Simplify caching logic in 'src batch exec' (#653)

This is part of https://github.com/sourcegraph/sourcegraph/issues/26929
and simplifies the caching-related logic in `src batch exec` to only
what's needed: checking for per-step cache results.

Since server-side batch changes use server-side caching for changeset
specs we don't need to check for changeset specs.

Author: mrnugget

cmd/src/batch_exec.go

@@ -159,26 +159,16 @@ func executeBatchSpecInWorkspaces(ctx context.Context, ui *ui.JSONLines, opts ex
 		TempDir:       opts.flags.tempDir,
 	})
 
-	ui.CheckingCache()
+	// `src batch exec` uses server-side caching for changeset specs, so we
+	// only need to call `CheckStepResultsCache` to make sure that per-step cache entries
+	// are loaded and set on the tasks.
 	tasks := svc.BuildTasks(ctx, batchSpec, repoWorkspaces)
-	var (
-		specs         []*batcheslib.ChangesetSpec
-		uncachedTasks []*executor.Task
-	)
-	// TODO: We don't need to support clear cache here at all.
-	if opts.flags.clearCache {
-		coord.ClearCache(ctx, tasks)
-		uncachedTasks = tasks
-	} else {
-		uncachedTasks, specs, err = coord.CheckCache(ctx, tasks)
-		if err != nil {
-			return err
-		}
+	if err := coord.CheckStepResultsCache(ctx, tasks); err != nil {
+		return err
 	}
-	ui.CheckingCacheSuccess(len(specs), len(uncachedTasks))
 
 	taskExecUI := ui.ExecutingTasks(*verbose, opts.flags.parallelism)
-	freshSpecs, _, err := coord.Execute(ctx, uncachedTasks, batchSpec, taskExecUI)
+	specs, _, err := coord.Execute(ctx, tasks, batchSpec, taskExecUI)
 	if err == nil || opts.flags.skipErrors {
 		if err == nil {
 			taskExecUI.Success()
@@ -191,9 +181,6 @@ func executeBatchSpecInWorkspaces(ctx context.Context, ui *ui.JSONLines, opts ex
 			return err
 		}
 	}
-
-	specs = append(specs, freshSpecs...)
-
 	ids := make([]graphql.ChangesetSpecID, len(specs))
 
 	ui.UploadingChangesetSpecs(len(specs))

internal/batches/executor/coordinator.go

@@ -103,6 +103,17 @@ func (c *Coordinator) CheckCache(ctx context.Context, tasks []*Task) (uncached [
 	return uncached, specs, nil
 }
 
+// CheckStepResultsCache checks the cache for each Task, but only for cached
+// step results. This is used by `src batch exec` when executing server-side.
+func (c *Coordinator) CheckStepResultsCache(ctx context.Context, tasks []*Task) error {
+	for _, t := range tasks {
+		if err := c.loadCachedStepResults(ctx, t); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
 func (c *Coordinator) ClearCache(ctx context.Context, tasks []*Task) error {
 	for _, task := range tasks {
 		cacheKey := task.cacheKey()