fn: introducing 503 responses for out of capacity case (#518)

* fn: introducing 503 responses for out of capacity case

*) Adding 503 with Retry-After header case if request failed
during waiting for slots.
*) TODO: return 503 without Retry-After if the request can
never be met by this fn server.
*) fn: runner test docker pull fixup
*) fn: MaxMemory for routes is now a variable to allow
testing and adjusting it according to fleet memory sizes.

api/server/runner.go

@@ -2,7 +2,6 @@ package server
 
 import (
 	"bytes"
-	"context"
 	"net/http"
 	"path"
 	"strings"
@@ -90,12 +89,10 @@ func (s *Server) serve(c *gin.Context, appName, path string) {
 	if err != nil {
 		// NOTE if they cancel the request then it will stop the call (kind of cool),
 		// we could filter that error out here too as right now it yells a little
-		if err == context.DeadlineExceeded {
+		if err == models.ErrCallTimeoutServerBusy || err == models.ErrCallTimeout {
 			// TODO maneuver
 			// 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())
-
-			err = models.ErrCallTimeout // 504 w/ friendly note
 		}
 		// 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!)