From 3e4c1aa2d9880c716b566e100033f57c625094a1 Mon Sep 17 00:00:00 2001
From: "Sascha L. Teichmann" <sascha.teichmann@intevation.de>
Date: Sat, 20 Sep 2014 12:28:38 +0200
Subject: [PATCH] mtwebmapper: To avoid possible races when serving tiles the
 same time as re-generating them the re-generation write them to temp files
 and rename them afterwards.

---
 cmd/mtwebmapper/tilesupdater.go | 40 +++++++++++++++++++++++++--------
 1 file changed, 31 insertions(+), 9 deletions(-)

diff --git a/cmd/mtwebmapper/tilesupdater.go b/cmd/mtwebmapper/tilesupdater.go
index a77745e..ba65080 100644
--- a/cmd/mtwebmapper/tilesupdater.go
+++ b/cmd/mtwebmapper/tilesupdater.go
@@ -10,7 +10,9 @@ import (
 	"image/color"
 	"log"
 	"net/http"
+	"os"
 	"path/filepath"
+	"strconv"
 	"sync"
 
 	"bitbucket.org/s_l_teichmann/mtredisalize/common"
@@ -107,7 +109,7 @@ func (tu *tileUpdater) doUpdates() (err error) {
 		if client, err = common.NewRedisClient("tcp", tu.redisAddress); err != nil {
 			return
 		}
-		btc := NewBaseTileCreator(client, tu.colors, baseDir)
+		btc := NewBaseTileCreator(client, tu.colors, baseDir, true)
 		go btc.run(jobs)
 	}
 
@@ -137,17 +139,22 @@ type BaseTileCreator struct {
 	renderer *common.Renderer
 	yOrder   *common.YOrder
 	baseDir  string
+	update   bool
 }
 
-func NewBaseTileCreator(client *common.RedisClient,
-	colors *common.Colors, baseDir string) *BaseTileCreator {
+func NewBaseTileCreator(
+	client *common.RedisClient,
+	colors *common.Colors,
+	baseDir string,
+	update bool) *BaseTileCreator {
 	renderer := common.NewRenderer(tileWidth, tileHeight)
 	return &BaseTileCreator{
 		client:   client,
 		colors:   colors,
-		baseDir:  baseDir,
 		renderer: renderer,
-		yOrder:   common.NewYOrder(renderer, yOrderCapacity)}
+		yOrder:   common.NewYOrder(renderer, yOrderCapacity),
+		baseDir:  baseDir,
+		update:   update}
 }
 
 func (btc *BaseTileCreator) run(jobs chan xz) {
@@ -215,10 +222,25 @@ func (btc *BaseTileCreator) createTile(x, z int16, i, j int) error {
 		16, 16, (tileWidth-2)*16, (tileHeight-2)*16,
 		btc.colors.Colors, color.RGBA{R: 0xff, G: 0xff, B: 0xff, A: 0xff})
 
-	path := filepath.Join(btc.baseDir, fmt.Sprintf("%d", i), fmt.Sprintf("%d.png", j))
-	log.Printf("file path: %s", path)
+	path := filepath.Join(btc.baseDir, strconv.Itoa(i), fmt.Sprintf("%d.png", j))
 
-	log.Printf("Writing (%d, %d) (%d, %d)", i, j, x, z)
+	log.Printf("Writing (%d, %d) to file %s", x, z, path)
 
-	return common.SaveAsPNG(path, image)
+	if !btc.update {
+		return common.SaveAsPNG(path, image)
+	}
+
+	// Try to make creation of update "atomic" by first writing to tmp file
+	// and rename the tmp file to the original one afterwards.
+	pathTmp := path + "tmp"
+	pathTmpPre := pathTmp
+	tc := 0
+	for _, err := os.Stat(pathTmp); err == nil; _, err = os.Stat(pathTmp) {
+		pathTmp = fmt.Sprintf("%s%d", pathTmpPre, tc)
+		tc++
+	}
+	if err := common.SaveAsPNG(pathTmp, image); err != nil {
+		return err
+	}
+	return os.Rename(pathTmp, path)
 }