1 files changed, 147 insertions, 0 deletions
diff --git a/modules/util/filebuffer/file_backed_buffer.go b/modules/util/filebuffer/file_backed_buffer.go
new file mode 100644
index 0000000000..128030b4c5
--- /dev/null
+++ b/modules/util/filebuffer/file_backed_buffer.go
@@ -0,0 +1,147 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package filebuffer
+
+import (
+	"bytes"
+	"errors"
+	"io"
+	"os"
+)
+
+const maxInt = int(^uint(0) >> 1) // taken from bytes.Buffer
+
+var (
+	// ErrInvalidMemorySize occurs if the memory size is not in a valid range
+	ErrInvalidMemorySize = errors.New("Memory size must be greater 0 and lower math.MaxInt32")
+	// ErrWriteAfterRead occurs if Write is called after a read operation
+	ErrWriteAfterRead = errors.New("Write is unsupported after a read operation")
+)
+
+type readAtSeeker interface {
+	io.ReadSeeker
+	io.ReaderAt
+}
+
+// FileBackedBuffer uses a memory buffer with a fixed size.
+// If more data is written a temporary file is used instead.
+// It implements io.ReadWriteCloser, io.ReadSeekCloser and io.ReaderAt
+type FileBackedBuffer struct {
+	maxMemorySize int64
+	size          int64
+	buffer        bytes.Buffer
+	file          *os.File
+	reader        readAtSeeker
+}
+
+// New creates a file backed buffer with a specific maximum memory size
+func New(maxMemorySize int) (*FileBackedBuffer, error) {
+	if maxMemorySize < 0 || maxMemorySize > maxInt {
+		return nil, ErrInvalidMemorySize
+	}
+
+	return &FileBackedBuffer{
+		maxMemorySize: int64(maxMemorySize),
+	}, nil
+}
+
+// CreateFromReader creates a file backed buffer and copies the provided reader data into it.
+func CreateFromReader(r io.Reader, maxMemorySize int) (*FileBackedBuffer, error) {
+	b, err := New(maxMemorySize)
+	if err != nil {
+		return nil, err
+	}
+
+	_, err = io.Copy(b, r)
+	if err != nil {
+		return nil, err
+	}
+
+	return b, nil
+}
+
+// Write implements io.Writer
+func (b *FileBackedBuffer) Write(p []byte) (int, error) {
+	if b.reader != nil {
+		return 0, ErrWriteAfterRead
+	}
+
+	var n int
+	var err error
+
+	if b.file != nil {
+		n, err = b.file.Write(p)
+	} else {
+		if b.size+int64(len(p)) > b.maxMemorySize {
+			b.file, err = os.CreateTemp("", "gitea-buffer-")
+			if err != nil {
+				return 0, err
+			}
+
+			_, err = io.Copy(b.file, &b.buffer)
+			if err != nil {
+				return 0, err
+			}
+
+			return b.Write(p)
+		}
+
+		n, err = b.buffer.Write(p)
+	}
+
+	if err != nil {
+		return n, err
+	}
+	b.size += int64(n)
+	return n, nil
+}
+
+// Size returns the byte size of the buffered data
+func (b *FileBackedBuffer) Size() int64 {
+	return b.size
+}
+
+func (b *FileBackedBuffer) switchToReader() {
+	if b.reader != nil {
+		return
+	}
+
+	if b.file != nil {
+		b.reader = b.file
+	} else {
+		b.reader = bytes.NewReader(b.buffer.Bytes())
+	}
+}
+
+// Read implements io.Reader
+func (b *FileBackedBuffer) Read(p []byte) (int, error) {
+	b.switchToReader()
+
+	return b.reader.Read(p)
+}
+
+// ReadAt implements io.ReaderAt
+func (b *FileBackedBuffer) ReadAt(p []byte, off int64) (int, error) {
+	b.switchToReader()
+
+	return b.reader.ReadAt(p, off)
+}
+
+// Seek implements io.Seeker
+func (b *FileBackedBuffer) Seek(offset int64, whence int) (int64, error) {
+	b.switchToReader()
+
+	return b.reader.Seek(offset, whence)
+}
+
+// Close implements io.Closer
+func (b *FileBackedBuffer) Close() error {
+	if b.file != nil {
+		err := b.file.Close()
+		os.Remove(b.file.Name())
+		return err
+	}
+	return nil
+}