Truncate files on Write

handler/paste/paster.go

@@ -10,18 +10,19 @@ import (
 
 const pasteFileMode = 0644
 
-// ReadWriteSeekCloser implements reading, writing, seeking, and closing
-type ReadWriteSeekCloser interface {
+// TruncatableFile implements reading, writing, seeking, truncation, and closing
+type TruncatableFile interface {
+	Truncate(size int64) error
 	io.ReadWriteSeeker
 	io.Closer
 }
 
 // Paster holds a single paste from the system
-// Implements io.ReadWriteCloser
+// Implements TruncatableFile
 type Paster struct {
 	Paste    repository.Paste
 	pasteDir string
-	file     ReadWriteSeekCloser
+	file     TruncatableFile
 }
 
 func (p *Paster) getPath() string {
@@ -56,11 +57,6 @@ func (p *Paster) prepareAndDo(buffer []byte, action func([]byte) (int, error)) (
 		}
 	}
 
-	_, err := p.file.Seek(0, io.SeekStart)
-	if err != nil {
-		return 0, err
-	}
-
 	return action(buffer)
 }
 
@@ -76,6 +72,16 @@ func (p *Paster) Read(buffer []byte) (int, error) {
 func (p *Paster) Write(contents []byte) (int, error) {
 	// Wrap in closure to handle the nil file case
 	return p.prepareAndDo(contents, func(buffer []byte) (int, error) {
+		// Start writing at the beginning.
+		_, err := p.file.Seek(0, io.SeekStart)
+		if err != nil {
+			return 0, err
+		}
+		err = p.file.Truncate(0)
+		if err != nil {
+			return 0, err
+		}
+
 		return p.file.Write(buffer)
 	})
 }

handler/paste/paster_test.go

@@ -26,6 +26,12 @@ func (file *MockFile) Write(buffer []byte) (int, error) {
 	return args.Int(0), args.Error(1)
 }
 
+func (file *MockFile) Truncate(size int64) error {
+	args := file.Called(size)
+
+	return args.Error(0)
+}
+
 func (file *MockFile) Seek(offset int64, whence int) (int64, error) {
 	args := file.Called(offset, whence)
 
@@ -64,10 +70,10 @@ func TestRead(t *testing.T) {
 			buffer[i] = byte(char)
 		}
 	}).Once()
-	// Ensure we seek to the start of the file
-	mockFile.On("Seek", int64(0), 0).Return(int64(0), nil).Once()
+	mockFile.AssertNotCalled(t, "Seek")
 	mockFile.AssertNotCalled(t, "Close")
 	mockFile.AssertNotCalled(t, "Write")
+	mockFile.AssertNotCalled(t, "Truncate")
 
 	paster.Read(readBuffer)
 	mockFile.AssertExpectations(t)
@@ -82,6 +88,7 @@ func TestWrite(t *testing.T) {
 	mockFile.On("Write", testPaste).Return(len(testPaste), nil).Once()
 	// Ensure we seek to the start of the file
 	mockFile.On("Seek", int64(0), 0).Return(int64(0), nil).Once()
+	mockFile.On("Truncate", int64(0)).Return(nil).Once()
 	mockFile.AssertNotCalled(t, "Close")
 
 	paster.Write(testPaste)
@@ -93,7 +100,8 @@ func TestClose(t *testing.T) {
 	mockFile := paster.file.(*MockFile)
 
 	mockFile.On("Close").Return(nil).Once()
-	mockFile.AssertNotCalled(t, "Close")
+	mockFile.AssertNotCalled(t, "Write")
+	mockFile.AssertNotCalled(t, "Truncate")
 	mockFile.AssertNotCalled(t, "Seek")
 	mockFile.AssertNotCalled(t, "Read")
 	// If we write to the file on close, that isn't the worst thing in the world and we won't enforce it being a problem