fix(s3api): return 503 + Retry-After when remote object not cached yet (#9233)

* fix(s3api): return 503 with Retry-After when remote object not cached yet

When a GET hits a remote-only object whose cache fill timed out or was
canceled, the handler returned 500 InternalError. SDK clients treat 500
as a server bug and surface it as a fatal error (boto3
S3DownloadFailedError), even though the cache is often still filling in
the background and the next request would succeed.

Return 503 ServiceUnavailable with Retry-After: 5 instead, matching
AWS S3's "try again later" semantics. AWS SDKs already classify 503 as
retryable and apply exponential backoff transparently, so clients
recover without changes.

Refs https://github.com/seaweedfs/seaweedfs/discussions/9174

* treat client cancel as cancellation, not 503

If r.Context() is already canceled when the cache attempt returns no
chunks, the cache failure is almost certainly a side-effect of the
client disconnecting, not real backpressure. Surface the context error
so GetObjectHandler logs at V(3) and skips writing a response, instead
of synthesizing a 503 that nobody will read.

Addresses Gemini review feedback on #9233.

* simplify comments

weed/s3api/s3api_object_handlers.go

@@ -1000,10 +1000,15 @@ func (s3a *S3ApiServer) streamFromVolumeServers(w http.ResponseWriter, r *http.R
 				entry = cachedEntry
 				glog.V(1).Infof("streamFromVolumeServers: successfully cached remote object, got %d chunks", len(chunks))
 			} else {
-				// Caching failed - return error to client
-				glog.Errorf("streamFromVolumeServers: failed to cache remote object for streaming")
-				s3err.WriteErrorResponse(w, r, s3err.ErrInternalError)
-				return newStreamErrorWithResponse(fmt.Errorf("failed to cache remote object for streaming"))
+				// Client disconnected: report cancellation, not 503.
+				if ctxErr := r.Context().Err(); ctxErr != nil {
+					return ctxErr
+				}
+				// Cache still filling: 503 + Retry-After so SDKs back off and retry.
+				glog.V(1).Infof("streamFromVolumeServers: remote object %s/%s not cached yet, returning 503 for retry", bucket, object)
+				w.Header().Set("Retry-After", "5")
+				s3err.WriteErrorResponse(w, r, s3err.ErrServiceUnavailable)
+				return newStreamErrorWithResponse(fmt.Errorf("remote object not cached yet"))
 			}
 		} else if totalSize > 0 && len(entry.Content) == 0 {
 			// Not a remote entry but has size without content - this is a data integrity issue