syscall: fix Fchdir on js/wasm

NodeJS does not support fchdir so it has to be emulated with chdir by
saving the path when opening a directory.

However, if the path opened is relative, saving this path is not
sufficient, because after changing the working directory the path
does not resolve correctly any more, thus a subsequent fd.Chdir() fails.

This change fixes the issue by resolving a relative path when
opening the directory and saving the absolute path instead.

Fixes #37448

Change-Id: Id6bc8c4232b0019fc11e850599a526336608ce54
Reviewed-on: https://go-review.googlesource.com/c/go/+/221717
Run-TryBot: Richard Musiol <neelance@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

src/os/os_test.go

@@ -1242,6 +1242,41 @@ func testChtimes(t *testing.T, name string) {
 	}
 }
 
+func TestFileChdir(t *testing.T) {
+	// TODO(brainman): file.Chdir() is not implemented on windows.
+	if runtime.GOOS == "windows" {
+		return
+	}
+
+	wd, err := Getwd()
+	if err != nil {
+		t.Fatalf("Getwd: %s", err)
+	}
+	defer Chdir(wd)
+
+	fd, err := Open(".")
+	if err != nil {
+		t.Fatalf("Open .: %s", err)
+	}
+	defer fd.Close()
+
+	if err := Chdir("/"); err != nil {
+		t.Fatalf("Chdir /: %s", err)
+	}
+
+	if err := fd.Chdir(); err != nil {
+		t.Fatalf("fd.Chdir: %s", err)
+	}
+
+	wdNew, err := Getwd()
+	if err != nil {
+		t.Fatalf("Getwd: %s", err)
+	}
+	if wdNew != wd {
+		t.Fatalf("fd.Chdir failed, got %s, want %s", wdNew, wd)
+	}
+}
+
 func TestChdirAndGetwd(t *testing.T) {
 	// TODO(brainman): file.Chdir() is not implemented on windows.
 	if runtime.GOOS == "windows" {

src/syscall/fs_js.go

@@ -102,6 +102,10 @@ func Open(path string, openmode int, perm uint32) (int, error) {
 		}
 	}
 
+	if path[0] != '/' {
+		cwd := jsProcess.Call("cwd").String()
+		path = cwd + "/" + path
+	}
 	f := &jsFile{
 		path:    path,
 		entries: entries,