mtwebmapper: First version of pyramid tile updater. Needs testing.

common/basetilecreator.go

@@ -8,7 +8,6 @@ import (
 	"fmt"
 	"image/color"
 	"log"
-	"os"
 	"path/filepath"
 	"strconv"
 )
@@ -129,17 +128,5 @@ func (btc *BaseTileCreator) CreateTile(x, z int16, i, j int) error {
 		return 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 := SaveAsPNG(pathTmp, image); err != nil {
-		return err
-	}
-	return os.Rename(pathTmp, path)
+	return SaveAsPNGAtomic(path, image)
 }

common/image.go

@@ -6,12 +6,35 @@ package common
 
 import (
 	"bufio"
+	"errors"
 	"image"
 	"image/png"
 	"log"
 	"os"
+	"strconv"
+	"sync"
+	"time"
 )
 
+var rand uint32
+var randmu sync.Mutex
+
+func reseed() uint32 {
+	return uint32(time.Now().UnixNano() + int64(os.Getpid()))
+}
+
+func nextSuffix() string {
+	randmu.Lock()
+	r := rand
+	if r == 0 {
+		r = reseed()
+	}
+	r = r*1664525 + 1013904223 // constants from Numerical Recipes
+	rand = r
+	randmu.Unlock()
+	return strconv.Itoa(int(1e9 + r%1e9))[1:]
+}
+
 func SaveAsPNG(path string, img image.Image) (err error) {
 	var file *os.File
 	if file, err = os.Create(path); err != nil {
@@ -24,6 +47,38 @@ func SaveAsPNG(path string, img image.Image) (err error) {
 	return
 }
 
+func tmpName(tmpl string) (string, error) {
+	tmpPre := tmpl + ".tmp"
+	nconflict := 0
+	for i := 0; i < 10000; i++ {
+		tmp := tmpPre + nextSuffix()
+		if _, err := os.Stat(tmp); err != nil {
+			if os.IsNotExist(err) {
+				return tmp, nil
+			}
+			return "", err
+		}
+		if nconflict++; nconflict > 10 {
+			nconflict = 0
+			rand = reseed()
+		}
+	}
+	return "", errors.New("Cannot create temp name")
+}
+
+func SaveAsPNGAtomic(path string, img image.Image) (err error) {
+
+	var tmpPath string
+	if tmpPath, err = tmpName(path); err != nil {
+		return
+	}
+	// Still a bit racy
+	if err = SaveAsPNG(tmpPath, img); err != nil {
+		return
+	}
+	return os.Rename(tmpPath, path)
+}
+
 func LoadPNG(path string) image.Image {
 	var err error
 	var file *os.File