Redesign sidecar sync: baseline bootstrap + incremental patch

acceptance/sidecar_test.go

@@ -365,7 +365,11 @@ func TestSidecarsSshSyncFlags(t *testing.T) {
 			env := testenv.NewTestEnv(t)
 			env.CircleCIURL = srv.URL
 
-			result := binary.RunCLI(t, tt.args, env, env.HomeDir)
+			// sync runs git operations (branch detection) before opening SSH, so
+			// the working directory must be a git repo.
+			workDir := gitrepo.SetupGitRepo(t, "test-org", "test-repo")
+
+			result := binary.RunCLI(t, tt.args, env, workDir)
 
 			// Commands should fail at SSH key step, not at flag parsing
 			assert.Assert(t, result.ExitCode != 0, "expected non-zero exit (SSH fails)")

internal/cmd/sidecar.go

@@ -409,7 +409,11 @@ func newSidecarSyncCmd() *cobra.Command {
 			if err != nil {
 				return err
 			}
-			err = sidecar.Sync(cmd.Context(), client, sidecarID, identityFile, authSock, workdir, newStatusFunc(io))
+			cwd, err := os.Getwd()
+			if err != nil {
+				return fmt.Errorf("sync: %w", err)
+			}
+			err = sidecar.Sync(cmd.Context(), client, sidecarID, identityFile, authSock, workdir, cwd, newStatusFunc(io))
 			if err != nil {
 				if _, ok := errors.AsType[*sidecar.RemoteBaseError](err); ok {
 					return &userError{
@@ -827,7 +831,7 @@ Example:
 
 			// Step 4: Sync files to sidecar.
 			if !skipSync {
-				if err := sidecarSetupSync(cmd.Context(), client, sidecarID, identityFile, authSock, status); err != nil {
+				if err := sidecarSetupSync(cmd.Context(), client, sidecarID, identityFile, authSock, dir, status); err != nil {
 					return err
 				}
 			}
@@ -917,11 +921,11 @@ func sidecarSetupEnsureSSHKey(identityFile string, status iostream.StatusFunc) e
 func sidecarSetupSync(
 	ctx context.Context,
 	client *circleci.Client,
-	sidecarID, identityFile, authSock string,
+	sidecarID, identityFile, authSock, cwd string,
 	status iostream.StatusFunc,
 ) error {
 	status(iostream.LevelStep, "Syncing files to sidecar...")
-	err := sidecar.Sync(ctx, client, sidecarID, identityFile, authSock, "", status)
+	err := sidecar.Bootstrap(ctx, client, sidecarID, identityFile, authSock, "", cwd, status)
 	if err == nil {
 		return nil
 	}

internal/gitutil/gitutil.go

@@ -60,17 +60,39 @@ func MergeBase() (string, error) {
 		}
 	}
 
-	out, err = exec.Command("git", "rev-parse", "origin/HEAD").Output()
+	out, err = exec.Command("git", "merge-base", "HEAD", "origin/HEAD").Output()
 	if err != nil {
 		return "", fmt.Errorf("resolve remote base: no upstream tracking branch or origin/HEAD found")
 	}
 	sha := strings.TrimSpace(string(out))
 	if sha == "" {
-		return "", fmt.Errorf("resolve remote base: origin/HEAD is empty")
+		return "", fmt.Errorf("resolve remote base: could not determine merge-base with origin/HEAD")
 	}
 	return sha, nil
 }
 
+// HeadRef returns the SHA of the current HEAD commit in the repo at cwd.
+func HeadRef(cwd string) (string, error) {
+	cmd := exec.Command("git", "rev-parse", "HEAD")
+	cmd.Dir = cwd
+	out, err := cmd.Output()
+	if err != nil {
+		return "", fmt.Errorf("resolve HEAD: %w", err)
+	}
+	sha := strings.TrimSpace(string(out))
+	if sha == "" {
+		return "", fmt.Errorf("resolve HEAD: empty output")
+	}
+	return sha, nil
+}
+
+// IsAncestor returns true if candidate is an ancestor of head in the repo at cwd.
+func IsAncestor(candidate, head, cwd string) bool {
+	cmd := exec.Command("git", "merge-base", "--is-ancestor", candidate, head)
+	cmd.Dir = cwd
+	return cmd.Run() == nil
+}
+
 // GeneratePatch generates a binary diff from the given base commit,
 // including untracked files. It temporarily stages untracked files
 // with git add -N and resets them after generating the diff.

internal/sidecar/active.go

@@ -13,9 +13,11 @@ import (
 
 // ActiveSidecar holds the currently active sidecar for a project.
 type ActiveSidecar struct {
-	SidecarID string `json:"sidecar_id"`
-	Name      string `json:"name,omitempty"`
-	Workspace string `json:"workspace,omitempty"`
+	SidecarID      string `json:"sidecar_id"`
+	Name           string `json:"name,omitempty"`
+	Workspace      string `json:"workspace,omitempty"`
+	BaselineRef    string `json:"baseline_ref,omitempty"`    // commit SHA checked out on sidecar
+	BaselineBranch string `json:"baseline_branch,omitempty"` // local branch name at bootstrap time
 }
 
 // sidecarFileName returns the name of the sidecar state file. When sessionID