Issue:
Bug: refactor(user): Make password nullable for `users`



Currently `password` is required field in `users` table. This is causing us to create temporary passwords for `users` in some APIs which inserts `users` without password.

So, we have to make password nullable to solve this.

Pull request description:
## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [ ] Bugfix
- [ ] New feature
- [ ] Enhancement
- [x] Refactoring
- [ ] Dependency updates
- [ ] Documentation
- [ ] CI/CD

## Description
<!-- Describe your changes in detail -->
This API will make the password column in the `users` table nullable.

### Additional Changes

- [ ] This PR modifies the API contract
- [x] This PR modifies the database schema
- [ ] This PR modifies application configuration/environment variables

<!--
Provide links to the files with corresponding changes.

Following are the paths where you can find config files:
1. `config`
2. `crates/router/src/configs`
3. `loadtest/config`
-->


## Motivation and Context
<!--
Why is this change required? What problem does it solve?
If it fixes an open issue, please link to the issue here.

If you don't have an issue, we'd recommend starting with one first so the PR
can focus on the implementation (unless it is an obvious bug or documentation fix
that will have little conversation).
-->
Closes #4901.

## How did you test it?
<!--
Did you write an integration/unit/API test to verify the code changes?
Or did you test this change manually (provide relevant screenshots)?
-->
1. Create user with `connect_account` API
	```
	curl --location 'http://localhost:8080/user/connect_account' \
	--header 'Content-Type: application/json' \
	--data-raw '{
	    "email": "unregistered email"
	}'
	```

2. Invite a unregistered user
	```
	curl --location 'http://localhost:8080/user/user/invite_multiple' \
	--header 'Content-Type: application/json' \
	--header 'Authorization: Bearer JWT' \
	--data-raw '[
	    {
	        "email": "new user email",
	        "name": "name",
	        "role_id": "merchant_view_only"
	    }
	]'
	```

3. Check the DB entry for those users using the following query
	```sql
	select * from users where email = 'email used in previous API'
	```
	Password should be null in all the cases.

## Checklist
<!-- Put an `x` in the boxes that apply -->

- [x] I formatted the code `cargo +nightly fmt --all`
- [x] I addressed lints thrown by `cargo clippy`
- [x] I reviewed the submitted code
- [ ] I added unit tests for my changes where possible


Testing:

1. Create user with `connect_account` API
	```
	curl --location 'http://localhost:8080/user/connect_account' \
	--header 'Content-Type: application/json' \
	--data-raw '{
	    "email": "unregistered email"
	}'
	```

2. Invite a unregistered user
	```
	curl --location 'http://localhost:8080/user/user/invite_multiple' \
	--header 'Content-Type: application/json' \
	--header 'Authorization: Bearer JWT' \
	--data-raw '[
	    {
	        "email": "new user email",
	        "name": "name",
	        "role_id": "merchant_view_only"
	    }
	]'
	```

3. Check the DB entry for those users using the following query
	```sql
	select * from users where email = 'email used in previous API'
	```
	Password should be null in all the cases.

Patch:
diff --git a/crates/diesel_models/src/schema.rs b/crates/diesel_models/src/schema.rs
index 15e662fe438..a7d5a8c02bc 100644
--- a/crates/diesel_models/src/schema.rs
+++ b/crates/diesel_models/src/schema.rs
@@ -1223,7 +1223,7 @@ diesel::table! {
         #[max_length = 255]
         name -> Varchar,
         #[max_length = 255]
-        password -> Varchar,
+        password -> Nullable<Varchar>,
         is_verified -> Bool,
         created_at -> Timestamp,
         last_modified_at -> Timestamp,
diff --git a/crates/diesel_models/src/user.rs b/crates/diesel_models/src/user.rs
index 6a040e41468..b1bbb66256c 100644
--- a/crates/diesel_models/src/user.rs
+++ b/crates/diesel_models/src/user.rs
@@ -17,7 +17,7 @@ pub struct User {
     pub user_id: String,
     pub email: pii::Email,
     pub name: Secret<String>,
-    pub password: Secret<String>,
+    pub password: Option<Secret<String>>,
     pub is_verified: bool,
     pub created_at: PrimitiveDateTime,
     pub last_modified_at: PrimitiveDateTime,
@@ -37,7 +37,7 @@ pub struct UserNew {
     pub user_id: String,
     pub email: pii::Email,
     pub name: Secret<String>,
-    pub password: Secret<String>,
+    pub password: Option<Secret<String>>,
     pub is_verified: bool,
     pub created_at: Option<PrimitiveDateTime>,
     pub last_modified_at: Option<PrimitiveDateTime>,
@@ -76,7 +76,7 @@ pub enum UserUpdate {
         totp_recovery_codes: Option<Vec<Secret<String>>>,
     },
     PasswordUpdate {
-        password: Option<Secret<String>>,
+        password: Secret<String>,
     },
 }
 
@@ -127,7 +127,7 @@ impl From<UserUpdate> for UserUpdateInternal {
             },
             UserUpdate::PasswordUpdate { password } => Self {
                 name: None,
-                password,
+                password: Some(password),
                 is_verified: None,
                 last_modified_at,
                 preferred_merchant_id: None,
diff --git a/crates/router/src/core/user.rs b/crates/router/src/core/user.rs
index a4bb4fb4b03..c78f2c8080f 100644
--- a/crates/router/src/core/user.rs
+++ b/crates/router/src/core/user.rs
@@ -368,7 +368,7 @@ pub async fn change_password(
         .update_user_by_user_id(
             user.get_user_id(),
             diesel_models::user::UserUpdate::PasswordUpdate {
-                password: Some(new_password_hash),
+                password: new_password_hash,
             },
         )
         .await
@@ -459,7 +459,7 @@ pub async fn rotate_password(
         .update_user_by_user_id(
             &user_token.user_id,
             storage_user::UserUpdate::PasswordUpdate {
-                password: Some(hash_password),
+                password: hash_password,
             },
         )
         .await
@@ -510,7 +510,7 @@ pub async fn reset_password_token_only_flow(
                 .get_email()
                 .change_context(UserErrors::InternalServerError)?,
             storage_user::UserUpdate::PasswordUpdate {
-                password: Some(hash_password),
+                password: hash_password,
             },
         )
         .await
@@ -548,7 +548,7 @@ pub async fn reset_password(
                 .get_email()
                 .change_context(UserErrors::InternalServerError)?,
             storage_user::UserUpdate::PasswordUpdate {
-                password: Some(hash_password),
+                password: hash_password,
             },
         )
         .await
@@ -838,11 +838,9 @@ async fn handle_new_user_invitation(
 
     Ok(InviteMultipleUserResponse {
         is_email_sent,
-        password: if cfg!(not(feature = "email")) {
-            Some(new_user.get_password().get_secret())
-        } else {
-            None
-        },
+        password: new_user
+            .get_password()
+            .map(|password| password.get_secret()),
         email: request.email.clone(),
         error: None,
     })
diff --git a/crates/router/src/db/user.rs b/crates/router/src/db/user.rs
index e8104e3a155..742944bd1e0 100644
--- a/crates/router/src/db/user.rs
+++ b/crates/router/src/db/user.rs
@@ -244,7 +244,7 @@ impl UserInterface for MockDb {
                         ..user.to_owned()
                     },
                     storage::UserUpdate::PasswordUpdate { password } => storage::User {
-                        password: password.clone().unwrap_or(user.password.clone()),
+                        password: Some(password.clone()),
                         last_password_modified_at: Some(common_utils::date_time::now()),
                         ..user.to_owned()
                     },
@@ -299,7 +299,7 @@ impl UserInterface for MockDb {
                         ..user.to_owned()
                     },
                     storage::UserUpdate::PasswordUpdate { password } => storage::User {
-                        password: password.clone().unwrap_or(user.password.clone()),
+                        password: Some(password.clone()),
                         last_password_modified_at: Some(common_utils::date_time::now()),
                         ..user.to_owned()
                     },
diff --git a/crates/router/src/types/domain/user.rs b/crates/router/src/types/domain/user.rs
index d9ef736347f..47f26ce04e5 100644
--- a/crates/router/src/types/domain/user.rs
+++ b/crates/router/src/types/domain/user.rs
@@ -1,8 +1,4 @@
-use std::{
-    collections::HashSet,
-    ops::{self, Not},
-    str::FromStr,
-};
+use std::{collections::HashSet, ops, str::FromStr};
 
 use api_models::{
     admin as admin_api, organization as api_org, user as user_api, user_role as user_role_api,
@@ -517,9 +513,8 @@ pub struct NewUser {
     user_id: String,
     name: UserName,
     email: UserEmail,
-    password: UserPassword,
+    password: Option<UserPassword>,
     new_merchant: NewUserMerchant,
-    is_temporary_password: bool,
 }
 
 impl NewUser {
@@ -539,7 +534,7 @@ impl NewUser {
         self.new_merchant.clone()
     }
 
-    pub fn get_password(&self) -> UserPassword {
+    pub fn get_password(&self) -> Option<UserPassword> {
         self.password.clone()
     }
 
@@ -623,7 +618,12 @@ impl TryFrom<NewUser> for storage_user::UserNew {
     type Error = error_stack::Report<UserErrors>;
 
     fn try_from(value: NewUser) -> UserResult<Self> {
-        let hashed_password = password::generate_password_hash(value.password.get_secret())?;
+        let hashed_password = value
+            .password
+            .as_ref()
+            .map(|password| password::generate_password_hash(password.get_secret()))
+            .transpose()?;
+
         let now = common_utils::date_time::now();
         Ok(Self {
             user_id: value.get_user_id(),
@@ -637,7 +637,7 @@ impl TryFrom<NewUser> for storage_user::UserNew {
             totp_status: TotpStatus::NotSet,
             totp_secret: None,
             totp_recovery_codes: None,
-            last_password_modified_at: value.is_temporary_password.not().then_some(now),
+            last_password_modified_at: value.password.is_some().then_some(now),
         })
     }
 }
@@ -655,10 +655,9 @@ impl TryFrom<user_api::SignUpWithMerchantIdRequest> for NewUser {
         Ok(Self {
             name,
             email,
-            password,
+            password: Some(password),
             user_id,
             new_merchant,
-            is_temporary_password: false,
         })
     }
 }
@@ -677,9 +676,8 @@ impl TryFrom<user_api::SignUpRequest> for NewUser {
             user_id,
             name,
             email,
-            password,
+            password: Some(password),
             new_merchant,
-            is_temporary_password: false,
         })
     }
 }
@@ -691,16 +689,14 @@ impl TryFrom<user_api::ConnectAccountRequest> for NewUser {
         let user_id = uuid::Uuid::new_v4().to_string();
         let email = value.email.clone().try_into()?;
         let name = UserName::try_from(value.email.clone())?;
-        let password = UserPassword::new(password::get_temp_password())?;
         let new_merchant = NewUserMerchant::try_from(value)?;
 
         Ok(Self {
             user_id,
             name,
             email,
-            password,
+            password: None,
             new_merchant,
-            is_temporary_password: true,
         })
     }
 }
@@ -721,9 +717,8 @@ impl TryFrom<(user_api::CreateInternalUserRequest, String)> for NewUser {
             user_id,
             name,
             email,
-            password,
+            password: Some(password),
             new_merchant,
-            is_temporary_password: false,
         })
     }
 }
@@ -739,11 +734,12 @@ impl TryFrom<UserMerchantCreateRequestWithToken> for NewUser {
             user_id: user.0.user_id,
             name: UserName::new(user.0.name)?,
             email: user.0.email.clone().try_into()?,
-            password: UserPassword::new_password_without_validation(user.0.password)?,
+            password: user
+                .0
+                .password
+                .map(UserPassword::new_password_without_validation)
+                .transpose()?,
             new_merchant,
-            // This is true because we are not creating a user with this request. And if it is set
-            // to false, last_password_modified_at will be overwritten if this user is inserted.
-            is_temporary_password: true,
         })
     }
 }
@@ -754,7 +750,8 @@ impl TryFrom<InviteeUserRequestWithInvitedUserToken> for NewUser {
         let user_id = uuid::Uuid::new_v4().to_string();
         let email = value.0.email.clone().try_into()?;
         let name = UserName::new(value.0.name.clone())?;
-        let password = UserPassword::new(password::get_temp_password())?;
+        let password = cfg!(not(feature = "email"))
+            .then_some(UserPassword::new(password::get_temp_password())?);
         let new_merchant = NewUserMerchant::try_from(value)?;
 
         Ok(Self {
@@ -763,7 +760,6 @@ impl TryFrom<InviteeUserRequestWithInvitedUserToken> for NewUser {
             email,
             password,
             new_merchant,
-            is_temporary_password: true,
         })
     }
 }
@@ -783,10 +779,14 @@ impl UserFromStorage {
     }
 
     pub fn compare_password(&self, candidate: &Secret<String>) -> UserResult<()> {
-        match password::is_correct_password(candidate, &self.0.password) {
-            Ok(true) => Ok(()),
-            Ok(false) => Err(UserErrors::InvalidCredentials.into()),
-            Err(e) => Err(e),
+        if let Some(password) = self.0.password.as_ref() {
+            match password::is_correct_password(candidate, password) {
+                Ok(true) => Ok(()),
+                Ok(false) => Err(UserErrors::InvalidCredentials.into()),
+                Err(e) => Err(e),
+            }
+        } else {
+            Err(UserErrors::InvalidCredentials.into())
         }
     }
 
diff --git a/migrations/2024-06-06-101812_user_optional_password/down.sql b/migrations/2024-06-06-101812_user_optional_password/down.sql
new file mode 100644
index 00000000000..acd0b1efca3
--- /dev/null
+++ b/migrations/2024-06-06-101812_user_optional_password/down.sql
@@ -0,0 +1,2 @@
+-- This file should undo anything in `up.sql`
+ALTER TABLE users ALTER COLUMN password SET NOT NULL;
diff --git a/migrations/2024-06-06-101812_user_optional_password/up.sql b/migrations/2024-06-06-101812_user_optional_password/up.sql
new file mode 100644
index 00000000000..e48b1b751ab
--- /dev/null
+++ b/migrations/2024-06-06-101812_user_optional_password/up.sql
@@ -0,0 +1,2 @@
+-- Your SQL goes here
+ALTER TABLE users ALTER COLUMN password DROP NOT NULL;