Fix turbo run when --filter is provided and no packages are affected (#…

…3901)

cli/integration_tests/basic_monorepo/filter-run.t

@@ -0,0 +1,16 @@
+Setup
+  $ . ${TESTDIR}/../setup.sh
+  $ . ${TESTDIR}/setup.sh $(pwd)
+
+# Running with --filter works and exits
+  $ ${TURBO} run build --filter=main
+  \xe2\x80\xa2 Packages in scope:  (esc)
+  \xe2\x80\xa2 Running build in 0 packages (esc)
+  \xe2\x80\xa2 Remote caching disabled (esc)
+
+  No tasks were executed as part of this run.
+
+   Tasks:    0 successful, 0 total
+  Cached:    0 cached, 0 total
+    Time:\s*[\.0-9]+m?s  (re)
+

cli/internal/core/engine.go

@@ -156,6 +156,13 @@ func (e *Engine) Prepare(options *EngineBuildingOptions) error {
 	taskNames := options.TaskNames
 	tasksOnly := options.TasksOnly
 
+	// If there are no affected packages, we don't need to go through all this work
+	// we can just exit early.
+	// TODO(mehulkar): but we still need to validate bad task names?
+	if len(pkgs) == 0 {
+		return nil
+	}
+
 	traversalQueue := []string{}
 
 	// get a set of taskNames passed in. we'll remove the ones that have a definition