Only apply cache result progress if resulted in diff (#778)

This was reported by a user, we store execution step results in the cache also when no diff was generated (rightfully), but then we should only apply the patch when there is actually anything to do.

Also, I initially couldn't reproduce this locally because we swallowed this error in volume mode. It would only happen in bind mode.

internal/batches/executor/run_steps.go

@@ -121,8 +121,12 @@ func runSteps(ctx context.Context, opts *executionOpts) (result execution.Result
 			stepContext.Steps.Changes = previousStepResult.Files
 			stepContext.Outputs = opts.task.CachedResult.Outputs
 
-			if err := ws.ApplyDiff(ctx, []byte(opts.task.CachedResult.Diff)); err != nil {
-				return execResult, nil, errors.Wrap(err, "getting changed files in step")
+			// If the previous steps made any modifications in the workspace yet,
+			// apply them.
+			if opts.task.CachedResult.Diff != "" {
+				if err := ws.ApplyDiff(ctx, []byte(opts.task.CachedResult.Diff)); err != nil {
+					return execResult, nil, errors.Wrap(err, "getting changed files in step")
+				}
 			}
 		}
 

internal/batches/workspace/bind_workspace.go

@@ -172,8 +172,8 @@ func (w *dockerBindWorkspace) ApplyDiff(ctx context.Context, diff []byte) error
 	}
 
 	// Apply diff
-	if _, err = runGitCmd(ctx, w.dir, "apply", "-p0", tmp.Name()); err != nil {
-		return errors.Wrap(err, "applying cached diff")
+	if out, err := runGitCmd(ctx, w.dir, "apply", "-p0", tmp.Name()); err != nil {
+		return errors.Wrapf(err, "applying cached diff: %s", string(out))
 	}
 
 	// Add all files to index

internal/batches/workspace/git.go

@@ -27,7 +27,7 @@ func runGitCmd(ctx context.Context, dir string, args ...string) ([]byte, error)
 	cmd.Dir = dir
 	out, err := cmd.CombinedOutput()
 	if err != nil {
-		return nil, errors.Wrapf(err, "'git %s' failed: %s", strings.Join(args, " "), out)
+		return out, errors.Wrapf(err, "'git %s' failed: %s", strings.Join(args, " "), out)
 	}
 	return out, nil
 }

internal/batches/workspace/volume_workspace.go

@@ -279,6 +279,8 @@ exec git diff --cached --no-prefix --binary
 func (w *dockerVolumeWorkspace) ApplyDiff(ctx context.Context, diff []byte) error {
 	script := fmt.Sprintf(`#!/bin/sh
 
+set -e
+
 cat <<'EOF' | exec git apply -p0 -
 %s
 EOF
@@ -322,11 +324,15 @@ func (w *dockerVolumeWorkspace) runScript(ctx context.Context, target, script st
 	if _, err := f.WriteString(script); err != nil {
 		return nil, errors.Wrap(err, "writing run script")
 	}
-	f.Close()
+	if err := f.Close(); err != nil {
+		return nil, errors.Wrap(err, "closing run script")
+	}
 
 	// Sidestep any umask issues on the temporary file by always making it
 	// executable by everyone.
-	os.Chmod(name, 0755)
+	if err := os.Chmod(name, 0755); err != nil {
+		return nil, errors.Wrap(err, "chmodding run script")
+	}
 
 	common, err := w.DockerRunOpts(ctx, target)
 	if err != nil {