Improve checkIfPRContentChanged (#22611) (#22644)

Backport #22611

The code for checking if a commit has caused a change in a PR is
extremely inefficient and affects the head repository instead of using a
temporary repository.

This PR therefore makes several significant improvements:

* A temporary repo like that used in merging.
* The diff code is then significant improved to use a three-way diff
instead of comparing diffs (possibly binary) line-by-line - in memory...

Ref #22578

Signed-off-by: Andrew Thornton <art27@cantab.net>

2 files changed, 61 insertions, 53 deletions

diff --git a/modules/util/io.go b/modules/util/io.go
index d765e27733..f3a24736a8 100644
--- a/modules/util/io.go
+++ b/modules/util/io.go
@@ -5,6 +5,7 @@
 package util
 
 import (
+	"errors"
 	"io"
 )
 
@@ -18,3 +19,24 @@ func ReadAtMost(r io.Reader, buf []byte) (n int, err error) {
 	}
 	return n, err
 }
+
+// ErrNotEmpty is an error reported when there is a non-empty reader
+var ErrNotEmpty = errors.New("not-empty")
+
+// IsEmptyReader reads a reader and ensures it is empty
+func IsEmptyReader(r io.Reader) (err error) {
+	var buf [1]byte
+
+	for {
+		n, err := r.Read(buf[:])
+		if err != nil {
+			if err == io.EOF {
+				return nil
+			}
+			return err
+		}
+		if n > 0 {
+			return ErrNotEmpty
+		}
+	}
+}
diff --git a/services/pull/pull.go b/services/pull/pull.go
index 5f8bd6b671..fbfc31d4e7 100644
--- a/services/pull/pull.go
+++ b/services/pull/pull.go
@@ -5,14 +5,12 @@
 package pull
 
 import (
-	"bufio"
-	"bytes"
 	"context"
 	"fmt"
 	"io"
+	"os"
 	"regexp"
 	"strings"
-	"time"
 
 	"code.gitea.io/gitea/models"
 	"code.gitea.io/gitea/models/db"
@@ -30,6 +28,7 @@ import (
 	repo_module "code.gitea.io/gitea/modules/repository"
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/sync"
+	"code.gitea.io/gitea/modules/util"
 	issue_service "code.gitea.io/gitea/services/issue"
 )
 
@@ -352,69 +351,56 @@ func AddTestPullRequestTask(doer *user_model.User, repoID int64, branch string,
 // checkIfPRContentChanged checks if diff to target branch has changed by push
 // A commit can be considered to leave the PR untouched if the patch/diff with its merge base is unchanged
 func checkIfPRContentChanged(ctx context.Context, pr *issues_model.PullRequest, oldCommitID, newCommitID string) (hasChanged bool, err error) {
-	if err = pr.LoadHeadRepoCtx(ctx); err != nil {
-		return false, fmt.Errorf("LoadHeadRepo: %w", err)
-	} else if pr.HeadRepo == nil {
-		// corrupt data assumed changed
-		return true, nil
-	}
-
-	if err = pr.LoadBaseRepoCtx(ctx); err != nil {
-		return false, fmt.Errorf("LoadBaseRepo: %w", err)
-	}
-
-	headGitRepo, err := git.OpenRepository(ctx, pr.HeadRepo.RepoPath())
+	tmpBasePath, err := createTemporaryRepo(ctx, pr)
 	if err != nil {
-		return false, fmt.Errorf("OpenRepository: %w", err)
-	}
-	defer headGitRepo.Close()
-
-	// Add a temporary remote.
-	tmpRemote := "checkIfPRContentChanged-" + fmt.Sprint(time.Now().UnixNano())
-	if err = headGitRepo.AddRemote(tmpRemote, pr.BaseRepo.RepoPath(), true); err != nil {
-		return false, fmt.Errorf("AddRemote: %s/%s-%s: %w", pr.HeadRepo.OwnerName, pr.HeadRepo.Name, tmpRemote, err)
+		log.Error("CreateTemporaryRepo: %v", err)
+		return false, err
 	}
 	defer func() {
-		if err := headGitRepo.RemoveRemote(tmpRemote); err != nil {
-			log.Error("checkIfPRContentChanged: RemoveRemote: %s/%s-%s: %v", pr.HeadRepo.OwnerName, pr.HeadRepo.Name, tmpRemote, err)
+		if err := repo_module.RemoveTemporaryPath(tmpBasePath); err != nil {
+			log.Error("checkIfPRContentChanged: RemoveTemporaryPath: %s", err)
 		}
 	}()
-	// To synchronize repo and get a base ref
-	_, base, err := headGitRepo.GetMergeBase(tmpRemote, pr.BaseBranch, pr.HeadBranch)
+
+	tmpRepo, err := git.OpenRepository(ctx, tmpBasePath)
 	if err != nil {
-		return false, fmt.Errorf("GetMergeBase: %w", err)
+		return false, fmt.Errorf("OpenRepository: %w", err)
 	}
+	defer tmpRepo.Close()
 
-	diffBefore := &bytes.Buffer{}
-	diffAfter := &bytes.Buffer{}
-	if err := headGitRepo.GetDiffFromMergeBase(base, oldCommitID, diffBefore); err != nil {
-		// If old commit not found, assume changed.
-		log.Debug("GetDiffFromMergeBase: %v", err)
-		return true, nil
-	}
-	if err := headGitRepo.GetDiffFromMergeBase(base, newCommitID, diffAfter); err != nil {
-		// New commit should be found
-		return false, fmt.Errorf("GetDiffFromMergeBase: %w", err)
+	// Find the merge-base
+	_, base, err := tmpRepo.GetMergeBase("", "base", "tracking")
+	if err != nil {
+		return false, fmt.Errorf("GetMergeBase: %w", err)
 	}
 
-	diffBeforeLines := bufio.NewScanner(diffBefore)
-	diffAfterLines := bufio.NewScanner(diffAfter)
-
-	for diffBeforeLines.Scan() && diffAfterLines.Scan() {
-		if strings.HasPrefix(diffBeforeLines.Text(), "index") && strings.HasPrefix(diffAfterLines.Text(), "index") {
-			// file hashes can change without the diff changing
-			continue
-		} else if strings.HasPrefix(diffBeforeLines.Text(), "@@") && strings.HasPrefix(diffAfterLines.Text(), "@@") {
-			// the location of the difference may change
-			continue
-		} else if !bytes.Equal(diffBeforeLines.Bytes(), diffAfterLines.Bytes()) {
+	cmd := git.NewCommand(ctx, "diff", "--name-only", "-z").AddDynamicArguments(newCommitID, oldCommitID, base)
+	stdoutReader, stdoutWriter, err := os.Pipe()
+	if err != nil {
+		return false, fmt.Errorf("unable to open pipe for to run diff: %w", err)
+	}
+
+	if err := cmd.Run(&git.RunOpts{
+		Dir:    tmpBasePath,
+		Stdout: stdoutWriter,
+		PipelineFunc: func(ctx context.Context, cancel context.CancelFunc) error {
+			_ = stdoutWriter.Close()
+			defer func() {
+				_ = stdoutReader.Close()
+			}()
+			return util.IsEmptyReader(stdoutReader)
+		},
+	}); err != nil {
+		if err == util.ErrNotEmpty {
 			return true, nil
 		}
-	}
 
-	if diffBeforeLines.Scan() || diffAfterLines.Scan() {
-		// Diffs not of equal length
-		return true, nil
+		log.Error("Unable to run diff on %s %s %s in tempRepo for PR[%d]%s/%s...%s/%s: Error: %v",
+			newCommitID, oldCommitID, base,
+			pr.ID, pr.BaseRepo.FullName(), pr.BaseBranch, pr.HeadRepo.FullName(), pr.HeadBranch,
+			err)
+
+		return false, fmt.Errorf("Unable to run git diff --name-only -z %s %s %s: %w", newCommitID, oldCommitID, base, err)
 	}
 
 	return false, nil