httptransport: add error helper

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

httptransport/error.go

@@ -0,0 +1,47 @@
+package httptransport
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"net/http"
+)
+
+// ApiError writes an untyped (that is, "application/json") error with the
+// provided HTTP status code and message.
+func apiError(w http.ResponseWriter, code int, f string, v ...interface{}) {
+	const errheader = `Clair-Error`
+	h := w.Header()
+	h.Del("link")
+	h.Set("content-type", "application/json")
+	h.Set("x-content-type-options", "nosniff")
+	h.Set("trailer", errheader)
+	w.WriteHeader(code)
+
+	var buf bytes.Buffer
+	buf.WriteString(`{"code":"`)
+	switch code {
+	case http.StatusBadRequest:
+		buf.WriteString("bad-request")
+	case http.StatusMethodNotAllowed:
+		buf.WriteString("method-not-allowed")
+	case http.StatusNotFound:
+		buf.WriteString("not-found")
+	default:
+		buf.WriteString("internal-error")
+	}
+	buf.WriteByte('"')
+	if f != "" {
+		buf.WriteString(`,"message":`)
+		b, _ := json.Marshal(fmt.Sprintf(f, v...)) // OK use of encoding/json.
+		buf.Write(b)
+	}
+	buf.WriteByte('}')
+
+	if _, err := buf.WriteTo(w); err != nil {
+		h.Set(errheader, err.Error())
+	}
+	if f, ok := w.(http.Flusher); ok {
+		f.Flush()
+	}
+}