buffer: Store potential resolved symlinks as AbsPath

Otherwise we can't identify if we have the same file open multiple times via
different symlinks.
Additionally this will also store the buffer backup and serialized buffer for
the same target file just once.

Author: JoeKar

internal/buffer/buffer.go

@@ -360,6 +360,10 @@ func NewBuffer(r io.Reader, size int64, path string, btype BufType, cmd Command)
 		absPath = path
 	}
 
+	// Ignore the returned error, since the checks are already performed in
+	// NewBufferFromFileWithCommand()
+	absPath, _ = util.ResolveSymlinks(absPath)
+
 	b := new(Buffer)
 
 	found := false

internal/buffer/save.go

@@ -336,7 +336,9 @@ func (b *Buffer) saveToFile(filename string, withSudo bool, autoSave bool) error
 	}
 
 	b.Path = filename
-	b.AbsPath = absFilename
+	// Ignore the returned error, since the checks are already performed in
+	// above
+	b.AbsPath, _ = util.ResolveSymlinks(absFilename)
 	b.isModified = false
 	b.UpdateModTime()
 

internal/util/util.go

@@ -662,6 +662,36 @@ func HttpRequest(method string, url string, headers []string) (resp *http.Respon
 	return client.Do(req)
 }
 
+func ResolveSymlinks(filename string) (string, error) {
+	// We use Stat() as trick to let the OS do symlink loop detection for us,
+	// since we are lazy to do it ourselves
+	fileInfo, err := os.Stat(filename)
+	if errors.Is(err, fs.ErrNotExist) {
+		return filename, nil
+	}
+	if err != nil {
+		return filename, err
+	}
+
+	for {
+		fileInfo, err = os.Lstat(filename)
+		if err != nil {
+			return filename, err
+		}
+		if fileInfo.Mode()&os.ModeSymlink != 0 {
+			dstFilename, err := os.Readlink(filename)
+			if err != nil {
+				return filename, err
+			}
+			if !filepath.IsAbs(dstFilename) {
+				filename = filepath.Join(filepath.Dir(filename), dstFilename)
+				continue
+			}
+		}
+		return filename, nil
+	}
+}
+
 // SafeWrite writes bytes to a file in a "safe" way, preventing loss of the
 // contents of the file if it fails to write the new contents.
 // This means that the file is not overwritten directly but by writing to a