Shaman: handle duplicate symlinks gracefully

Instead of erroring out when a symlink already exists, investigate it. If
the linked file is the one that's intended, just use it.

For some reason, BAT and/or the Flamenco add-on include some files twice
in the checkout request to Shaman. This is now handled gracefully.

pkg/shaman/checkout/manager.go

@@ -200,26 +200,47 @@ func (m *Manager) SymlinkToCheckout(blobPath, checkoutPath, symlinkRelativePath
 	// This is expected to fail sometimes, because we don't create parent directories yet.
 	// We only create those when we get a failure from symlinking.
 	err = os.Symlink(blobPath, symlinkPath)
-	if err == nil {
+	switch {
+	case err == nil:
+		return nil
+	case errors.Is(err, fs.ErrExist):
+		// The symlink already exists, which is weird. Investigate so we can log a
+		// more detailed warning.
+		linkTarget, readErr := os.Readlink(symlinkPath)
+		if readErr != nil {
+			logger.Error().
+				AnErr("symlinkError", err).
+				AnErr("symlinkReadError", readErr).
+				Msg("shaman: unable to create symlink as it already exists, but also it cannot be read")
 			return err
 		}
-	if !errors.Is(err, fs.ErrNotExist) {
+		if linkTarget != blobPath {
-		logger.Error().Err(err).Msg("shaman: unable to create symlink")
+			logger.Error().
+				AnErr("symlinkError", err).
+				Str("alreadyLinkedFrom", linkTarget).
+				Msg("shaman: unable to create simlink, as it already exists and links a different blob")
 			return err
 		}
-
+		// The right file is linked, so let's warn about the situation and otherwise ignore it.
+		logger.Warn().
+			AnErr("symlinkError", err).
+			Msg("shaman: symlink unexpectedly already exists, but it is linking the right path so let's just use it")
+	case errors.Is(err, fs.ErrNotExist):
+		// The directory doesn't exist yet.
 		logger.Debug().Msg("shaman: creating parent directory")
-
 		dir := filepath.Dir(symlinkPath)
 		if err := os.MkdirAll(dir, 0777); err != nil {
 			logger.Error().Err(err).Msg("shaman: unable to create parent directory")
 			return err
 		}
-
 		if err := os.Symlink(blobPath, symlinkPath); err != nil {
 			logger.Error().Err(err).Msg("shaman: unable to create symlink, after creating parent directory")
 			return err
 		}
+	default:
+		logger.Error().Err(err).Msg("shaman: unable to create symlink")
+		return err
+	}
 
 	// Change the modification time of the blob to mark it as 'referenced' just now.
 	m.wg.Add(1)

pkg/shaman/checkout/manager_test.go

@@ -58,6 +58,9 @@ func TestSymlinkToCheckout(t *testing.T) {
 	err = manager.SymlinkToCheckout(blobPath, manager.checkoutBasePath, symlinkRelativePath)
 	assert.NoError(t, err)
 
+	err = manager.SymlinkToCheckout(blobPath, manager.checkoutBasePath, symlinkRelativePath)
+	assert.NoError(t, err, "symlinking a file twice should not be an issue")
+
 	// Wait for touch() calls to be done.
 	manager.wg.Wait()