Multithread symlink forest removal.

This makes symlink forest creation ca. 2x faster again, taking 2-3
seconds instead of 5.

Who would have thought that os.RemoveAll() is slow.

Test: Presubmits.
Change-Id: I91e41319c972dbf1113cf723e383c785433c18b9

bp2build/java_binary_host_conversion_test.go

@@ -32,7 +32,7 @@ func runJavaBinaryHostTestCase(t *testing.T, tc Bp2buildTestCase) {
 	}, tc)
 }
 
-var fs = map[string]string{
+var testFs = map[string]string{
 	"test.mf": "Main-Class: com.android.test.MainClass",
 	"other/Android.bp": `cc_library_host_shared {
     name: "jni-lib-1",
@@ -43,7 +43,7 @@ var fs = map[string]string{
 func TestJavaBinaryHost(t *testing.T) {
 	runJavaBinaryHostTestCase(t, Bp2buildTestCase{
 		Description: "java_binary_host with srcs, exclude_srcs, jni_libs, javacflags, and manifest.",
-		Filesystem:  fs,
+		Filesystem:  testFs,
 		Blueprint: `java_binary_host {
     name: "java-binary-host-1",
     srcs: ["a.java", "b.java"],
@@ -76,7 +76,7 @@ func TestJavaBinaryHost(t *testing.T) {
 func TestJavaBinaryHostRuntimeDeps(t *testing.T) {
 	runJavaBinaryHostTestCase(t, Bp2buildTestCase{
 		Description: "java_binary_host with srcs, exclude_srcs, jni_libs, javacflags, and manifest.",
-		Filesystem:  fs,
+		Filesystem:  testFs,
 		Blueprint: `java_binary_host {
     name: "java-binary-host-1",
     static_libs: ["java-dep-1"],

bp2build/symlink_forest.go

@@ -15,7 +15,9 @@
 package bp2build
 
 import (
+	"errors"
 	"fmt"
+	"io/fs"
 	"io/ioutil"
 	"os"
 	"path/filepath"
@@ -317,6 +319,45 @@ func plantSymlinkForestRecursive(context *symlinkForestContext, instructions *in
 	}
 }
 
+func removeParallelRecursive(path string, fi os.FileInfo, wg *sync.WaitGroup) {
+	defer wg.Done()
+
+	if fi.IsDir() {
+		children := readdirToMap(path)
+		childrenWg := &sync.WaitGroup{}
+		childrenWg.Add(len(children))
+
+		for child, childFi := range children {
+			go removeParallelRecursive(shared.JoinPath(path, child), childFi, childrenWg)
+		}
+
+		childrenWg.Wait()
+	}
+
+	if err := os.Remove(path); err != nil {
+		fmt.Fprintf(os.Stderr, "Cannot unlink '%s': %s\n", path, err)
+		os.Exit(1)
+	}
+}
+
+func removeParallel(path string) {
+	fi, err := os.Lstat(path)
+	if err != nil {
+		if errors.Is(err, fs.ErrNotExist) {
+			return
+		}
+
+		fmt.Fprintf(os.Stderr, "Cannot lstat '%s': %s\n", path, err)
+		os.Exit(1)
+	}
+
+	wg := &sync.WaitGroup{}
+	wg.Add(1)
+	removeParallelRecursive(path, fi, wg)
+
+	wg.Wait()
+}
+
 // Creates a symlink forest by merging the directory tree at "buildFiles" and
 // "srcDir" while excluding paths listed in "exclude". Returns the set of paths
 // under srcDir on which readdir() had to be called to produce the symlink
@@ -330,7 +371,7 @@ func PlantSymlinkForest(verbose bool, topdir string, forest string, buildFiles s
 
 	context.okay.Store(true)
 
-	os.RemoveAll(shared.JoinPath(topdir, forest))
+	removeParallel(shared.JoinPath(topdir, forest))
 
 	instructions := instructionsFromExcludePathList(exclude)
 	go func() {