Fix misuse of TxContext (#30061)

Help #29999, or its tests cannot pass.

Also, add some comments to clarify the usage of `TxContext`.

I don't check all usages of `TxContext` because there are too many
(almost 140+). It's a better idea to replace them with `WithTx` instead
of checking them one by one. However, that may be another refactoring
PR.

(cherry picked from commit c6c4d66004c70b24abc8048b39b660b8361a0395)

models/db/context.go

@@ -120,6 +120,16 @@ func (c *halfCommitter) Close() error {
 
 // TxContext represents a transaction Context,
 // it will reuse the existing transaction in the parent context or create a new one.
+// Some tips to use:
+//
+//	1 It's always recommended to use `WithTx` in new code instead of `TxContext`, since `WithTx` will handle the transaction automatically.
+//	2. To maintain the old code which uses `TxContext`:
+//	  a. Always call `Close()` before returning regardless of whether `Commit()` has been called.
+//	  b. Always call `Commit()` before returning if there are no errors, even if the code did not change any data.
+//	  c. Remember the `Committer` will be a halfCommitter when a transaction is being reused.
+//	     So calling `Commit()` will do nothing, but calling `Close()` without calling `Commit()` will rollback the transaction.
+//	     And all operations submitted by the caller stack will be rollbacked as well, not only the operations in the current function.
+//	  d. It doesn't mean rollback is forbidden, but always do it only when there is an error, and you do want to rollback.
 func TxContext(parentCtx context.Context) (*Context, Committer, error) {
 	if sess, ok := inTransaction(parentCtx); ok {
 		return newContext(parentCtx, sess, true), &halfCommitter{committer: sess}, nil

models/issues/review.go

@@ -620,7 +620,7 @@ func AddReviewRequest(ctx context.Context, issue *Issue, reviewer, doer *user_mo
 
 	// skip it when reviewer hase been request to review
 	if review != nil && review.Type == ReviewTypeRequest {
-		return nil, nil
+		return nil, committer.Commit() // still commit the transaction, or committer.Close() will rollback it, even if it's a reused transaction.
 	}
 
 	// if the reviewer is an official reviewer,