build: limit concurrency of updateSymlinks

In rare cases, the unbounded concurrency here may lead to Go runtime
panics, due to the runtime spawning >10K threads.

Bug: 376466642
Test: `m` generates build files properly
Change-Id: Ib2e812b2fd56ebcee16154c69927821a4f379a87

1 files changed, 18 insertions, 3 deletions

diff --git a/ui/build/soong.go b/ui/build/soong.go
index e6d01dd1d..0963f76b7 100644
--- a/ui/build/soong.go
+++ b/ui/build/soong.go
@@ -433,13 +433,13 @@ func checkEnvironmentFile(ctx Context, currentEnv *Environment, envFile string)
 	}
 }
 
-func updateSymlinks(ctx Context, dir, prevCWD, cwd string) error {
+func updateSymlinks(ctx Context, dir, prevCWD, cwd string, updateSemaphore chan struct{}) error {
 	defer symlinkWg.Done()
 
 	visit := func(path string, d fs.DirEntry, err error) error {
 		if d.IsDir() && path != dir {
 			symlinkWg.Add(1)
-			go updateSymlinks(ctx, path, prevCWD, cwd)
+			go updateSymlinks(ctx, path, prevCWD, cwd, updateSemaphore)
 			return filepath.SkipDir
 		}
 		f, err := d.Info()
@@ -470,12 +470,27 @@ func updateSymlinks(ctx Context, dir, prevCWD, cwd string) error {
 		return nil
 	}
 
+	<-updateSemaphore
+	defer func() { updateSemaphore <- struct{}{} }()
 	if err := filepath.WalkDir(dir, visit); err != nil {
 		return err
 	}
 	return nil
 }
 
+// b/376466642: If the concurrency of updateSymlinks is unbounded, Go's runtime spawns a
+// theoretically unbounded number of threads to handle blocking syscalls. This causes the runtime to
+// panic due to hitting thread limits in rare cases. Limiting to GOMAXPROCS concurrent symlink
+// updates should make this a non-issue.
+func newUpdateSemaphore() chan struct{} {
+	numPermits := runtime.GOMAXPROCS(0)
+	c := make(chan struct{}, numPermits)
+	for i := 0; i < numPermits; i++ {
+		c <- struct{}{}
+	}
+	return c
+}
+
 func fixOutDirSymlinks(ctx Context, config Config, outDir string) error {
 	cwd, err := os.Getwd()
 	if err != nil {
@@ -508,7 +523,7 @@ func fixOutDirSymlinks(ctx Context, config Config, outDir string) error {
 	}
 
 	symlinkWg.Add(1)
-	if err := updateSymlinks(ctx, outDir, prevCWD, cwd); err != nil {
+	if err := updateSymlinks(ctx, outDir, prevCWD, cwd, newUpdateSemaphore()); err != nil {
 		return err
 	}
 	symlinkWg.Wait()