git2go: Add squash parameter to MergeCommand

This change adds support for squashing in gitaly-git2go merge.
This will allow developing new RPC for squash that would be
more optimized, and wouldn't have to use rebase before
performing merge.
We simply use the same strategy as for merge, the only difference
is that we set only one parent to the merge commit.

Changelog: added

3 files changed, 138 insertions, 1 deletions

diff --git a/cmd/gitaly-git2go/merge.go b/cmd/gitaly-git2go/merge.go
index f97125061..80c9662d1 100644
--- a/cmd/gitaly-git2go/merge.go
+++ b/cmd/gitaly-git2go/merge.go
@@ -98,7 +98,13 @@ func merge(request git2go.MergeCommand) (string, error) {
 		committer = git.Signature(git2go.NewSignature(request.CommitterName, request.CommitterMail, request.CommitterDate))
 	}
 
-	commit, err := repo.CreateCommitFromIds("", &author, &committer, request.Message, tree, ours.Id(), theirs.Id())
+	var parents []*git.Oid
+	if request.Squash {
+		parents = []*git.Oid{ours.Id()}
+	} else {
+		parents = []*git.Oid{ours.Id(), theirs.Id()}
+	}
+	commit, err := repo.CreateCommitFromIds("", &author, &committer, request.Message, tree, parents...)
 	if err != nil {
 		return "", fmt.Errorf("could not create merge commit: %w", err)
 	}
diff --git a/cmd/gitaly-git2go/merge_test.go b/cmd/gitaly-git2go/merge_test.go
index b9fff3f6e..da438e527 100644
--- a/cmd/gitaly-git2go/merge_test.go
+++ b/cmd/gitaly-git2go/merge_test.go
@@ -118,6 +118,7 @@ func TestMerge_trees(t *testing.T) {
 		theirs           map[string]string
 		expected         map[string]string
 		withCommitter    bool
+		squash           bool
 		expectedResponse git2go.MergeResult
 		expectedErr      error
 	}{
@@ -159,6 +160,25 @@ func TestMerge_trees(t *testing.T) {
 			},
 		},
 		{
+			desc: "trivial squash succeeds",
+			base: map[string]string{
+				"file": "a",
+			},
+			ours: map[string]string{
+				"file": "a",
+			},
+			theirs: map[string]string{
+				"file": "a",
+			},
+			expected: map[string]string{
+				"file": "a",
+			},
+			squash: true,
+			expectedResponse: git2go.MergeResult{
+				CommitID: "d4c52f063cd6544959d6b0d9a3d8fa8463c34086",
+			},
+		},
+		{
 			desc: "non-trivial merge succeeds",
 			base: map[string]string{
 				"file": "a\nb\nc\nd\ne\nf\n",
@@ -177,6 +197,25 @@ func TestMerge_trees(t *testing.T) {
 			},
 		},
 		{
+			desc: "non-trivial squash succeeds",
+			base: map[string]string{
+				"file": "a\nb\nc\nd\ne\nf\n",
+			},
+			ours: map[string]string{
+				"file": "0\na\nb\nc\nd\ne\nf\n",
+			},
+			theirs: map[string]string{
+				"file": "a\nb\nc\nd\ne\nf\n0\n",
+			},
+			expected: map[string]string{
+				"file": "0\na\nb\nc\nd\ne\nf\n0\n",
+			},
+			squash: true,
+			expectedResponse: git2go.MergeResult{
+				CommitID: "7ef7460f69503265a247e06218391cfa57c521fc",
+			},
+		},
+		{
 			desc: "multiple files succeed",
 			base: map[string]string{
 				"1": "foo",
@@ -203,6 +242,33 @@ func TestMerge_trees(t *testing.T) {
 			},
 		},
 		{
+			desc: "multiple files squash succeed",
+			base: map[string]string{
+				"1": "foo",
+				"2": "bar",
+				"3": "qux",
+			},
+			ours: map[string]string{
+				"1": "foo",
+				"2": "modified",
+				"3": "qux",
+			},
+			theirs: map[string]string{
+				"1": "modified",
+				"2": "bar",
+				"3": "qux",
+			},
+			expected: map[string]string{
+				"1": "modified",
+				"2": "modified",
+				"3": "qux",
+			},
+			squash: true,
+			expectedResponse: git2go.MergeResult{
+				CommitID: "a680459fe541be728c8494fb76c233a344c04460",
+			},
+		},
+		{
 			desc: "conflicting merge fails",
 			base: map[string]string{
 				"1": "foo",
@@ -240,6 +306,7 @@ func TestMerge_trees(t *testing.T) {
 				Message:    "Merge message",
 				Ours:       ours.String(),
 				Theirs:     theirs.String(),
+				Squash:     tc.squash,
 			}
 			if tc.withCommitter {
 				mergeCommand.CommitterName = "Jane Doe"
@@ -282,6 +349,66 @@ func TestMerge_trees(t *testing.T) {
 	}
 }
 
+func TestMerge_squash(t *testing.T) {
+	t.Parallel()
+
+	ctx := testhelper.Context(t)
+
+	cfg, repoProto, repoPath := testcfg.BuildWithRepo(t)
+	testcfg.BuildGitalyGit2Go(t, cfg)
+	executor := buildExecutor(t, cfg)
+
+	baseFiles := map[string]string{"file.txt": "b\nc"}
+	ourFiles := map[string]string{"file.txt": "a\nb\nc"}
+	theirFiles1 := map[string]string{"file.txt": "b\nc\nd"}
+	theirFiles2 := map[string]string{"file.txt": "b\nc\nd\ne"}
+
+	base := cmdtesthelper.BuildCommit(t, repoPath, []*git.Oid{nil}, baseFiles)
+	ours := cmdtesthelper.BuildCommit(t, repoPath, []*git.Oid{base}, ourFiles)
+	theirs1 := cmdtesthelper.BuildCommit(t, repoPath, []*git.Oid{base}, theirFiles1)
+	theirs2 := cmdtesthelper.BuildCommit(t, repoPath, []*git.Oid{theirs1}, theirFiles2)
+
+	date := time.Date(2020, 7, 30, 7, 45, 50, 0, time.FixedZone("UTC+2", +2*60*60))
+	response, err := executor.Merge(ctx, repoProto, git2go.MergeCommand{
+		Repository: repoPath,
+		AuthorName: "John Doe",
+		AuthorMail: "john.doe@example.com",
+		AuthorDate: date,
+		Message:    "Merge message",
+		Ours:       ours.String(),
+		Theirs:     theirs2.String(),
+		Squash:     true,
+	})
+	require.NoError(t, err)
+	assert.Equal(t, "027d909803fbb3d17c3b10c1dfe8f120d99392e4", response.CommitID)
+
+	repo, err := git2goutil.OpenRepository(repoPath)
+	require.NoError(t, err)
+
+	commitOid, err := git.NewOid(response.CommitID)
+	require.NoError(t, err)
+
+	isDescendant, err := repo.DescendantOf(commitOid, theirs2)
+	require.NoError(t, err)
+	require.False(t, isDescendant)
+
+	commit, err := repo.LookupCommit(commitOid)
+	require.NoError(t, err)
+
+	require.Equal(t, uint(1), commit.ParentCount())
+	require.Equal(t, ours, commit.ParentId(0))
+
+	tree, err := commit.Tree()
+	require.NoError(t, err)
+
+	entry := tree.EntryByName("file.txt")
+	require.NotNil(t, entry)
+
+	blob, err := repo.LookupBlob(entry.Id)
+	require.NoError(t, err)
+	require.Equal(t, "a\nb\nc\nd\ne", string(blob.Contents()))
+}
+
 func TestMerge_recursive(t *testing.T) {
 	t.Parallel()
 	ctx := testhelper.Context(t)
diff --git a/internal/git2go/merge.go b/internal/git2go/merge.go
index 17e9a4fe2..b0f2e5503 100644
--- a/internal/git2go/merge.go
+++ b/internal/git2go/merge.go
@@ -43,6 +43,10 @@ type MergeCommand struct {
 	// AllowConflicts controls whether conflicts are allowed. If they are,
 	// then conflicts will be committed as part of the result.
 	AllowConflicts bool
+	// Squash controls whether to perform squash merge.
+	// If set to `true`, then the resulting commit will have `Ours` as its only parent.
+	// Otherwise, a merge commit will be created with `Ours` and `Theirs` as its parents.
+	Squash bool
 }
 
 // MergeResult contains results from a merge.