internal/lsp/fake: in WriteFileData, retry writes that fail with ERROR_LOCK_VIOLATION on Windows

The 'go' command reads the main module's 'go.mod' file with the file
locked. If 'gopls' starts a 'go' command in the background that reads
the 'go.mod' file, there is a narrow window in which that read can
race with the test's ioutil.WriteFile.

This change retries the write (as a user would probably do), fixing
the test failure. However, I suspect that there may be a broader UX
issue here that it will mask: in order to actually avoid the race
without a locking error, something in the LSP protocol needs to be
synchronizing gopls-initiated reads with editor-initiated writes. The
test failure this fixes indicates that that synchronization seems not
to be happening.

Fixes golang/go#50971

Change-Id: Iba140277bf957bae4937cc23f62d3aac817a7d36
Reviewed-on: https://go-review.googlesource.com/c/tools/+/382414
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

internal/lsp/fake/workdir.go

@@ -14,6 +14,7 @@ import (
 	"path/filepath"
 	"strings"
 	"sync"
+	"time"
 
 	"golang.org/x/tools/internal/lsp/protocol"
 	"golang.org/x/tools/internal/span"
@@ -67,12 +68,25 @@ func WriteFileData(path string, content []byte, rel RelativeTo) error {
 	if err := os.MkdirAll(filepath.Dir(fp), 0755); err != nil {
 		return errors.Errorf("creating nested directory: %w", err)
 	}
-	if err := ioutil.WriteFile(fp, []byte(content), 0644); err != nil {
-		return errors.Errorf("writing %q: %w", path, err)
+	backoff := 1 * time.Millisecond
+	for {
+		err := ioutil.WriteFile(fp, []byte(content), 0644)
+		if err != nil {
+			if isWindowsErrLockViolation(err) {
+				time.Sleep(backoff)
+				backoff *= 2
+				continue
+			}
+			return errors.Errorf("writing %q: %w", path, err)
+		}
+		return nil
 	}
-	return nil
 }
 
+// isWindowsErrLockViolation reports whether err is ERROR_LOCK_VIOLATION
+// on Windows.
+var isWindowsErrLockViolation = func(err error) bool { return false }
+
 // Workdir is a temporary working directory for tests. It exposes file
 // operations in terms of relative paths, and fakes file watching by triggering
 // events on file operations.

internal/lsp/fake/workdir_windows.go

@@ -0,0 +1,20 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fake
+
+import (
+	"syscall"
+
+	errors "golang.org/x/xerrors"
+)
+
+func init() {
+	// from https://docs.microsoft.com/en-us/windows/win32/debug/system-error-codes--0-499-
+	const ERROR_LOCK_VIOLATION syscall.Errno = 33
+
+	isWindowsErrLockViolation = func(err error) bool {
+		return errors.Is(err, ERROR_LOCK_VIOLATION)
+	}
+}