reverseproxy: Add more debug logs

This makes debug logging very noisy when reverse proxying, but I guess
that's the point.

This has shown to be useful in troubleshooting infrastructure issues.

modules/caddyhttp/reverseproxy/reverseproxy.go

@@ -977,9 +977,11 @@ func (h Handler) finalizeResponse(
 	}
 
 	rw.WriteHeader(res.StatusCode)
+	logger.Debug("wrote header")
 
-	err := h.copyResponse(rw, res.Body, h.flushInterval(req, res))
-	res.Body.Close() // close now, instead of defer, to populate res.Trailer
+	err := h.copyResponse(rw, res.Body, h.flushInterval(req, res), logger)
+	errClose := res.Body.Close() // close now, instead of defer, to populate res.Trailer
+	logger.Debug("closed response body from upstream", zap.Error(errClose))
 	if err != nil {
 		// we're streaming the response and we've already written headers, so
 		// there's nothing an error handler can do to recover at this point;
@@ -1014,6 +1016,8 @@ func (h Handler) finalizeResponse(
 		}
 	}
 
+	logger.Debug("response finalized")
+
 	return nil
 }
 

modules/caddyhttp/reverseproxy/streaming.go

@@ -166,12 +166,13 @@ func (h Handler) isBidirectionalStream(req *http.Request, res *http.Response) bo
 		(ae == "identity" || ae == "")
 }
 
-func (h Handler) copyResponse(dst io.Writer, src io.Reader, flushInterval time.Duration) error {
+func (h Handler) copyResponse(dst io.Writer, src io.Reader, flushInterval time.Duration, logger *zap.Logger) error {
 	if flushInterval != 0 {
 		if wf, ok := dst.(writeFlusher); ok {
 			mlw := &maxLatencyWriter{
 				dst:     wf,
 				latency: flushInterval,
+				logger:  logger.Named("max_latency_writer"),
 			}
 			defer mlw.stop()
 
@@ -185,19 +186,22 @@ func (h Handler) copyResponse(dst io.Writer, src io.Reader, flushInterval time.D
 
 	buf := streamingBufPool.Get().(*[]byte)
 	defer streamingBufPool.Put(buf)
-	_, err := h.copyBuffer(dst, src, *buf)
+	_, err := h.copyBuffer(dst, src, *buf, logger)
 	return err
 }
 
 // copyBuffer returns any write errors or non-EOF read errors, and the amount
 // of bytes written.
-func (h Handler) copyBuffer(dst io.Writer, src io.Reader, buf []byte) (int64, error) {
+func (h Handler) copyBuffer(dst io.Writer, src io.Reader, buf []byte, logger *zap.Logger) (int64, error) {
 	if len(buf) == 0 {
 		buf = make([]byte, defaultBufferSize)
 	}
 	var written int64
 	for {
+		logger.Debug("waiting to read from upstream")
 		nr, rerr := src.Read(buf)
+		logger := logger.With(zap.Int("read", nr))
+		logger.Debug("read from upstream", zap.Error(rerr))
 		if rerr != nil && rerr != io.EOF && rerr != context.Canceled {
 			// TODO: this could be useful to know (indeed, it revealed an error in our
 			// fastcgi PoC earlier; but it's this single error report here that necessitates
@@ -209,10 +213,15 @@ func (h Handler) copyBuffer(dst io.Writer, src io.Reader, buf []byte) (int64, er
 			h.logger.Error("reading from backend", zap.Error(rerr))
 		}
 		if nr > 0 {
+			logger.Debug("writing to downstream")
 			nw, werr := dst.Write(buf[:nr])
 			if nw > 0 {
 				written += int64(nw)
 			}
+			logger.Debug("wrote to downstream",
+				zap.Int("written", nw),
+				zap.Int64("written_total", written),
+				zap.Error(werr))
 			if werr != nil {
 				return written, werr
 			}
@@ -295,17 +304,21 @@ type maxLatencyWriter struct {
 	mu           sync.Mutex // protects t, flushPending, and dst.Flush
 	t            *time.Timer
 	flushPending bool
+	logger       *zap.Logger
 }
 
 func (m *maxLatencyWriter) Write(p []byte) (n int, err error) {
 	m.mu.Lock()
 	defer m.mu.Unlock()
 	n, err = m.dst.Write(p)
+	m.logger.Debug("wrote bytes", zap.Int("n", n), zap.Error(err))
 	if m.latency < 0 {
+		m.logger.Debug("flushing immediately")
 		m.dst.Flush()
 		return
 	}
 	if m.flushPending {
+		m.logger.Debug("delayed flush already pending")
 		return
 	}
 	if m.t == nil {
@@ -313,6 +326,7 @@ func (m *maxLatencyWriter) Write(p []byte) (n int, err error) {
 	} else {
 		m.t.Reset(m.latency)
 	}
+	m.logger.Debug("timer set for delayed flush", zap.Duration("duration", m.latency))
 	m.flushPending = true
 	return
 }
@@ -321,8 +335,10 @@ func (m *maxLatencyWriter) delayedFlush() {
 	m.mu.Lock()
 	defer m.mu.Unlock()
 	if !m.flushPending { // if stop was called but AfterFunc already started this goroutine
+		m.logger.Debug("delayed flush is not pending")
 		return
 	}
+	m.logger.Debug("delayed flush")
 	m.dst.Flush()
 	m.flushPending = false
 }

modules/caddyhttp/reverseproxy/streaming_test.go

@@ -4,6 +4,8 @@ import (
 	"bytes"
 	"strings"
 	"testing"
+
+	"github.com/caddyserver/caddy/v2"
 )
 
 func TestHandlerCopyResponse(t *testing.T) {
@@ -18,7 +20,7 @@ func TestHandlerCopyResponse(t *testing.T) {
 	for _, d := range testdata {
 		src := bytes.NewBuffer([]byte(d))
 		dst.Reset()
-		err := h.copyResponse(dst, src, 0)
+		err := h.copyResponse(dst, src, 0, caddy.Log())
 		if err != nil {
 			t.Errorf("failed with error: %v", err)
 		}