text: // Copyright 2013 The Go Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style
  // license that can be found in the LICENSE file.
  
  package loader
  
  // See doc.go for package documentation and implementation notes.
  
  import (
  	"errors"
  	"fmt"
  	"go/ast"
  	"go/build"
  	"go/parser"
  	"go/token"
  	"go/types"
  	"os"
  	"path/filepath"
  	"sort"
  	"strings"
  	"sync"
  	"time"
  
  	"golang.org/x/tools/go/ast/astutil"
  )
  
  var ignoreVendor build.ImportMode
  
  const trace = false // show timing info for type-checking
  
  // Config specifies the configuration for loading a whole program from
  // Go source code.
  // The zero value for Config is a ready-to-use default configuration.
  type Config struct {
  	// Fset is the file set for the parser to use when loading the
  	// program.  If nil, it may be lazily initialized by any
  	// method of Config.
  	Fset *token.FileSet
  
  	// ParserMode specifies the mode to be used by the parser when
  	// loading source packages.
  	ParserMode parser.Mode
  
  	// TypeChecker contains options relating to the type checker.
  	//
  	// The supplied IgnoreFuncBodies is not used; the effective
  	// value comes from the TypeCheckFuncBodies func below.
  	// The supplied Import function is not used either.
  	TypeChecker types.Config
  
  	// TypeCheckFuncBodies is a predicate over package paths.
  	// A package for which the predicate is false will
  	// have its package-level declarations type checked, but not
  	// its function bodies; this can be used to quickly load
  	// dependencies from source.  If nil, all func bodies are type
  	// checked.
  	TypeCheckFuncBodies func(path string) bool
  
  	// If Build is non-nil, it is used to locate source packages.
  	// Otherwise &build.Default is used.
  	//
  	// By default, cgo is invoked to preprocess Go files that
  	// import the fake package "C".  This behaviour can be
  	// disabled by setting CGO_ENABLED=0 in the environment prior
  	// to startup, or by setting Build.CgoEnabled=false.
  	Build *build.Context
  
  	// The current directory, used for resolving relative package
  	// references such as "./go/loader".  If empty, os.Getwd will be
  	// used instead.
  	Cwd string
  
  	// If DisplayPath is non-nil, it is used to transform each
  	// file name obtained from Build.Import().  This can be used
  	// to prevent a virtualized build.Config's file names from
  	// leaking into the user interface.
  	DisplayPath func(path string) string
  
  	// If AllowErrors is true, Load will return a Program even
  	// if some of the its packages contained I/O, parser or type
  	// errors; such errors are accessible via PackageInfo.Errors.  If
  	// false, Load will fail if any package had an error.
  	AllowErrors bool
  
  	// CreatePkgs specifies a list of non-importable initial
  	// packages to create.  The resulting packages will appear in
  	// the corresponding elements of the Program.Created slice.
  	CreatePkgs []PkgSpec
  
  	// ImportPkgs specifies a set of initial packages to load.
  	// The map keys are package paths.
  	//
  	// The map value indicates whether to load tests.  If true, Load
  	// will add and type-check two lists of files to the package:
  	// non-test files followed by in-package *_test.go files.  In
  	// addition, it will append the external test package (if any)
  	// to Program.Created.
  	ImportPkgs map[string]bool
  
  	// FindPackage is called during Load to create the build.Package
  	// for a given import path from a given directory.
  	// If FindPackage is nil, (*build.Context).Import is used.
  	// A client may use this hook to adapt to a proprietary build
  	// system that does not follow the "go build" layout
  	// conventions, for example.
  	//
  	// It must be safe to call concurrently from multiple goroutines.
  	FindPackage func(ctxt *build.Context, importPath, fromDir string, mode build.ImportMode) (*build.Package, error)
  
  	// AfterTypeCheck is called immediately after a list of files
  	// has been type-checked and appended to info.Files.
  	//
  	// This optional hook function is the earliest opportunity for
  	// the client to observe the output of the type checker,
  	// which may be useful to reduce analysis latency when loading
  	// a large program.
  	//
  	// The function is permitted to modify info.Info, for instance
  	// to clear data structures that are no longer needed, which can
  	// dramatically reduce peak memory consumption.
  	//
  	// The function may be called twice for the same PackageInfo:
  	// once for the files of the package and again for the
  	// in-package test files.
  	//
  	// It must be safe to call concurrently from multiple goroutines.
  	AfterTypeCheck func(info *PackageInfo, files []*ast.File)
  }
  
  // A PkgSpec specifies a non-importable package to be created by Load.
  // Files are processed first, but typically only one of Files and
  // Filenames is provided.  The path needn't be globally unique.
  //
  // For vendoring purposes, the package's directory is the one that
  // contains the first file.
  type PkgSpec struct {
  	Path      string      // package path ("" => use package declaration)
  	Files     []*ast.File // ASTs of already-parsed files
  	Filenames []string    // names of files to be parsed
  }
  
  // A Program is a Go program loaded from source as specified by a Config.
  type Program struct {
  	Fset *token.FileSet // the file set for this program
  
  	// Created[i] contains the initial package whose ASTs or
  	// filenames were supplied by Config.CreatePkgs[i], followed by
  	// the external test package, if any, of each package in
  	// Config.ImportPkgs ordered by ImportPath.
  	//
  	// NOTE: these files must not import "C".  Cgo preprocessing is
  	// only performed on imported packages, not ad hoc packages.
  	//
  	// TODO(adonovan): we need to copy and adapt the logic of
  	// goFilesPackage (from $GOROOT/src/cmd/go/build.go) and make
  	// Config.Import and Config.Create methods return the same kind
  	// of entity, essentially a build.Package.
  	// Perhaps we can even reuse that type directly.
  	Created []*PackageInfo
  
  	// Imported contains the initially imported packages,
  	// as specified by Config.ImportPkgs.
  	Imported map[string]*PackageInfo
  
  	// AllPackages contains the PackageInfo of every package
  	// encountered by Load: all initial packages and all
  	// dependencies, including incomplete ones.
  	AllPackages map[*types.Package]*PackageInfo
  
  	// importMap is the canonical mapping of package paths to
  	// packages.  It contains all Imported initial packages, but not
  	// Created ones, and all imported dependencies.
  	importMap map[string]*types.Package
  }
  
  // PackageInfo holds the ASTs and facts derived by the type-checker
  // for a single package.
  //
  // Not mutated once exposed via the API.
  //
  type PackageInfo struct {
  	Pkg                   *types.Package
  	Importable            bool        // true if 'import "Pkg.Path()"' would resolve to this
  	TransitivelyErrorFree bool        // true if Pkg and all its dependencies are free of errors
  	Files                 []*ast.File // syntax trees for the package's files
  	Errors                []error     // non-nil if the package had errors
  	types.Info                        // type-checker deductions.
  	dir                   string      // package directory
  
  	checker   *types.Checker // transient type-checker state
  	errorFunc func(error)
  }
  
  func (info *PackageInfo) String() string { return info.Pkg.Path() }
  
  func (info *PackageInfo) appendError(err error) {
  	if info.errorFunc != nil {
  		info.errorFunc(err)
  	} else {
  		fmt.Fprintln(os.Stderr, err)
  	}
  	info.Errors = append(info.Errors, err)
  }
  
  func (conf *Config) fset() *token.FileSet {
  	if conf.Fset == nil {
  		conf.Fset = token.NewFileSet()
  	}
  	return conf.Fset
  }
  
  // ParseFile is a convenience function (intended for testing) that invokes
  // the parser using the Config's FileSet, which is initialized if nil.
  //
  // src specifies the parser input as a string, []byte, or io.Reader, and
  // filename is its apparent name.  If src is nil, the contents of
  // filename are read from the file system.
  //
  func (conf *Config) ParseFile(filename string, src interface{}) (*ast.File, error) {
  	// TODO(adonovan): use conf.build() etc like parseFiles does.
  	return parser.ParseFile(conf.fset(), filename, src, conf.ParserMode)
  }
  
  // FromArgsUsage is a partial usage message that applications calling
  // FromArgs may wish to include in their -help output.
  const FromArgsUsage = `
  <args> is a list of arguments denoting a set of initial packages.
  It may take one of two forms:
  
  1. A list of *.go source files.
  
     All of the specified files are loaded, parsed and type-checked
     as a single package.  All the files must belong to the same directory.
  
  2. A list of import paths, each denoting a package.
  
     The package's directory is found relative to the $GOROOT and
     $GOPATH using similar logic to 'go build', and the *.go files in
     that directory are loaded, parsed and type-checked as a single
     package.
  
     In addition, all *_test.go files in the directory are then loaded
     and parsed.  Those files whose package declaration equals that of
     the non-*_test.go files are included in the primary package.  Test
     files whose package declaration ends with "_test" are type-checked
     as another package, the 'external' test package, so that a single
     import path may denote two packages.  (Whether this behaviour is
     enabled is tool-specific, and may depend on additional flags.)
  
  A '--' argument terminates the list of packages.
  `
  
  // FromArgs interprets args as a set of initial packages to load from
  // source and updates the configuration.  It returns the list of
  // unconsumed arguments.
  //
  // It is intended for use in command-line interfaces that require a
  // set of initial packages to be specified; see FromArgsUsage message
  // for details.
  //
  // Only superficial errors are reported at this stage; errors dependent
  // on I/O are detected during Load.
  //
  func (conf *Config) FromArgs(args []string, xtest bool) ([]string, error) {
  	var rest []string
  	for i, arg := range args {
  		if arg == "--" {
  			rest = args[i+1:]
  			args = args[:i]
  			break // consume "--" and return the remaining args
  		}
  	}
  
  	if len(args) > 0 && strings.HasSuffix(args[0], ".go") {
  		// Assume args is a list of a *.go files
  		// denoting a single ad hoc package.
  		for _, arg := range args {
  			if !strings.HasSuffix(arg, ".go") {
  				return nil, fmt.Errorf("named files must be .go files: %s", arg)
  			}
  		}
  		conf.CreateFromFilenames("", args...)
  	} else {
  		// Assume args are directories each denoting a
  		// package and (perhaps) an external test, iff xtest.
  		for _, arg := range args {
  			if xtest {
  				conf.ImportWithTests(arg)
  			} else {
  				conf.Import(arg)
  			}
  		}
  	}
  
  	return rest, nil
  }
  
  // CreateFromFilenames is a convenience function that adds
  // a conf.CreatePkgs entry to create a package of the specified *.go
  // files.
  //
  func (conf *Config) CreateFromFilenames(path string, filenames ...string) {
  	conf.CreatePkgs = append(conf.CreatePkgs, PkgSpec{Path: path, Filenames: filenames})
  }
  
  // CreateFromFiles is a convenience function that adds a conf.CreatePkgs
  // entry to create package of the specified path and parsed files.
  //
  func (conf *Config) CreateFromFiles(path string, files ...*ast.File) {
  	conf.CreatePkgs = append(conf.CreatePkgs, PkgSpec{Path: path, Files: files})
  }
  
  // ImportWithTests is a convenience function that adds path to
  // ImportPkgs, the set of initial source packages located relative to
  // $GOPATH.  The package will be augmented by any *_test.go files in
  // its directory that contain a "package x" (not "package x_test")
  // declaration.
  //
  // In addition, if any *_test.go files contain a "package x_test"
  // declaration, an additional package comprising just those files will
  // be added to CreatePkgs.
  //
  func (conf *Config) ImportWithTests(path string) { conf.addImport(path, true) }
  
  // Import is a convenience function that adds path to ImportPkgs, the
  // set of initial packages that will be imported from source.
  //
  func (conf *Config) Import(path string) { conf.addImport(path, false) }
  
  func (conf *Config) addImport(path string, tests bool) {
  	if path == "C" {
  		return // ignore; not a real package
  	}
  	if conf.ImportPkgs == nil {
  		conf.ImportPkgs = make(map[string]bool)
  	}
  	conf.ImportPkgs[path] = conf.ImportPkgs[path] || tests
  }
  
  // PathEnclosingInterval returns the PackageInfo and ast.Node that
  // contain source interval [start, end), and all the node's ancestors
  // up to the AST root.  It searches all ast.Files of all packages in prog.
  // exact is defined as for astutil.PathEnclosingInterval.
  //
  // The zero value is returned if not found.
  //
  func (prog *Program) PathEnclosingInterval(start, end token.Pos) (pkg *PackageInfo, path []ast.Node, exact bool) {
  	for _, info := range prog.AllPackages {
  		for _, f := range info.Files {
  			if f.Pos() == token.NoPos {
  				// This can happen if the parser saw
  				// too many errors and bailed out.
  				// (Use parser.AllErrors to prevent that.)
  				continue
  			}
  			if !tokenFileContainsPos(prog.Fset.File(f.Pos()), start) {
  				continue
  			}
  			if path, exact := astutil.PathEnclosingInterval(f, start, end); path != nil {
  				return info, path, exact
  			}
  		}
  	}
  	return nil, nil, false
  }
  
  // InitialPackages returns a new slice containing the set of initial
  // packages (Created + Imported) in unspecified order.
  //
  func (prog *Program) InitialPackages() []*PackageInfo {
  	infos := make([]*PackageInfo, 0, len(prog.Created)+len(prog.Imported))
  	infos = append(infos, prog.Created...)
  	for _, info := range prog.Imported {
  		infos = append(infos, info)
  	}
  	return infos
  }
  
  // Package returns the ASTs and results of type checking for the
  // specified package.
  func (prog *Program) Package(path string) *PackageInfo {
  	if info, ok := prog.AllPackages[prog.importMap[path]]; ok {
  		return info
  	}
  	for _, info := range prog.Created {
  		if path == info.Pkg.Path() {
  			return info
  		}
  	}
  	return nil
  }
  
  // ---------- Implementation ----------
  
  // importer holds the working state of the algorithm.
  type importer struct {
  	conf  *Config   // the client configuration
  	start time.Time // for logging
  
  	progMu sync.Mutex // guards prog
  	prog   *Program   // the resulting program
  
  	// findpkg is a memoization of FindPackage.
  	findpkgMu sync.Mutex // guards findpkg
  	findpkg   map[findpkgKey]*findpkgValue
  
  	importedMu sync.Mutex             // guards imported
  	imported   map[string]*importInfo // all imported packages (incl. failures) by import path
  
  	// import dependency graph: graph[x][y] => x imports y
  	//
  	// Since non-importable packages cannot be cyclic, we ignore
  	// their imports, thus we only need the subgraph over importable
  	// packages.  Nodes are identified by their import paths.
  	graphMu sync.Mutex
  	graph   map[string]map[string]bool
  }
  
  type findpkgKey struct {
  	importPath string
  	fromDir    string
  	mode       build.ImportMode
  }
  
  type findpkgValue struct {
  	ready chan struct{} // closed to broadcast readiness
  	bp    *build.Package
  	err   error
  }
  
  // importInfo tracks the success or failure of a single import.
  //
  // Upon completion, exactly one of info and err is non-nil:
  // info on successful creation of a package, err otherwise.
  // A successful package may still contain type errors.
  //
  type importInfo struct {
  	path     string        // import path
  	info     *PackageInfo  // results of typechecking (including errors)
  	complete chan struct{} // closed to broadcast that info is set.
  }
  
  // awaitCompletion blocks until ii is complete,
  // i.e. the info field is safe to inspect.
  func (ii *importInfo) awaitCompletion() {
  	<-ii.complete // wait for close
  }
  
  // Complete marks ii as complete.
  // Its info and err fields will not be subsequently updated.
  func (ii *importInfo) Complete(info *PackageInfo) {
  	if info == nil {
  		panic("info == nil")
  	}
  	ii.info = info
  	close(ii.complete)
  }
  
  type importError struct {
  	path string // import path
  	err  error  // reason for failure to create a package
  }
  
  // Load creates the initial packages specified by conf.{Create,Import}Pkgs,
  // loading their dependencies packages as needed.
  //
  // On success, Load returns a Program containing a PackageInfo for
  // each package.  On failure, it returns an error.
  //
  // If AllowErrors is true, Load will return a Program even if some
  // packages contained I/O, parser or type errors, or if dependencies
  // were missing.  (Such errors are accessible via PackageInfo.Errors.  If
  // false, Load will fail if any package had an error.
  //
  // It is an error if no packages were loaded.
  //
  func (conf *Config) Load() (*Program, error) {
  	// Create a simple default error handler for parse/type errors.
  	if conf.TypeChecker.Error == nil {
  		conf.TypeChecker.Error = func(e error) { fmt.Fprintln(os.Stderr, e) }
  	}
  
  	// Set default working directory for relative package references.
  	if conf.Cwd == "" {
  		var err error
  		conf.Cwd, err = os.Getwd()
  		if err != nil {
  			return nil, err
  		}
  	}
  
  	// Install default FindPackage hook using go/build logic.
  	if conf.FindPackage == nil {
  		conf.FindPackage = (*build.Context).Import
  	}
  
  	prog := &Program{
  		Fset:        conf.fset(),
  		Imported:    make(map[string]*PackageInfo),
  		importMap:   make(map[string]*types.Package),
  		AllPackages: make(map[*types.Package]*PackageInfo),
  	}
  
  	imp := importer{
  		conf:     conf,
  		prog:     prog,
  		findpkg:  make(map[findpkgKey]*findpkgValue),
  		imported: make(map[string]*importInfo),
  		start:    time.Now(),
  		graph:    make(map[string]map[string]bool),
  	}
  
  	// -- loading proper (concurrent phase) --------------------------------
  
  	var errpkgs []string // packages that contained errors
  
  	// Load the initially imported packages and their dependencies,
  	// in parallel.
  	// No vendor check on packages imported from the command line.
  	infos, importErrors := imp.importAll("", conf.Cwd, conf.ImportPkgs, ignoreVendor)
  	for _, ie := range importErrors {
  		conf.TypeChecker.Error(ie.err) // failed to create package
  		errpkgs = append(errpkgs, ie.path)
  	}
  	for _, info := range infos {
  		prog.Imported[info.Pkg.Path()] = info
  	}
  
  	// Augment the designated initial packages by their tests.
  	// Dependencies are loaded in parallel.
  	var xtestPkgs []*build.Package
  	for importPath, augment := range conf.ImportPkgs {
  		if !augment {
  			continue
  		}
  
  		// No vendor check on packages imported from command line.
  		bp, err := imp.findPackage(importPath, conf.Cwd, ignoreVendor)
  		if err != nil {
  			// Package not found, or can't even parse package declaration.
  			// Already reported by previous loop; ignore it.
  			continue
  		}
  
  		// Needs external test package?
  		if len(bp.XTestGoFiles) > 0 {
  			xtestPkgs = append(xtestPkgs, bp)
  		}
  
  		// Consult the cache using the canonical package path.
  		path := bp.ImportPath
  		imp.importedMu.Lock() // (unnecessary, we're sequential here)
  		ii, ok := imp.imported[path]
  		// Paranoid checks added due to issue #11012.
  		if !ok {
  			// Unreachable.
  			// The previous loop called importAll and thus
  			// startLoad for each path in ImportPkgs, which
  			// populates imp.imported[path] with a non-zero value.
  			panic(fmt.Sprintf("imported[%q] not found", path))
  		}
  		if ii == nil {
  			// Unreachable.
  			// The ii values in this loop are the same as in
  			// the previous loop, which enforced the invariant
  			// that at least one of ii.err and ii.info is non-nil.
  			panic(fmt.Sprintf("imported[%q] == nil", path))
  		}
  		if ii.info == nil {
  			// Unreachable.
  			// awaitCompletion has the postcondition
  			// ii.info != nil.
  			panic(fmt.Sprintf("imported[%q].info = nil", path))
  		}
  		info := ii.info
  		imp.importedMu.Unlock()
  
  		// Parse the in-package test files.
  		files, errs := imp.conf.parsePackageFiles(bp, 't')
  		for _, err := range errs {
  			info.appendError(err)
  		}
  
  		// The test files augmenting package P cannot be imported,
  		// but may import packages that import P,
  		// so we must disable the cycle check.
  		imp.addFiles(info, files, false)
  	}
  
  	createPkg := func(path, dir string, files []*ast.File, errs []error) {
  		info := imp.newPackageInfo(path, dir)
  		for _, err := range errs {
  			info.appendError(err)
  		}
  
  		// Ad hoc packages are non-importable,
  		// so no cycle check is needed.
  		// addFiles loads dependencies in parallel.
  		imp.addFiles(info, files, false)
  		prog.Created = append(prog.Created, info)
  	}
  
  	// Create packages specified by conf.CreatePkgs.
  	for _, cp := range conf.CreatePkgs {
  		files, errs := parseFiles(conf.fset(), conf.build(), nil, conf.Cwd, cp.Filenames, conf.ParserMode)
  		files = append(files, cp.Files...)
  
  		path := cp.Path
  		if path == "" {
  			if len(files) > 0 {
  				path = files[0].Name.Name
  			} else {
  				path = "(unnamed)"
  			}
  		}
  
  		dir := conf.Cwd
  		if len(files) > 0 && files[0].Pos().IsValid() {
  			dir = filepath.Dir(conf.fset().File(files[0].Pos()).Name())
  		}
  		createPkg(path, dir, files, errs)
  	}
  
  	// Create external test packages.
  	sort.Sort(byImportPath(xtestPkgs))
  	for _, bp := range xtestPkgs {
  		files, errs := imp.conf.parsePackageFiles(bp, 'x')
  		createPkg(bp.ImportPath+"_test", bp.Dir, files, errs)
  	}
  
  	// -- finishing up (sequential) ----------------------------------------
  
  	if len(prog.Imported)+len(prog.Created) == 0 {
  		return nil, errors.New("no initial packages were loaded")
  	}
  
  	// Create infos for indirectly imported packages.
  	// e.g. incomplete packages without syntax, loaded from export data.
  	for _, obj := range prog.importMap {
  		info := prog.AllPackages[obj]
  		if info == nil {
  			prog.AllPackages[obj] = &PackageInfo{Pkg: obj, Importable: true}
  		} else {
  			// finished
  			info.checker = nil
  			info.errorFunc = nil
  		}
  	}
  
  	if !conf.AllowErrors {
  		// Report errors in indirectly imported packages.
  		for _, info := range prog.AllPackages {
  			if len(info.Errors) > 0 {
  				errpkgs = append(errpkgs, info.Pkg.Path())
  			}
  		}
  		if errpkgs != nil {
  			var more string
  			if len(errpkgs) > 3 {
  				more = fmt.Sprintf(" and %d more", len(errpkgs)-3)
  				errpkgs = errpkgs[:3]
  			}
  			return nil, fmt.Errorf("couldn't load packages due to errors: %s%s",
  				strings.Join(errpkgs, ", "), more)
  		}
  	}
  
  	markErrorFreePackages(prog.AllPackages)
  
  	return prog, nil
  }
  
  type byImportPath []*build.Package
  
  func (b byImportPath) Len() int           { return len(b) }
  func (b byImportPath) Less(i, j int) bool { return b[i].ImportPath < b[j].ImportPath }
  func (b byImportPath) Swap(i, j int)      { b[i], b[j] = b[j], b[i] }
  
  // markErrorFreePackages sets the TransitivelyErrorFree flag on all
  // applicable packages.
  func markErrorFreePackages(allPackages map[*types.Package]*PackageInfo) {
  	// Build the transpose of the import graph.
  	importedBy := make(map[*types.Package]map[*types.Package]bool)
  	for P := range allPackages {
  		for _, Q := range P.Imports() {
  			clients, ok := importedBy[Q]
  			if !ok {
  				clients = make(map[*types.Package]bool)
  				importedBy[Q] = clients
  			}
  			clients[P] = true
  		}
  	}
  
  	// Find all packages reachable from some error package.
  	reachable := make(map[*types.Package]bool)
  	var visit func(*types.Package)
  	visit = func(p *types.Package) {
  		if !reachable[p] {
  			reachable[p] = true
  			for q := range importedBy[p] {
  				visit(q)
  			}
  		}
  	}
  	for _, info := range allPackages {
  		if len(info.Errors) > 0 {
  			visit(info.Pkg)
  		}
  	}
  
  	// Mark the others as "transitively error-free".
  	for _, info := range allPackages {
  		if !reachable[info.Pkg] {
  			info.TransitivelyErrorFree = true
  		}
  	}
  }
  
  // build returns the effective build context.
  func (conf *Config) build() *build.Context {
  	if conf.Build != nil {
  		return conf.Build
  	}
  	return &build.Default
  }
  
  // parsePackageFiles enumerates the files belonging to package path,
  // then loads, parses and returns them, plus a list of I/O or parse
  // errors that were encountered.
  //
  // 'which' indicates which files to include:
  //    'g': include non-test *.go source files (GoFiles + processed CgoFiles)
  //    't': include in-package *_test.go source files (TestGoFiles)
  //    'x': include external *_test.go source files. (XTestGoFiles)
  //
  func (conf *Config) parsePackageFiles(bp *build.Package, which rune) ([]*ast.File, []error) {
  	if bp.ImportPath == "unsafe" {
  		return nil, nil
  	}
  	var filenames []string
  	switch which {
  	case 'g':
  		filenames = bp.GoFiles
  	case 't':
  		filenames = bp.TestGoFiles
  	case 'x':
  		filenames = bp.XTestGoFiles
  	default:
  		panic(which)
  	}
  
  	files, errs := parseFiles(conf.fset(), conf.build(), conf.DisplayPath, bp.Dir, filenames, conf.ParserMode)
  
  	// Preprocess CgoFiles and parse the outputs (sequentially).
  	if which == 'g' && bp.CgoFiles != nil {
  		cgofiles, err := processCgoFiles(bp, conf.fset(), conf.DisplayPath, conf.ParserMode)
  		if err != nil {
  			errs = append(errs, err)
  		} else {
  			files = append(files, cgofiles...)
  		}
  	}
  
  	return files, errs
  }
  
  // doImport imports the package denoted by path.
  // It implements the types.Importer signature.
  //
  // It returns an error if a package could not be created
  // (e.g. go/build or parse error), but type errors are reported via
  // the types.Config.Error callback (the first of which is also saved
  // in the package's PackageInfo).
  //
  // Idempotent.
  //
  func (imp *importer) doImport(from *PackageInfo, to string) (*types.Package, error) {
  	if to == "C" {
  		// This should be unreachable, but ad hoc packages are
  		// not currently subject to cgo preprocessing.
  		// See https://github.com/golang/go/issues/11627.
  		return nil, fmt.Errorf(`the loader doesn't cgo-process ad hoc packages like %q; see Go issue 11627`,
  			from.Pkg.Path())
  	}
  
  	bp, err := imp.findPackage(to, from.dir, 0)
  	if err != nil {
  		return nil, err
  	}
  
  	// The standard unsafe package is handled specially,
  	// and has no PackageInfo.
  	if bp.ImportPath == "unsafe" {
  		return types.Unsafe, nil
  	}
  
  	// Look for the package in the cache using its canonical path.
  	path := bp.ImportPath
  	imp.importedMu.Lock()
  	ii := imp.imported[path]
  	imp.importedMu.Unlock()
  	if ii == nil {
  		panic("internal error: unexpected import: " + path)
  	}
  	if ii.info != nil {
  		return ii.info.Pkg, nil
  	}
  
  	// Import of incomplete package: this indicates a cycle.
  	fromPath := from.Pkg.Path()
  	if cycle := imp.findPath(path, fromPath); cycle != nil {
  		cycle = append([]string{fromPath}, cycle...)
  		return nil, fmt.Errorf("import cycle: %s", strings.Join(cycle, " -> "))
  	}
  
  	panic("internal error: import of incomplete (yet acyclic) package: " + fromPath)
  }
  
  // findPackage locates the package denoted by the importPath in the
  // specified directory.
  func (imp *importer) findPackage(importPath, fromDir string, mode build.ImportMode) (*build.Package, error) {
  	// We use a non-blocking duplicate-suppressing cache (gopl.io §9.7)
  	// to avoid holding the lock around FindPackage.
  	key := findpkgKey{importPath, fromDir, mode}
  	imp.findpkgMu.Lock()
  	v, ok := imp.findpkg[key]
  	if ok {
  		// cache hit
  		imp.findpkgMu.Unlock()
  
  		<-v.ready // wait for entry to become ready
  	} else {
  		// Cache miss: this goroutine becomes responsible for
  		// populating the map entry and broadcasting its readiness.
  		v = &findpkgValue{ready: make(chan struct{})}
  		imp.findpkg[key] = v
  		imp.findpkgMu.Unlock()
  
  		ioLimit <- true
  		v.bp, v.err = imp.conf.FindPackage(imp.conf.build(), importPath, fromDir, mode)
  		<-ioLimit
  
  		if _, ok := v.err.(*build.NoGoError); ok {
  			v.err = nil // empty directory is not an error
  		}
  
  		close(v.ready) // broadcast ready condition
  	}
  	return v.bp, v.err
  }
  
  // importAll loads, parses, and type-checks the specified packages in
  // parallel and returns their completed importInfos in unspecified order.
  //
  // fromPath is the package path of the importing package, if it is
  // importable, "" otherwise.  It is used for cycle detection.
  //
  // fromDir is the directory containing the import declaration that
  // caused these imports.
  //
  func (imp *importer) importAll(fromPath, fromDir string, imports map[string]bool, mode build.ImportMode) (infos []*PackageInfo, errors []importError) {
  	// TODO(adonovan): opt: do the loop in parallel once
  	// findPackage is non-blocking.
  	var pending []*importInfo
  	for importPath := range imports {
  		bp, err := imp.findPackage(importPath, fromDir, mode)
  		if err != nil {
  			errors = append(errors, importError{
  				path: importPath,
  				err:  err,
  			})
  			continue
  		}
  		pending = append(pending, imp.startLoad(bp))
  	}
  
  	if fromPath != "" {
  		// We're loading a set of imports.
  		//
  		// We must record graph edges from the importing package
  		// to its dependencies, and check for cycles.
  		imp.graphMu.Lock()
  		deps, ok := imp.graph[fromPath]
  		if !ok {
  			deps = make(map[string]bool)
  			imp.graph[fromPath] = deps
  		}
  		for _, ii := range pending {
  			deps[ii.path] = true
  		}
  		imp.graphMu.Unlock()
  	}
  
  	for _, ii := range pending {
  		if fromPath != "" {
  			if cycle := imp.findPath(ii.path, fromPath); cycle != nil {
  				// Cycle-forming import: we must not await its
  				// completion since it would deadlock.
  				//
  				// We don't record the error in ii since
  				// the error is really associated with the
  				// cycle-forming edge, not the package itself.
  				// (Also it would complicate the
  				// invariants of importPath completion.)
  				if trace {
  					fmt.Fprintf(os.Stderr, "import cycle: %q\n", cycle)
  				}
  				continue
  			}
  		}
  		ii.awaitCompletion()
  		infos = append(infos, ii.info)
  	}
  
  	return infos, errors
  }
  
  // findPath returns an arbitrary path from 'from' to 'to' in the import
  // graph, or nil if there was none.
  func (imp *importer) findPath(from, to string) []string {
  	imp.graphMu.Lock()
  	defer imp.graphMu.Unlock()
  
  	seen := make(map[string]bool)
  	var search func(stack []string, importPath string) []string
  	search = func(stack []string, importPath string) []string {
  		if !seen[importPath] {
  			seen[importPath] = true
  			stack = append(stack, importPath)
  			if importPath == to {
  				return stack
  			}
  			for x := range imp.graph[importPath] {
  				if p := search(stack, x); p != nil {
  					return p
  				}
  			}
  		}
  		return nil
  	}
  	return search(make([]string, 0, 20), from)
  }
  
  // startLoad initiates the loading, parsing and type-checking of the
  // specified package and its dependencies, if it has not already begun.
  //
  // It returns an importInfo, not necessarily in a completed state.  The
  // caller must call awaitCompletion() before accessing its info field.
  //
  // startLoad is concurrency-safe and idempotent.
  //
  func (imp *importer) startLoad(bp *build.Package) *importInfo {
  	path := bp.ImportPath
  	imp.importedMu.Lock()
  	ii, ok := imp.imported[path]
  	if !ok {
  		ii = &importInfo{path: path, complete: make(chan struct{})}
  		imp.imported[path] = ii
  		go func() {
  			info := imp.load(bp)
  			ii.Complete(info)
  		}()
  	}
  	imp.importedMu.Unlock()
  
  	return ii
  }
  
  // load implements package loading by parsing Go source files
  // located by go/build.
  func (imp *importer) load(bp *build.Package) *PackageInfo {
  	info := imp.newPackageInfo(bp.ImportPath, bp.Dir)
  	info.Importable = true
  	files, errs := imp.conf.parsePackageFiles(bp, 'g')
  	for _, err := range errs {
  		info.appendError(err)
  	}
  
  	imp.addFiles(info, files, true)
  
  	imp.progMu.Lock()
  	imp.prog.importMap[bp.ImportPath] = info.Pkg
  	imp.progMu.Unlock()
  
  	return info
  }
  
  // addFiles adds and type-checks the specified files to info, loading
  // their dependencies if needed.  The order of files determines the
  // package initialization order.  It may be called multiple times on the
  // same package.  Errors are appended to the info.Errors field.
  //
  // cycleCheck determines whether the imports within files create
  // dependency edges that should be checked for potential cycles.
  //
  func (imp *importer) addFiles(info *PackageInfo, files []*ast.File, cycleCheck bool) {
  	// Ensure the dependencies are loaded, in parallel.
  	var fromPath string
  	if cycleCheck {
  		fromPath = info.Pkg.Path()
  	}
  	// TODO(adonovan): opt: make the caller do scanImports.
  	// Callers with a build.Package can skip it.
  	imp.importAll(fromPath, info.dir, scanImports(files), 0)
  
  	if trace {
  		fmt.Fprintf(os.Stderr, "%s: start %q (%d)\n",
  			time.Since(imp.start), info.Pkg.Path(), len(files))
  	}
  
  	// Don't call checker.Files on Unsafe, even with zero files,
  	// because it would mutate the package, which is a global.
  	if info.Pkg == types.Unsafe {
  		if len(files) > 0 {
  			panic(`"unsafe" package contains unexpected files`)
  		}
  	} else {
  		// Ignore the returned (first) error since we
  		// already collect them all in the PackageInfo.
  		info.checker.Files(files)
  		info.Files = append(info.Files, files...)
  	}
  
  	if imp.conf.AfterTypeCheck != nil {
  		imp.conf.AfterTypeCheck(info, files)
  	}
  
  	if trace {
  		fmt.Fprintf(os.Stderr, "%s: stop %q\n",
  			time.Since(imp.start), info.Pkg.Path())
  	}
  }
  
  func (imp *importer) newPackageInfo(path, dir string) *PackageInfo {
  	var pkg *types.Package
  	if path == "unsafe" {
  		pkg = types.Unsafe
  	} else {
  		pkg = types.NewPackage(path, "")
  	}
  	info := &PackageInfo{
  		Pkg: pkg,
  		Info: types.Info{
  			Types:      make(map[ast.Expr]types.TypeAndValue),
  			Defs:       make(map[*ast.Ident]types.Object),
  			Uses:       make(map[*ast.Ident]types.Object),
  			Implicits:  make(map[ast.Node]types.Object),
  			Scopes:     make(map[ast.Node]*types.Scope),
  			Selections: make(map[*ast.SelectorExpr]*types.Selection),
  		},
  		errorFunc: imp.conf.TypeChecker.Error,
  		dir:       dir,
  	}
  
  	// Copy the types.Config so we can vary it across PackageInfos.
  	tc := imp.conf.TypeChecker
  	tc.IgnoreFuncBodies = false
  	if f := imp.conf.TypeCheckFuncBodies; f != nil {
  		tc.IgnoreFuncBodies = !f(path)
  	}
  	tc.Importer = closure{imp, info}
  	tc.Error = info.appendError // appendError wraps the user's Error function
  
  	info.checker = types.NewChecker(&tc, imp.conf.fset(), pkg, &info.Info)
  	imp.progMu.Lock()
  	imp.prog.AllPackages[pkg] = info
  	imp.progMu.Unlock()
  	return info
  }
  
  type closure struct {
  	imp  *importer
  	info *PackageInfo
  }
  
  func (c closure) Import(to string) (*types.Package, error) { return c.imp.doImport(c.info, to) }
  