httptransport: report write errors via trailer

Signed-off-by: Hank Donnay <hdonnay@redhat.com>

httptransport/indexhandler.go

@@ -92,15 +92,8 @@ func IndexHandler(serv indexer.StateIndexer) http.HandlerFunc {
 
 		w.Header().Set("etag", validator)
 		w.Header().Set("location", next)
+		defer writerError(w, &err)()
 		w.WriteHeader(http.StatusCreated)
 		err = json.NewEncoder(w).Encode(report)
-		if err != nil {
-			resp := &je.Response{
-				Code:    "encoding-error",
-				Message: fmt.Sprintf("failed to encode scan report: %v", err),
-			}
-			je.Error(w, resp, http.StatusInternalServerError)
-			return
-		}
 	}
 }

httptransport/indexreporthandler.go

@@ -91,14 +91,7 @@ func IndexReportHandler(serv indexer.StateReporter) http.HandlerFunc {
 		}
 
 		w.Header().Add("etag", validator)
+		defer writerError(w, &err)()
 		err = json.NewEncoder(w).Encode(report)
-		if err != nil {
-			resp := &je.Response{
-				Code:    "encoding-error",
-				Message: fmt.Sprintf("failed to encode scan report: %v", err),
-			}
-			je.Error(w, resp, http.StatusInternalServerError)
-			return
-		}
 	}
 }

httptransport/server.go

@@ -255,3 +255,19 @@ func (t *Server) configureWithAuth() error {
 	}
 	panic("should not reach")
 }
+
+// WriterError is a helper that closes over an error that may be returned after
+// writing a response body starts.
+//
+// The normal error flow can't be used, because the HTTP status code will have
+// been sent and some amount of body data may have been written.
+func writerError(w http.ResponseWriter, e *error) func() {
+	const errHeader = `Clair-Error`
+	w.Header().Add("trailer", errHeader)
+	return func() {
+		if *e == nil {
+			return
+		}
+		w.Header().Add(errHeader, (*e).Error())
+	}
+}

httptransport/statehandler.go

@@ -2,7 +2,6 @@ package httptransport
 
 import (
 	"encoding/json"
-	"fmt"
 	"net/http"
 
 	je "github.com/quay/claircore/pkg/jsonerr"
@@ -43,18 +42,11 @@ func StateHandler(service indexer.Stater) http.HandlerFunc {
 
 		w.Header().Set("content-type", "application/json")
 
+		defer writerError(w, &err)()
 		err = json.NewEncoder(w).Encode(struct {
 			State string `json:"state"`
 		}{
 			State: s,
 		})
-		if err != nil {
-			resp := &je.Response{
-				Code:    "encoding-error",
-				Message: fmt.Sprintf("failed to encode state: %v", err),
-			}
-			je.Error(w, resp, http.StatusInternalServerError)
-		}
-		return
 	}
 }

httptransport/vulnerabilityreporthandler.go

@@ -80,16 +80,8 @@ func VulnerabilityReportHandler(service matcher.Service, indexer indexer.Service
 			return
 		}
 
+		defer writerError(w, &err)()
 		w.WriteHeader(http.StatusOK)
 		err = json.NewEncoder(w).Encode(vulnReport)
-		if err != nil {
-			resp := &je.Response{
-				Code:    "encoding-error",
-				Message: fmt.Sprintf("failed to encode vulnerability report: %v", err),
-			}
-			je.Error(w, resp, http.StatusInternalServerError)
-			return
-		}
-		return
 	}
 }