syscall: Fix Getwd on Windows to correctly handle long paths.

Fixes #60051.

Change-Id: Ia68ca0493912cb09d8c1d36a144bf0725842af1d
Reviewed-on: https://go-review.googlesource.com/c/go/+/502415
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>

src/syscall/syscall_windows.go

@@ -533,11 +533,21 @@ const ImplementsGetwd = true
 
 func Getwd() (wd string, err error) {
 	b := make([]uint16, 300)
-	n, e := GetCurrentDirectory(uint32(len(b)), &b[0])
-	if e != nil {
-		return "", e
+	// The path of the current directory may not fit in the initial 300-word
+	// buffer when long path support is enabled. The current directory may also
+	// change between subsequent calls of GetCurrentDirectory. As a result, we
+	// need to retry the call in a loop until the current directory fits, each
+	// time with a bigger buffer.
+	for {
+		n, e := GetCurrentDirectory(uint32(len(b)), &b[0])
+		if e != nil {
+			return "", e
+		}
+		if int(n) <= len(b) {
+			return UTF16ToString(b[:n]), nil
+		}
+		b = make([]uint16, n)
 	}
-	return UTF16ToString(b[0:n]), nil
 }
 
 func Chdir(path string) (err error) {

src/syscall/syscall_windows_test.go

@@ -180,6 +180,30 @@ int main(int argc, char *argv[])
 	}
 }
 
+func TestGetwd_DoesNotPanicWhenPathIsLong(t *testing.T) {
+	// Regression test for https://github.com/golang/go/issues/60051.
+
+	// The length of a filename is also limited, so we can't reproduce the
+	// crash by creating a single directory with a very long name; we need two
+	// layers.
+	a200 := strings.Repeat("a", 200)
+	dirname := filepath.Join(t.TempDir(), a200, a200)
+
+	err := os.MkdirAll(dirname, 0o700)
+	if err != nil {
+		t.Skipf("MkdirAll failed: %v", err)
+	}
+	err = os.Chdir(dirname)
+	if err != nil {
+		t.Skipf("Chdir failed: %v", err)
+	}
+	// Change out of the temporary directory so that we don't inhibit its
+	// removal during test cleanup.
+	defer os.Chdir(`\`)
+
+	syscall.Getwd()
+}
+
 func FuzzUTF16FromString(f *testing.F) {
 	f.Add("hi")           // ASCII
 	f.Add("â")            // latin1