Use cap() instead of len() to compute the spooling threshold (#62)

* feat: use cap() instead of len() to compute the spooling threshold

* feat: if the buffer is above the s.maxInMemorySize, don't return it in the pool but just discard it

* fix: compute the len of the buffer AND the cap when deciding to spool

* fix: TestThresholdCrossing as a buffer grow is expressed in a power of 2 and the buffer.cap() check when writing was making the test fail

* feat: also discard the buffer pointer if it's cap exceeded memory limit when closing

pkg/spooledtempfile/spooled.go

@@ -176,7 +176,7 @@ func (s *spooledTempFile) Write(p []byte) (n int, err error) {
 	// Otherwise, check if system memory usage is above threshold
 	// or if we've exceeded our own in-memory limit, or if user forced on-disk.
 	aboveRAMThreshold := s.isSystemMemoryUsageHigh()
-	if aboveRAMThreshold || s.fullOnDisk || (s.buf.Len()+len(p) > s.maxInMemorySize) {
+	if aboveRAMThreshold || s.fullOnDisk || (s.buf.Len()+len(p) > s.maxInMemorySize) || (s.buf.Cap() > s.maxInMemorySize) {
 		// Switch to file if we haven't already
 		s.file, err = os.CreateTemp(s.tempDir, s.filePrefix+"-")
 		if err != nil {
@@ -191,10 +191,15 @@ func (s *spooledTempFile) Write(p []byte) (n int, err error) {
 			return 0, err
 		}
 
-		// Release the buffer
-		s.buf.Reset()
-		spooledPool.Put(s.buf)
-		s.buf = nil
+		// If we're above the RAM threshold, we don't want to keep the buffer around.
+		if s.buf.Cap() > s.maxInMemorySize {
+			s.buf = nil
+		} else {
+			// Release the buffer
+			s.buf.Reset()
+			spooledPool.Put(s.buf)
+			s.buf = nil
+		}
 
 		// Write incoming bytes directly to file
 		n, err = s.file.Write(p)
@@ -214,7 +219,11 @@ func (s *spooledTempFile) Close() error {
 	s.closed = true
 	s.mem = nil
 
-	if s.buf != nil {
+	// If we're above the RAM threshold, we don't want to keep the buffer around.
+	if s.buf != nil && s.buf.Cap() > s.maxInMemorySize {
+		s.buf = nil
+	} else {
+		// Release the buffer
 		s.buf.Reset()
 		spooledPool.Put(s.buf)
 		s.buf = nil

pkg/spooledtempfile/spooled_test.go

@@ -62,11 +62,11 @@ func TestInMemoryBasic(t *testing.T) {
 
 // TestThresholdCrossing writes enough data to switch from in-memory to disk.
 func TestThresholdCrossing(t *testing.T) {
-	spool := NewSpooledTempFile("test", os.TempDir(), 10, false, -1)
+	spool := NewSpooledTempFile("test", os.TempDir(), 16, false, -1)
 	defer spool.Close()
 
 	data1 := []byte("12345")
-	data2 := []byte("67890ABCD") // total length > 10
+	data2 := []byte("67890ABCDEFGHIJKLM") // total length > 16
 
 	_, err := spool.Write(data1)
 	if err != nil {