Improve TestPatch to use git read-tree -m and implement git-merge-one-file functionality (#18004)

The current TestPatch conflict code uses a plain git apply which does not properly
account for 3-way merging. However, we can improve things using `git read-tree -m` to
do a three-way merge then follow the algorithm used in merge-one-file. We can also use 
`--patience` and/or `--histogram` to generate a nicer diff for applying patches too.

Fix #13679
Fix #6417

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

services/pull/patch_unmerged.go

@@ -0,0 +1,180 @@
+// Copyright 2021 The Gitea Authors.
+// All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package pull
+
+import (
+	"bufio"
+	"context"
+	"fmt"
+	"io"
+	"os"
+	"strconv"
+	"strings"
+
+	"code.gitea.io/gitea/modules/git"
+	"code.gitea.io/gitea/modules/log"
+)
+
+// lsFileLine is a Quadruplet struct (+error) representing a partially parsed line from ls-files
+type lsFileLine struct {
+	mode  string
+	sha   string
+	stage int
+	path  string
+	err   error
+}
+
+// SameAs checks if two lsFileLines are referring to the same path, sha and mode (ignoring stage)
+func (line *lsFileLine) SameAs(other *lsFileLine) bool {
+	if line == nil || other == nil {
+		return false
+	}
+
+	if line.err != nil || other.err != nil {
+		return false
+	}
+
+	return line.mode == other.mode &&
+		line.sha == other.sha &&
+		line.path == other.path
+}
+
+// readUnmergedLsFileLines calls git ls-files -u -z and parses the lines into mode-sha-stage-path quadruplets
+// it will push these to the provided channel closing it at the end
+func readUnmergedLsFileLines(ctx context.Context, tmpBasePath string, outputChan chan *lsFileLine) {
+	defer func() {
+		// Always close the outputChan at the end of this function
+		close(outputChan)
+	}()
+
+	lsFilesReader, lsFilesWriter, err := os.Pipe()
+	if err != nil {
+		log.Error("Unable to open stderr pipe: %v", err)
+		outputChan <- &lsFileLine{err: fmt.Errorf("unable to open stderr pipe: %v", err)}
+		return
+	}
+	defer func() {
+		_ = lsFilesWriter.Close()
+		_ = lsFilesReader.Close()
+	}()
+
+	stderr := &strings.Builder{}
+	err = git.NewCommandContext(ctx, "ls-files", "-u", "-z").
+		RunInDirTimeoutEnvFullPipelineFunc(
+			nil, -1, tmpBasePath,
+			lsFilesWriter, stderr, nil,
+			func(_ context.Context, _ context.CancelFunc) error {
+				_ = lsFilesWriter.Close()
+				defer func() {
+					_ = lsFilesReader.Close()
+				}()
+				bufferedReader := bufio.NewReader(lsFilesReader)
+
+				for {
+					line, err := bufferedReader.ReadString('\000')
+					if err != nil {
+						if err == io.EOF {
+							return nil
+						}
+						return err
+					}
+					toemit := &lsFileLine{}
+
+					split := strings.SplitN(line, " ", 3)
+					if len(split) < 3 {
+						return fmt.Errorf("malformed line: %s", line)
+					}
+					toemit.mode = split[0]
+					toemit.sha = split[1]
+
+					if len(split[2]) < 4 {
+						return fmt.Errorf("malformed line: %s", line)
+					}
+
+					toemit.stage, err = strconv.Atoi(split[2][0:1])
+					if err != nil {
+						return fmt.Errorf("malformed line: %s", line)
+					}
+
+					toemit.path = split[2][2 : len(split[2])-1]
+					outputChan <- toemit
+				}
+			})
+
+	if err != nil {
+		outputChan <- &lsFileLine{err: fmt.Errorf("git ls-files -u -z: %v", git.ConcatenateError(err, stderr.String()))}
+	}
+}
+
+// unmergedFile is triple (+error) of lsFileLines split into stages 1,2 & 3.
+type unmergedFile struct {
+	stage1 *lsFileLine
+	stage2 *lsFileLine
+	stage3 *lsFileLine
+	err    error
+}
+
+// unmergedFiles will collate the output from readUnstagedLsFileLines in to file triplets and send them
+// to the provided channel, closing at the end.
+func unmergedFiles(ctx context.Context, tmpBasePath string, unmerged chan *unmergedFile) {
+	defer func() {
+		// Always close the channel
+		close(unmerged)
+	}()
+
+	ctx, cancel := context.WithCancel(ctx)
+	lsFileLineChan := make(chan *lsFileLine, 10) // give lsFileLineChan a buffer
+	go readUnmergedLsFileLines(ctx, tmpBasePath, lsFileLineChan)
+	defer func() {
+		cancel()
+		for range lsFileLineChan {
+			// empty channel
+		}
+	}()
+
+	next := &unmergedFile{}
+	for line := range lsFileLineChan {
+		if line.err != nil {
+			log.Error("Unable to run ls-files -u -z! Error: %v", line.err)
+			unmerged <- &unmergedFile{err: fmt.Errorf("unable to run ls-files -u -z! Error: %v", line.err)}
+			return
+		}
+
+		// stages are always emitted 1,2,3 but sometimes 1, 2 or 3 are dropped
+		switch line.stage {
+		case 0:
+			// Should not happen as this represents successfully merged file - we will tolerate and ignore though
+		case 1:
+			if next.stage1 != nil {
+				// We need to handle the unstaged file stage1,stage2,stage3
+				unmerged <- next
+			}
+			next = &unmergedFile{stage1: line}
+		case 2:
+			if next.stage3 != nil || next.stage2 != nil || (next.stage1 != nil && next.stage1.path != line.path) {
+				// We need to handle the unstaged file stage1,stage2,stage3
+				unmerged <- next
+				next = &unmergedFile{}
+			}
+			next.stage2 = line
+		case 3:
+			if next.stage3 != nil || (next.stage1 != nil && next.stage1.path != line.path) || (next.stage2 != nil && next.stage2.path != line.path) {
+				// We need to handle the unstaged file stage1,stage2,stage3
+				unmerged <- next
+				next = &unmergedFile{}
+			}
+			next.stage3 = line
+		default:
+			log.Error("Unexpected stage %d for path %s in run ls-files -u -z!", line.stage, line.path)
+			unmerged <- &unmergedFile{err: fmt.Errorf("unexpected stage %d for path %s in git ls-files -u -z", line.stage, line.path)}
+			return
+		}
+	}
+	// We need to handle the unstaged file stage1,stage2,stage3
+	if next.stage1 != nil || next.stage2 != nil || next.stage3 != nil {
+		unmerged <- next
+	}
+}