pkg/covermerger: optimize checkouts

Every commit checkout takes >3 seconds.
Let's optimize this operation to save on large merges.

3 files changed, 23 insertions, 0 deletions

diff --git a/pkg/vcs/fuchsia.go b/pkg/vcs/fuchsia.go
index 223c0b860..2ad0e1878 100644
--- a/pkg/vcs/fuchsia.go
+++ b/pkg/vcs/fuchsia.go
@@ -103,3 +103,7 @@ func (ctx *fuchsia) Object(name, commit string) ([]byte, error) {
 func (ctx *fuchsia) MergeBases(firstCommit, secondCommit string) ([]*Commit, error) {
 	return ctx.repo.MergeBases(firstCommit, secondCommit)
 }
+
+func (ctx *fuchsia) CommitExists(string) (bool, error) {
+	return false, fmt.Errorf("not implemented for fuchsia")
+}
diff --git a/pkg/vcs/git.go b/pkg/vcs/git.go
index 840a8592e..d1a91404c 100644
--- a/pkg/vcs/git.go
+++ b/pkg/vcs/git.go
@@ -619,3 +619,19 @@ func (git *git) MergeBases(firstCommit, secondCommit string) ([]*Commit, error)
 	}
 	return ret, nil
 }
+
+// CommitExists relies on 'git cat-file -e'.
+// If object exists its exit status is 0.
+// If object doesn't exist its exit status is 1 (not documented).
+// Otherwise, the exit status is 128 (not documented).
+func (git *git) CommitExists(commit string) (bool, error) {
+	_, err := git.git("cat-file", "-e", commit)
+	var vErr *osutil.VerboseError
+	if errors.As(err, &vErr) && vErr.ExitCode == 1 {
+		return false, nil
+	}
+	if err != nil {
+		return false, err
+	}
+	return true, nil
+}
diff --git a/pkg/vcs/vcs.go b/pkg/vcs/vcs.go
index faf1cf85d..0730ba721 100644
--- a/pkg/vcs/vcs.go
+++ b/pkg/vcs/vcs.go
@@ -69,6 +69,9 @@ type Repo interface {
 
 	// MergeBases returns good common ancestors of the two commits.
 	MergeBases(firstCommit, secondCommit string) ([]*Commit, error)
+
+	// CommitExists check for the commit presence in local checkout.
+	CommitExists(commit string) (bool, error)
 }
 
 // Bisecter may be optionally implemented by Repo.