Fix error handling for ExternalAuth

This corrects an issue where the error body was being hidden
for the external auth handler. It also adds the ca-certs into
the runtime Docker image for when the gateway is calling an
external plugin exposed over HTTPS.

Tested with OAuth2 plugin.

Signed-off-by: Alex Ellis <alexellis2@gmail.com>

gateway/Dockerfile

@@ -33,7 +33,7 @@ RUN license-check -path ./ --verbose=false "Alex Ellis" "OpenFaaS Project" "Open
     -X github.com/openfaas/faas/gateway/version.Version=${VERSION}" \
     -a -installsuffix cgo -o gateway .
 
-FROM alpine:3.8
+FROM alpine:3.9
 
 LABEL org.label-schema.license="MIT" \
     org.label-schema.vcs-url="https://github.com/openfaas/faas" \
@@ -43,7 +43,8 @@ LABEL org.label-schema.license="MIT" \
     org.label-schema.docker.schema-version="1.0"
 
 RUN addgroup -S app \
-    && adduser -S -g app app
+    && adduser -S -g app app \
+    && apk add --no-cache ca-certificates
 
 WORKDIR /home/app
 

gateway/handlers/external_auth.go

@@ -3,6 +3,7 @@ package handlers
 import (
 	"context"
 	"io"
+	"log"
 	"net/http"
 	"time"
 )
@@ -22,7 +23,8 @@ func MakeExternalAuthHandler(next http.HandlerFunc, upstreamTimeout time.Duratio
 
 		res, err := http.DefaultClient.Do(req.WithContext(deadlineContext))
 		if err != nil {
-			w.WriteHeader(http.StatusInternalServerError)
+			http.Error(w, err.Error(), http.StatusInternalServerError)
+			log.Printf("ExternalAuthHandler: %s", err.Error())
 			return
 		}
 

gateway/handlers/external_auth_test.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"net/http"
 	"net/http/httptest"
+	"strings"
 	"testing"
 	"time"
 )
@@ -206,6 +207,10 @@ func Test_External_Auth_Wrapper_TimeoutGivesInternalServerError(t *testing.T) {
 	if rr.Code != want {
 		t.Errorf("Status incorrect, want: %d, but got %d", want, rr.Code)
 	}
+	wantSubstring := "context deadline exceeded\n"
+	if !strings.HasSuffix(string(rr.Body.Bytes()), wantSubstring) {
+		t.Errorf("Body incorrect, want to have suffix: %q, but got %q", []byte(wantSubstring), rr.Body)
+	}
 }
 
 // // Test_External_Auth_Wrapper_PassesValidAuthButOnly200IsValid this test exists