http: Fix "Internal Server Error" regressions

If the database is not reachable for some reason, mirrorbits must not
return "500 Internal Server Error", instead it must redirect users to
the fallback mirror(s).

This was broken in ed1db9e, here's the
fix.

Closes: #195

Author: elboulangero

http/http.go

@@ -333,12 +333,11 @@ func (h *HTTP) mirrorHandler(w http.ResponseWriter, r *http.Request, ctx *Contex
 		return
 	}
 
-	// Get details about the requested file
+	// Get details about the requested file. Errors are not fatal, and
+	// expected when the database is not ready: fallbacks will handle it.
 	fileInfo, err := h.cache.GetFileInfo(urlPath)
 	if err != nil {
-		log.Errorf("Error while fetching Fileinfo: %s", err.Error())
-		http.Error(w, err.Error(), http.StatusInternalServerError)
-		return
+		//log.Debugf("Error while fetching Fileinfo: %s", err.Error())
 	}
 
 	if checkIfModifiedSince(r, fileInfo.ModTime) == condFalse {

http/selection.go

@@ -4,6 +4,7 @@
 package http
 
 import (
+	"errors"
 	"fmt"
 	"math"
 	"math/rand"
@@ -18,6 +19,10 @@ import (
 	"github.com/etix/mirrorbits/utils"
 )
 
+var (
+	ErrInvalidFileInfo = errors.New("Invalid file info (modtime is zero)")
+)
+
 type mirrorSelection interface {
 	// Selection must return an ordered list of selected mirror,
 	// a list of rejected mirrors and and an error code.
@@ -29,6 +34,12 @@ type DefaultEngine struct{}
 
 // Selection returns an ordered list of selected mirror, a list of rejected mirrors and and an error code
 func (h DefaultEngine) Selection(ctx *Context, cache *mirrors.Cache, fileInfo *filesystem.FileInfo, clientInfo network.GeoIPRecord) (mlist mirrors.Mirrors, excluded mirrors.Mirrors, err error) {
+	// Bail out early if we don't have valid file details
+	if fileInfo.ModTime.IsZero() {
+		err = ErrInvalidFileInfo
+		return
+	}
+
 	// Prepare and return the list of all potential mirrors
 	mlist, err = cache.GetMirrors(fileInfo.Path, clientInfo)
 	if err != nil {