fix(mg): allow resetting dependency cache in tests

mg/deps.go

@@ -44,11 +44,26 @@ func (o *onceMap) LoadOrStore(f Fn) *onceFun {
 	return one
 }
 
+func (o *onceMap) Reset() {
+	o.mu.Lock()
+	defer o.mu.Unlock()
+
+	o.m = map[onceKey]*onceFun{}
+}
+
 var onces = &onceMap{
 	mu: &sync.Mutex{},
 	m:  map[onceKey]*onceFun{},
 }
 
+// ResetDeps clears the dependency cache used by Deps, CtxDeps, SerialDeps, and
+// SerialCtxDeps. It is intended for tests that need to run the same dependencies
+// more than once in a single process. Do not call ResetDeps while dependencies
+// are running.
+func ResetDeps() {
+	onces.Reset()
+}
+
 // SerialDeps is like Deps except it runs each dependency serially, instead of
 // in parallel. This can be useful for resource intensive dependencies that
 // shouldn't be run at the same time.

mg/deps_test.go

@@ -29,6 +29,25 @@ func TestDepsRunOnce(t *testing.T) {
 	}
 }
 
+func TestResetDepsAllowsDependenciesToRunAgain(t *testing.T) {
+	var calls int64
+	f := func() {
+		atomic.AddInt64(&calls, 1)
+	}
+
+	Deps(f)
+	Deps(f)
+	if calls != 1 {
+		t.Fatalf("expected dependency to run once before reset, ran %d times", calls)
+	}
+
+	ResetDeps()
+	Deps(f)
+	if calls != 2 {
+		t.Fatalf("expected dependency to run again after reset, ran %d times", calls)
+	}
+}
+
 func TestDepsOfDeps(t *testing.T) {
 	ch := make(chan string, 3)
 	// this->f->g->h