fix up response headers (#788)

* fix up response headers

* stops defaulting to application/json. this was something awful, go stdlib has
a func to detect content type. sadly, it doesn't contain json, but we can do a
pretty good job by checking for an opening '{'... there are other fish in the
sea, and now we handle them nicely instead of saying it's a json [when it's
not]. a test confirms this, there should be no breakage for any routes
returning a json blob that were relying on us defaulting to this format
(granted that they start with a '{').
* buffers output now to a buffer for all protocol types (default is no longer
left out in the cold). use a little response writer so that we can still let
users write headers from their functions. this is useful for content type
detection instead of having to do it in multiple places.
* plumbs the little content type bit into fn-test-util just so we can test it,
we don't want to put this in the fdk since it's redundant.

I am totally in favor of getting rid of content type from the top level json
blurb. it's redundant, at best, and can have confusing behaviors if a user
uses both the headers and the content_type field (we override with the latter,
now). it's client protocol specific to http to a certain degree, other
protocols may use this concept but have their own way to set it (like http
does in headers..). I realize that it mostly exists because it's somewhat gross
to have to index a list from the headers in certain languages more than
others, but with the ^ behavior, is it really worth it?

closes #782

* reset idle timeouts back

* move json prefix to stack / next to use

api/server/runner.go

@@ -2,8 +2,11 @@ package server
 
 import (
 	"bytes"
+	"io"
 	"net/http"
 	"path"
+	"strconv"
+	"sync"
 	"time"
 
 	"github.com/fnproject/fn/api"
@@ -46,18 +49,29 @@ func (s *Server) handleFunctionCall2(c *gin.Context) error {
 
 	// gin sets this to 404 on NoRoute, so we'll just ensure it's 200 by default.
 	c.Status(200) // this doesn't write the header yet
-	c.Header("Content-Type", "application/json")
 
 	return s.serve(c, a, path.Clean(p))
 }
 
+var (
+	bufPool = &sync.Pool{New: func() interface{} { return new(bytes.Buffer) }}
+)
+
 // TODO it would be nice if we could make this have nothing to do with the gin.Context but meh
 // TODO make async store an *http.Request? would be sexy until we have different api format...
 func (s *Server) serve(c *gin.Context, appName, path string) error {
+	buf := bufPool.Get().(*bytes.Buffer)
+	buf.Reset()
+	writer := syncResponseWriter{
+		Buffer:  buf,
+		headers: c.Writer.Header(), // copy ref
+	}
+	defer bufPool.Put(buf) // TODO need to ensure this is safe with Dispatch?
+
 	// GetCall can mod headers, assign an id, look up the route/app (cached),
 	// strip params, etc.
 	call, err := s.agent.GetCall(
-		agent.WithWriter(c.Writer), // XXX (reed): order matters [for now]
+		agent.WithWriter(&writer), // XXX (reed): order matters [for now]
 		agent.FromRequest(appName, path, c.Request),
 	)
 	if err != nil {
@@ -72,11 +86,9 @@ func (s *Server) serve(c *gin.Context, appName, path string) error {
 
 	if model.Type == "async" {
 		// TODO we should push this into GetCall somehow (CallOpt maybe) or maybe agent.Queue(Call) ?
-		contentLength := c.Request.ContentLength
-		if contentLength < 128 { // contentLength could be -1 or really small, sanitize
-			contentLength = 128
+		if c.Request.ContentLength > 0 {
+			buf.Grow(int(c.Request.ContentLength))
 		}
-		buf := bytes.NewBuffer(make([]byte, int(contentLength))[:0]) // TODO sync.Pool me
 		_, err := buf.ReadFrom(c.Request.Body)
 		if err != nil {
 			return models.ErrInvalidPayload
@@ -102,16 +114,45 @@ func (s *Server) serve(c *gin.Context, appName, path string) error {
 			// add this, since it means that start may not have been called [and it's relevant]
 			c.Writer.Header().Add("XXX-FXLB-WAIT", time.Now().Sub(time.Time(model.CreatedAt)).String())
 		}
-		// NOTE: if the task wrote the headers already then this will fail to write
-		// a 5xx (and log about it to us) -- that's fine (nice, even!)
 		return err
 	}
 
-	// TODO plumb FXLB-WAIT somehow (api?)
+	// if they don't set a content-type - detect it
+	if writer.Header().Get("Content-Type") == "" {
+		// see http.DetectContentType, the go server is supposed to do this for us but doesn't appear to?
+		var contentType string
+		jsonPrefix := [1]byte{'{'} // stack allocated
+		if bytes.HasPrefix(buf.Bytes(), jsonPrefix[:]) {
+			// try to detect json, since DetectContentType isn't a hipster.
+			contentType = "application/json; charset=utf-8"
+		} else {
+			contentType = http.DetectContentType(buf.Bytes())
+		}
+		writer.Header().Set("Content-Type", contentType)
+	}
 
-	// TODO we need to watch the response writer and if no bytes written
-	// then write a 200 at this point?
-	// c.Data(http.StatusOK)
+	writer.Header().Set("Content-Length", strconv.Itoa(int(buf.Len())))
+
+	if writer.status > 0 {
+		c.Writer.WriteHeader(writer.status)
+	}
+	io.Copy(c.Writer, &writer)
 
 	return nil
 }
+
+var _ http.ResponseWriter = new(syncResponseWriter)
+
+// implements http.ResponseWriter
+// this little guy buffers responses from user containers and lets them still
+// set headers and such without us risking writing partial output [as much, the
+// server could still die while we're copying the buffer]. this lets us set
+// content length and content type nicely, as a bonus. it is sad, yes.
+type syncResponseWriter struct {
+	headers http.Header
+	status  int
+	*bytes.Buffer
+}
+
+func (s *syncResponseWriter) Header() http.Header  { return s.headers }
+func (s *syncResponseWriter) WriteHeader(code int) { s.status = code }