Merge pull request #3923 from MichaelEischer/fix-flaky-cache-test

cache: fix flaky TestFileSaveConcurrent on windows

internal/cache/file_test.go

@@ -6,6 +6,7 @@ import (
 	"io/ioutil"
 	"math/rand"
 	"os"
+	"runtime"
 	"testing"
 	"time"
 
@@ -204,7 +205,22 @@ func TestFileLoad(t *testing.T) {
 }
 
 // Simulate multiple processes writing to a cache, using goroutines.
+//
+// The possibility of sharing a cache between multiple concurrent restic
+// processes isn't guaranteed in the docs and doesn't always work on Windows, hence the
+// check on GOOS. Cache sharing is considered a "nice to have" on POSIX, for now.
+//
+// The cache first creates a temporary file and then renames it to its final name.
+// On Windows renaming internally creates a file handle with a shareMode which
+// includes FILE_SHARE_DELETE. The Go runtime opens files without FILE_SHARE_DELETE,
+// thus Open(fn) will fail until the file handle used for renaming was closed.
+// See https://devblogs.microsoft.com/oldnewthing/20211022-00/?p=105822
+// for hints on how to fix this properly.
 func TestFileSaveConcurrent(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("may not work due to FILE_SHARE_DELETE issue")
+	}
+
 	const nproc = 40
 
 	c, cleanup := TestNewCache(t)