Replace nil-able *sync.Waitgroup with sync.Once

Simplifies synchronization and eliminates lock for nil waitroup.

Test: m droid

Test: m out/soong/.intermediates/packages/modules/StatsD/apex/com.android.os.statsd/android_common_com.android.os.statsd_image/NOTICE.html.gz

Change-Id: I381ee79e142214e7331241071f076db2f7960ba6

tools/compliance/graph.go

@@ -58,13 +58,11 @@ type LicenseGraph struct {
 	/// (guarded by mu)
 	targets map[string]*TargetNode
 
-	// wgBU becomes non-nil when the bottom-up resolve begins and reaches 0
+	// onceBottomUp makes sure the bottom-up resolve walk only happens one time.
-	// (i.e. Wait() proceeds) when the bottom-up resolve completes. (guarded by mu)
+	onceBottomUp sync.Once
-	wgBU *sync.WaitGroup
 
-	// wgTD becomes non-nil when the top-down resolve begins and reaches 0 (i.e. Wait()
+	// onceTopDown makes sure the top-down resolve walk only happens one time.
-	// proceeds) when the top-down resolve completes. (guarded by mu)
+	onceTopDown sync.Once
-	wgTD *sync.WaitGroup
 
 	// shippedNodes caches the results of a full walk of nodes identifying targets
 	// distributed either directly or as derivative works. (creation guarded by mu)

tools/compliance/policy_resolve.go

@@ -49,19 +49,7 @@ func ResolveBottomUpConditions(lg *LicenseGraph) {
 func TraceBottomUpConditions(lg *LicenseGraph, conditionsFn TraceConditions) {
 
 	// short-cut if already walked and cached
-	lg.mu.Lock()
+	lg.onceBottomUp.Do(func() {
-	wg := lg.wgBU
-
-	if wg != nil {
-		lg.mu.Unlock()
-		wg.Wait()
-		return
-	}
-	wg = &sync.WaitGroup{}
-	wg.Add(1)
-	lg.wgBU = wg
-	lg.mu.Unlock()
-
 		// amap identifes targets previously walked. (guarded by mu)
 		amap := make(map[*TargetNode]struct{})
 
@@ -125,8 +113,7 @@ func TraceBottomUpConditions(lg *LicenseGraph, conditionsFn TraceConditions) {
 			rnode := lg.targets[rname]
 			_ = walk(rnode, rnode.IsContainer())
 		}
-
+	})
-	wg.Done()
 }
 
 // ResolveTopDownCondtions performs a top-down walk of the LicenseGraph
@@ -145,18 +132,9 @@ func ResolveTopDownConditions(lg *LicenseGraph) {
 func TraceTopDownConditions(lg *LicenseGraph, conditionsFn TraceConditions) {
 
 	// short-cut if already walked and cached
-	lg.mu.Lock()
+	lg.onceTopDown.Do(func() {
-	wg := lg.wgTD
+		wg := &sync.WaitGroup{}
-
-	if wg != nil {
-		lg.mu.Unlock()
-		wg.Wait()
-		return
-	}
-	wg = &sync.WaitGroup{}
 		wg.Add(1)
-	lg.wgTD = wg
-	lg.mu.Unlock()
 
 		// start with the conditions propagated up the graph
 		TraceBottomUpConditions(lg, conditionsFn)
@@ -174,6 +152,7 @@ func TraceTopDownConditions(lg *LicenseGraph, conditionsFn TraceConditions) {
 			continueWalk := func() bool {
 				mu.Lock()
 				defer mu.Unlock()
+
 				depcs := fnode.resolution
 				_, alreadyWalked := amap[fnode]
 				if alreadyWalked {
@@ -224,4 +203,5 @@ func TraceTopDownConditions(lg *LicenseGraph, conditionsFn TraceConditions) {
 		}
 		wg.Done()
 		wg.Wait()
+	})
 }