Speed up conflict checking in pull request creation (#276)

* Speed up conflict checking in pull request creation

In order to check conflicts of a PR, we set up a working tree by
cloning the base branch, which is quite time-consuming when the repository is huge.
Instead, this PR uses `git read-tree` and `git apply --check --cached` to check conflicts.

For #258

* Use $GIT_INDEX_FILE instead of --index-output to avoid lockfile problem

The lockfile gets renamed to the final destination after the operation
finishes. But it must be located in the same filesystem, which prevents
us from using /tmp.

* Temporary file names should not prefixed with '-'

models/pull.go

@@ -8,6 +8,8 @@ import (
 	"fmt"
 	"os"
 	"path"
+	"path/filepath"
+	"strconv"
 	"strings"
 	"time"
 
@@ -428,17 +430,22 @@ func (pr *PullRequest) testPatch() (err error) {
 
 	log.Trace("PullRequest[%d].testPatch (patchPath): %s", pr.ID, patchPath)
 
-	// Delete old temp local copy before we create a new temp local copy
-	RemoveAllWithNotice("Deleting old local copy", pr.BaseRepo.LocalCopyPath())
+	pr.Status = PullRequestStatusChecking
+
+	indexTmpPath := filepath.Join(os.TempDir(), "gitea-"+pr.BaseRepo.Name+"-"+strconv.Itoa(time.Now().Nanosecond()))
+	defer os.Remove(indexTmpPath)
 
-	if err := pr.BaseRepo.UpdateLocalCopyBranch(pr.BaseBranch); err != nil {
-		return fmt.Errorf("UpdateLocalCopy: %v", err)
+	var stderr string
+	_, stderr, err = process.ExecDirEnv(-1, "", fmt.Sprintf("testPatch (git read-tree): %d", pr.BaseRepo.ID),
+		[]string{"GIT_DIR=" + pr.BaseRepo.RepoPath(), "GIT_INDEX_FILE=" + indexTmpPath},
+		"git", "read-tree", pr.BaseBranch)
+	if err != nil {
+		return fmt.Errorf("git read-tree --index-output=%s %s: %v - %s", indexTmpPath, pr.BaseBranch, err, stderr)
 	}
 
-	pr.Status = PullRequestStatusChecking
-	_, stderr, err := process.ExecDir(-1, pr.BaseRepo.LocalCopyPath(),
-		fmt.Sprintf("testPatch (git apply --check): %d", pr.BaseRepo.ID),
-		"git", "apply", "--check", patchPath)
+	_, stderr, err = process.ExecDirEnv(-1, "", fmt.Sprintf("testPatch (git apply --check): %d", pr.BaseRepo.ID),
+		[]string{"GIT_INDEX_FILE=" + indexTmpPath, "GIT_DIR=" + pr.BaseRepo.RepoPath()},
+		"git", "apply", "--check", "--cached", patchPath)
 	if err != nil {
 		for i := range patchConflicts {
 			if strings.Contains(stderr, patchConflicts[i]) {