internal/task: improve how fakes report nonexistent file and no tags

The TagXReposTasks.readRepo method calls ReadBranchHead and ReadFile,
and handles errors matching gerrit.ErrResourceNotExist that the real
Gerrit implementation returns. Improve the fakes accordingly, enough
to make the upcoming changes to the tagging workflow its tests happy.

For golang/go#56530.

Change-Id: I4d92d578210b20752e65bdc2719b74bc5c7259ff
Reviewed-on: https://go-review.googlesource.com/c/build/+/523155
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Heschi Kreinick <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
Run-TryBot: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: dmitshur

internal/task/fakes.go

@@ -421,7 +421,11 @@ func (repo *FakeRepo) Branch(branch, commit string) {
 }
 
 func (repo *FakeRepo) ReadFile(commit, file string) ([]byte, error) {
-	return repo.dir.RunCommand(context.Background(), "show", commit+":"+file)
+	b, err := repo.dir.RunCommand(context.Background(), "show", commit+":"+file)
+	if err != nil && strings.Contains(err.Error(), " does not exist ") {
+		err = errors.Join(gerrit.ErrResourceNotExist, err)
+	}
+	return b, err
 }
 
 var _ GerritClient = (*FakeGerrit)(nil)
@@ -447,6 +451,7 @@ func (g *FakeGerrit) ReadBranchHead(ctx context.Context, project, branch string)
 	if err != nil {
 		return "", err
 	}
+	// TODO: If the branch doesn't exist, return an error matching gerrit.ErrResourceNotExist.
 	out, err := repo.dir.RunCommand(ctx, "rev-parse", "refs/heads/"+branch)
 	return strings.TrimSpace(string(out)), err
 }
@@ -465,7 +470,13 @@ func (g *FakeGerrit) ListTags(ctx context.Context, project string) ([]string, er
 		return nil, err
 	}
 	out, err := repo.dir.RunCommand(ctx, "tag", "-l")
-	return strings.Split(strings.TrimSpace(string(out)), "\n"), err
+	if err != nil {
+		return nil, err
+	}
+	if len(out) == 0 {
+		return nil, nil // No tags.
+	}
+	return strings.Split(strings.TrimSpace(string(out)), "\n"), nil
 }
 
 func (g *FakeGerrit) GetTag(ctx context.Context, project string, tag string) (gerrit.TagInfo, error) {

internal/task/gerrit.go

@@ -34,6 +34,7 @@ type GerritClient interface {
 	// ListTags returns all the tags on project.
 	ListTags(ctx context.Context, project string) ([]string, error)
 	// ReadBranchHead returns the head of a branch in project.
+	// If the branch doesn't exist, it returns an error matching gerrit.ErrResourceNotExist.
 	ReadBranchHead(ctx context.Context, project, branch string) (string, error)
 	// ListProjects lists all the projects on the server.
 	ListProjects(ctx context.Context) ([]string, error)