprise copper transmission cables, optical transmission fibers, wireless transmission, routers, firewalls, switches, gateway computers and/or edge servers. A network adapter card or network interface in each computing/processing device receives computer readable program instructions from the network and forwards the computer readable program instructions for storage in a computer readable storage medium within the respective computing/processing device. 
     Computer readable program instructions for carrying out operations of the present invention may be assembler instructions, instruction-set-architecture (ISA) instructions, machine instructions, machine dependent instructions, microcode, firmware instructions, state-setting data, or either source code or object code written in any combination of one or more programming languages, including an object oriented programming language such as Smalltalk, C++ or the like, and conventional procedural programming languages, such as the “C” programming language or similar programming languages. The computer readable program instructions may execute entirely on the user&#39;s computer, partly on the user&#39;s computer, as a stand-alone software package, partly on the user&#39;s computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user&#39;s computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). In some embodiments, electronic circuitry including, for example, programmable logic circuitry, field-programmable gate arrays (FPGA), or programmable logic arrays (PLA) may execute the computer readable program instructions by utilizing state information of the computer readable program instructions to personalize the electronic circuitry, in order to perform aspects of the present invention. 
     Aspects of the present invention are described herein with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer readable program instructions. 
     These computer readable program instructions may be provided to a processor of a general purpose computer, a special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. These computer readable program instructions may also be stored in a computer readable storage medium that can direct a computer, a programmable data processing apparatus, and/or other devices to function in a particular manner, such that the computer readable storage medium having instructions stored therein comprises an article of manufacture including instructions which implement aspects of the function/act specified in the flowchart and/or block diagram block or blocks. 
     The computer readable program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other device to cause a series of operational steps to be performed on the computer, other programmable apparatus or other device to produce a computer implemented process, such that the instructions which execute on the computer, other programmable apparatus, or other device implement the functions/acts specified in the flowchart and/or block diagram block or blocks. 
     The flowchart and block diagrams in the Figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer program products according to various embodiments of the present invention. In this regard, each block in the flowchart or block diagrams may represent a module, a segment, or a portion of instructions, which comprises one or more executable instructions for implementing the specified logical function(s). In some alternative implementations, the functions noted in the blocks may occur out of the order noted in the Figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts or carry out combinations of special purpose hardware and computer instructions. 
     The foregoing descriptions of the various embodiments of the present invention have been presented for purposes of illustration and example, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The terminology used herein was chosen to best explain the principles of the embodiment, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.