safe: Introduce `ErrFileAlreadyLocked` error

We've started to check for locking failures in multiple packages' tests
by doing a string comparison. Introduce a new `ErrFileAlreadyLocked`
error which we can return and test for instead.

3 files changed, 10 insertions, 6 deletions

diff --git a/internal/git/localrepo/config_test.go b/internal/git/localrepo/config_test.go
index a4b5ce26a..e0d188cf4 100644
--- a/internal/git/localrepo/config_test.go
+++ b/internal/git/localrepo/config_test.go
@@ -2,7 +2,6 @@ package localrepo
 
 import (
 	"context"
-	"errors"
 	"fmt"
 	"path/filepath"
 	"runtime"
@@ -86,7 +85,7 @@ func TestRepo_SetConfig(t *testing.T) {
 			value:           "value",
 			locked:          true,
 			expectedEntries: standardEntries,
-			expectedErr:     fmt.Errorf("committing config: %w", fmt.Errorf("locking file: %w", errors.New("file already locked"))),
+			expectedErr:     fmt.Errorf("committing config: %w", fmt.Errorf("locking file: %w", safe.ErrFileAlreadyLocked)),
 		},
 	} {
 		t.Run(tc.desc, func(t *testing.T) {
@@ -224,7 +223,7 @@ func TestRepo_UnsetMatchingConfig(t *testing.T) {
 			desc:         "locked",
 			regex:        ".*",
 			locked:       true,
-			expectedErr:  fmt.Errorf("committing config: %w", fmt.Errorf("locking file: %w", errors.New("file already locked"))),
+			expectedErr:  fmt.Errorf("committing config: %w", fmt.Errorf("locking file: %w", safe.ErrFileAlreadyLocked)),
 			expectedKeys: standardKeys,
 		},
 	} {
diff --git a/internal/safe/locking_file_writer.go b/internal/safe/locking_file_writer.go
index b7e8405c2..c43720cbe 100644
--- a/internal/safe/locking_file_writer.go
+++ b/internal/safe/locking_file_writer.go
@@ -1,6 +1,7 @@
 package safe
 
 import (
+	"errors"
 	"fmt"
 	"io"
 	"os"
@@ -14,6 +15,10 @@ const (
 	lockingFileWriterStateClosed
 )
 
+// ErrFileAlreadyLocked is returned when trying to lock a file which has already been locked by
+// another concurrent process.
+var ErrFileAlreadyLocked = errors.New("file already locked")
+
 // LockingFileWriter is a FileWriter which locks the target file on commit and checks whether it
 // has been modified since the LockingFileWriter has been created. The user must first create a new
 // LockingFileWriter via `NewLockingFileWriter()`, at which point it is open for writes. The writer
@@ -134,7 +139,7 @@ func (fw *LockingFileWriter) Lock() error {
 	lock, err := os.OpenFile(fw.lockPath(), os.O_CREATE|os.O_EXCL|os.O_RDONLY, 0o400)
 	if err != nil {
 		if os.IsExist(err) {
-			return fmt.Errorf("file already locked")
+			return ErrFileAlreadyLocked
 		}
 
 		return fmt.Errorf("creating lock file: %w", err)
diff --git a/internal/safe/locking_file_writer_test.go b/internal/safe/locking_file_writer_test.go
index 152c63d94..7632a9d69 100644
--- a/internal/safe/locking_file_writer_test.go
+++ b/internal/safe/locking_file_writer_test.go
@@ -261,7 +261,7 @@ func TestLockingFileWriter_concurrentLocking(t *testing.T) {
 	require.NoError(t, err)
 
 	require.NoError(t, first.Lock())
-	require.Equal(t, fmt.Errorf("file already locked"), second.Lock())
+	require.Equal(t, safe.ErrFileAlreadyLocked, second.Lock())
 	require.NoError(t, first.Commit())
 
 	require.Equal(t, []byte("first"), testhelper.MustReadFile(t, file))
@@ -279,7 +279,7 @@ func TestLockingFileWriter_locked(t *testing.T) {
 	// Concurrently lock the file.
 	require.NoError(t, os.WriteFile(target+".lock", nil, 0o644))
 
-	require.Equal(t, fmt.Errorf("file already locked"), writer.Lock())
+	require.Equal(t, safe.ErrFileAlreadyLocked, writer.Lock())
 
 	require.Equal(t, []byte("base"), testhelper.MustReadFile(t, target))
 }