Answer 499 to a client that goes away while waiting for an image (closes #339)

When a request waits for an image another request is making and its
client goes away, the image proxy returns context.Canceled. serveImage
now answers it 499, the status nginx calls "client closed request". It
logs it once at info level with the request_id, host and path. It was
answered 500 and logged at error level, as pixa's own failure. A
cancellation while the request's context is live is still answered 500.
The request making the image is still answered as before. README.md
lists the 499 on both image routes.

Model: opus-5-5

README.md

@@ -310,8 +310,8 @@ path under `/v1/` answers 200, in maintenance mode too.
     - 403 when the upstream host, or a host it redirects to, has an address in a
       blocked network (see `blocked_networks`).
     - 429 over `image_requests_per_minute` (below).
-    - 499 when the client goes away while waiting for an image another request
-      is making (below).
+    - 499 when the client goes away before its image is ready, unless its
+      request is the one making the image (below).
     - 415 when the upstream answers with a `Content-Type` that is not one of the
       source formats (below).
     - 415 when the upstream sends a body whose first bytes are not those of the

TODO.md

@@ -31,9 +31,9 @@ P2: security: per-origin rate limiting
 # Completed Steps
 
 - 2026-10-26 on both image routes, a request is answered 499 and logged at info
-  level when its client goes away while waiting for an image another request is
-  making (closes #339). It was answered 500 and logged at error level, as pixa's
-  own failure.
+  level when its client goes away before its image is ready, unless it is the
+  request making the image (closes #339). It was answered 500 and logged at
+  error level, as pixa's own failure.
     - The line is "client went away before the image was ready". It carries the
       `request_id`, the host and the path.
     - The 499 has no body, since nobody reads it.

internal/handlers/image.go

@@ -21,9 +21,9 @@ import (
 )
 
 // statusClientClosedRequest is the status nginx calls "client closed
-// request". The image routes answer it when the client goes away while
-// waiting for an image another request is making. Nobody reads it; it is for
-// logs and metrics.
+// request". The image routes answer it when the client goes away before its
+// image is ready, unless its request is the one making the image. Nobody reads
+// it; it is for logs and metrics.
 const statusClientClosedRequest = 499
 
 // HandleImage handles the main image proxy route:
@@ -353,8 +353,8 @@ func notModified(w http.ResponseWriter, r *http.Request, etag string) bool {
 // once they have the request and its URL's expiry, which ends the
 // Cache-Control max-age. It chooses the format for the format auto, answers
 // If-None-Match with 304 and HEAD with the headers only. It answers 499 when
-// the client goes away while waiting for an image another request is making.
-// It answers any other error of the image service with respondImageError.
+// the image service stopped because the client went away, and any other error
+// of the image service with respondImageError.
 func (s *Handlers) serveImage(
 	w http.ResponseWriter, r *http.Request,
 	req *imagerequest.Request, expires time.Time,