chore: log where and after how many bytes a file transfer breaks

internal/server/fsstream.go

@@ -6,6 +6,7 @@ import (
 	"encoding/json"
 	"errors"
 	"io"
+	"log"
 	"net/http"
 	"path"
 	"strconv"
@@ -154,7 +155,10 @@ func (s *Server) handleFSUpload(w http.ResponseWriter, r *http.Request) {
 	copied := make(chan struct{})
 	go func() {
 		defer close(copied)
-		_, err := io.Copy(st.w, body)
+		n, err := io.Copy(st.w, body)
+		if err != nil && !errors.Is(err, io.ErrClosedPipe) {
+			log.Printf("fs upload %s to %s: browser stream broke after %d of %d bytes: %v", p, agentID, n, r.ContentLength, err)
+		}
 		st.w.CloseWithError(err) // nil: the reader sees the end of the file
 	}()
 
@@ -246,9 +250,10 @@ func (s *Server) handleFSDownload(w http.ResponseWriter, r *http.Request) {
 		if st.size >= 0 {
 			w.Header().Set("Content-Length", strconv.FormatInt(st.size, 10))
 		}
-		if _, err := io.Copy(w, st.r); err != nil {
+		if n, err := io.Copy(w, st.r); err != nil {
 			// Cut the response off, so the browser does not take a part of
 			// the file for all of it.
+			log.Printf("fs download %s from %s: cut off after %d of %d bytes: %v", p, agentID, n, st.size, err)
 			st.r.CloseWithError(err)
 			panic(http.ErrAbortHandler)
 		}
@@ -310,9 +315,10 @@ func (s *Server) handleAgentFSStreamGet(w http.ResponseWriter, r *http.Request)
 	if st.size >= 0 {
 		w.Header().Set("Content-Length", strconv.FormatInt(st.size, 10))
 	}
-	if _, err := io.Copy(w, st.r); err != nil {
+	if n, err := io.Copy(w, st.r); err != nil {
 		// The browser broke off: cut the response, so the agent does not
 		// take a part of the file for all of it.
+		log.Printf("fs upload to %s: cut off after %d of %d bytes: %v", a.ID, n, st.size, err)
 		panic(http.ErrAbortHandler)
 	}
 }
@@ -338,9 +344,10 @@ func (s *Server) handleAgentFSStreamPost(w http.ResponseWriter, r *http.Request)
 		http.Error(w, "gone", http.StatusGone)
 		return
 	}
-	_, err = io.Copy(st.w, http.MaxBytesReader(w, r.Body, proto.FSFileCap))
+	n, err := io.Copy(st.w, http.MaxBytesReader(w, r.Body, proto.FSFileCap))
 	st.w.CloseWithError(err)
 	if err != nil {
+		log.Printf("fs download from %s: agent stream broke after %d of %d bytes: %v", a.ID, n, size, err)
 		http.Error(w, "gone", http.StatusGone)
 		return
 	}