test(cache): fix Windows flake in the torn-file concurrency test

pkg/cache/cache_test.go

@@ -260,7 +260,11 @@ func TestFileCache_concurrentStoreNeverYieldsTornFile(t *testing.T) {
 	// half-written content and json.Unmarshal would error.
 	for range 100 {
 		data, err := os.ReadFile(path)
-		if errors.Is(err, os.ErrNotExist) {
+		// Tolerate transient open failures caused by the writer itself:
+		// the file may not exist yet, and on Windows the open can race a
+		// rename replacing it. Only successfully read bytes are asserted
+		// on — the torn-write check is about content, not availability.
+		if errors.Is(err, os.ErrNotExist) || isTransientSharingViolation(err) {
 			continue
 		}
 		require.NoError(t, err)

pkg/cache/sharing_violation_other_test.go

@@ -0,0 +1,10 @@
+//go:build !windows
+
+package cache
+
+// isTransientSharingViolation only exists on Windows, where opening a file
+// can transiently fail while a concurrent rename replaces it. POSIX renames
+// never make the destination unopenable.
+func isTransientSharingViolation(error) bool {
+	return false
+}

pkg/cache/sharing_violation_windows_test.go

@@ -0,0 +1,18 @@
+//go:build windows
+
+package cache
+
+import (
+	"errors"
+
+	"golang.org/x/sys/windows"
+)
+
+// isTransientSharingViolation reports whether a file open failed because a
+// concurrent rename-over-existing briefly held the destination. During
+// MoveFileEx(MOVEFILE_REPLACE_EXISTING) the target can be transiently
+// unopenable, so a reader racing Store's temp-file+rename publish sees
+// ERROR_SHARING_VIOLATION; that is not a torn write.
+func isTransientSharingViolation(err error) bool {
+	return errors.Is(err, windows.ERROR_SHARING_VIOLATION)
+}