pkg/fs: optimize GetObject syscalls for common case

In the common case, GetObject is called on a bucket that exists and an
object that exists and is not a directory. It should be optimized for
this case, thus error-related syscalls are pushed back until they are
necessary.

This should not impact performance negatively in the uncommon case, and
instead drops two otherwise unnecessary os.Stat's in the common case.

The race conditions around a proper error being returned were present
beforehand.

It also renames 'err' to 'e'.

pkg/fs/fs-object.go

@@ -54,50 +54,53 @@ func (fs Filesystem) GetObject(w io.Writer, bucket, object string, start, length
 
 	// normalize buckets.
 	bucket = fs.denormalizeBucket(bucket)
-	bucketPath := filepath.Join(fs.path, bucket)
-	if _, e := os.Stat(bucketPath); e != nil {
+	objectPath := filepath.Join(fs.path, bucket, object)
+
+	file, e := os.Open(objectPath)
+	if e != nil {
+		// If the object doesn't exist, the bucket might not exist either. Stat for
+		// the bucket and give a better error message if that is true.
+		if os.IsNotExist(e) {
+			_, e = os.Stat(filepath.Join(fs.path, bucket))
 			if os.IsNotExist(e) {
 				return 0, probe.NewError(BucketNotFound{Bucket: bucket})
 			}
-		return 0, probe.NewError(e)
-	}
 
-	objectPath := filepath.Join(bucketPath, object)
-	filestat, err := os.Stat(objectPath)
-	switch err := err.(type) {
-	case nil:
-		if filestat.IsDir() {
 			return 0, probe.NewError(ObjectNotFound{Bucket: bucket, Object: object})
 		}
-	default:
-		if os.IsNotExist(err) {
-			return 0, probe.NewError(ObjectNotFound{Bucket: bucket, Object: object})
-		}
-		return 0, probe.NewError(err)
-	}
-	file, err := os.Open(objectPath)
-	if err != nil {
-		return 0, probe.NewError(err)
+		return 0, probe.NewError(e)
 	}
 	defer file.Close()
 
-	_, err = file.Seek(start, os.SEEK_SET)
-	if err != nil {
-		return 0, probe.NewError(err)
+	_, e = file.Seek(start, os.SEEK_SET)
+	if e != nil {
+		// When the "handle is invalid", the file might be a directory on Windows.
+		if runtime.GOOS == "windows" && strings.Contains(e.Error(), "handle is invalid") {
+			return 0, probe.NewError(ObjectNotFound{Bucket: bucket, Object: object})
+		}
+
+		return 0, probe.NewError(e)
 	}
 
 	var count int64
+	// Copy over the whole file if the length is non-positive.
 	if length > 0 {
-		count, err = io.CopyN(w, file, length)
-		if err != nil {
-			return count, probe.NewError(err)
-		}
+		count, e = io.CopyN(w, file, length)
 	} else {
-		count, err = io.Copy(w, file)
-		if err != nil {
-			return count, probe.NewError(err)
+		count, e = io.Copy(w, file)
 	}
+
+	if e != nil {
+		// This call will fail if the object is a directory. Stat the file to see if
+		// this is true, if so, return an ObjectNotFound error.
+		stat, e := os.Stat(objectPath)
+		if e == nil && stat.IsDir() {
+			return count, probe.NewError(ObjectNotFound{Bucket: bucket, Object: object})
 		}
+
+		return count, probe.NewError(e)
+	}
+
 	return count, nil
 }