runtime/pprof: fix TestCPUProfileWithFork for GOOS=android.

1) Large allocation in this test caused crash. This was not
detected by builder because builder runs tests with -test.short.

2) The command "go" for forking doesn't exist in some platforms
including android. This change uses the test binary itself which
is guaranteed to exist.

This change also adds logging of the total samples collected in
TestCPUProfileMultithreaded test that is flaky in android-arm
builder.

Change-Id: I225c6b7877d811edef8b25e7eb00559450640c42
Reviewed-on: https://go-review.googlesource.com/8131Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/pprof/pprof_test.go

@@ -143,7 +143,9 @@ func testCPUProfile(t *testing.T, need []string, f func()) {
 
 	// Check that profile is well formed and contains need.
 	have := make([]uintptr, len(need))
+	var samples uintptr
 	parseProfile(t, prof.Bytes(), func(count uintptr, stk []uintptr) {
+		samples += count
 		for _, pc := range stk {
 			f := runtime.FuncForPC(pc)
 			if f == nil {
@@ -156,6 +158,7 @@ func testCPUProfile(t *testing.T, need []string, f func()) {
 			}
 		}
 	})
+	t.Logf("total %d CPU profile samples collected", samples)
 
 	if len(need) == 0 {
 		return
@@ -200,6 +203,8 @@ func testCPUProfile(t *testing.T, need []string, f func()) {
 	}
 }
 
+// Fork can hang if preempted with signals frequently enough (see issue 5517).
+// Ensure that we do not do this.
 func TestCPUProfileWithFork(t *testing.T) {
 	if runtime.GOOS == "darwin" {
 		if runtime.GOARCH == "arm" {
@@ -207,9 +212,11 @@ func TestCPUProfileWithFork(t *testing.T) {
 		}
 	}
 
-	// Fork can hang if preempted with signals frequently enough (see issue 5517).
-	// Ensure that we do not do this.
 	heap := 1 << 30
+	if runtime.GOOS == "android" {
+		// Use smaller size for Android to avoid crash.
+		heap = 100 << 20
+	}
 	if testing.Short() {
 		heap = 100 << 20
 	}
@@ -232,7 +239,7 @@ func TestCPUProfileWithFork(t *testing.T) {
 	defer StopCPUProfile()
 
 	for i := 0; i < 10; i++ {
-		exec.Command("go").CombinedOutput()
+		exec.Command(os.Args[0], "-h").CombinedOutput()
 	}
 }