posixtest: ReadDir: check directory listing after each file

There is an unhandled corner case in the directory stream
handling. The stricter test catches it:

  go-fuse/fs$ go test
  --- FAIL: TestPosix (0.20s)
      --- FAIL: TestPosix/ReadDir (0.09s)
          test.go:342: got 19 entries, want 20
          test.go:342: got 40 entries, want 41
          test.go:342: got 61 entries, want 62
          test.go:342: got 82 entries, want 83
          test.go:342: got 103 entries, want 104
  FAIL
  exit status 1
  FAIL	github.com/hanwen/go-fuse/v2/fs	0.610s

Fix comes next.

Change-Id: I3e22904fce96bf2264ede47656ebd2936a7d13cd

fs/simple_test.go

@@ -196,8 +196,14 @@ func TestNotifyEntry(t *testing.T) {
 func TestReadDirStress(t *testing.T) {
 	tc := newTestCase(t, &testOptions{suppressDebug: true, attrCache: true, entryCache: true})
 	defer tc.Clean()
-	// (ab)use posixtest.ReadDir to create 110 test files
-	posixtest.ReadDir(t, tc.mntDir)
+
+	// Create 110 entries
+	for i := 0; i < 110; i++ {
+		name := fmt.Sprintf("file%036x", i)
+		if err := ioutil.WriteFile(filepath.Join(tc.mntDir, name), []byte("hello"), 0644); err != nil {
+			t.Fatalf("WriteFile %q: %v", name, err)
+		}
+	}
 
 	var wg sync.WaitGroup
 	stress := func(gr int) {

posixtest/test.go

@@ -312,29 +312,28 @@ func RenameOverwrite(t *testing.T, mnt string, destExists bool) {
 	}
 }
 
+// ReadDir creates 110 files one by one, checking that we get the expected
+// entries after each file creation.
 func ReadDir(t *testing.T, mnt string) {
-	f, err := os.Open(mnt)
-	if err != nil {
-		t.Fatalf("Open: %v", err)
-	}
-	defer f.Close()
-
-	// add entries after opening the directory
 	want := map[string]bool{}
-	for i := 0; i < 110; i++ {
 	// 40 bytes of filename, so 110 entries overflows a
 	// 4096 page.
+	for i := 0; i < 110; i++ {
 		nm := fmt.Sprintf("file%036x", i)
 		want[nm] = true
 		if err := ioutil.WriteFile(filepath.Join(mnt, nm), []byte("hello"), 0644); err != nil {
 			t.Fatalf("WriteFile %q: %v", nm, err)
 		}
+		// Verify that we get the expected entries
+		f, err := os.Open(mnt)
+		if err != nil {
+			t.Fatalf("Open: %v", err)
 		}
-
 		names, err := f.Readdirnames(-1)
 		if err != nil {
 			t.Fatalf("ReadDir: %v", err)
 		}
+		f.Close()
 		got := map[string]bool{}
 		for _, e := range names {
 			got[e] = true
@@ -347,6 +346,7 @@ func ReadDir(t *testing.T, mnt string) {
 				t.Errorf("got unknown name %q", k)
 			}
 		}
+	}
 }
 
 // LinkUnlinkRename implements rename with a link/unlink sequence