Add contrib/atomicrename

$ ./contrib/atomicrename/atomicrename -h
atomicrename creates 100 "src" files in the current directory, renames
them in random order over a single "dst" file while reading the "dst"
file concurrently in a loop.

Progress and errors are reported as they occour in addition to a summary
printed at the end. cifs and fuse filesystems are known to fail, local
filesystems and nfs seem ok.

See https://github.com/hanwen/go-fuse/issues/398 for background info.

build.bash

@@ -94,7 +94,7 @@ fi
 # Actual "go build" call for gocryptfs
 go build "-ldflags=$GO_LDFLAGS" "$@"
 # Additional binaries
-for d in gocryptfs-xray contrib/statfs contrib/findholes ; do
+for d in gocryptfs-xray contrib/statfs contrib/findholes contrib/atomicrename ; do
 	(cd "$d"; go build "-ldflags=$GO_LDFLAGS" "$@")
 done
 

contrib/atomicrename/.gitignore

@@ -0,0 +1 @@
+/atomicrename

contrib/atomicrename/main.go

@@ -0,0 +1,101 @@
+package main
+
+import (
+	"bytes"
+	"flag"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"strings"
+	"sync/atomic"
+	"syscall"
+)
+
+const fileCount = 100
+
+type stats struct {
+	renameOk            int
+	renameError         int
+	readOk              int
+	readError           int
+	readContentMismatch int
+}
+
+func usage() {
+	fmt.Printf(`atomicrename creates %d "src" files in the current directory, renames
+them in random order over a single "dst" file while reading the "dst"
+file concurrently in a loop.
+
+Progress and errors are reported as they occour in addition to a summary
+printed at the end. cifs and fuse filesystems are known to fail, local
+filesystems and nfs seem ok.
+
+See https://github.com/hanwen/go-fuse/issues/398 for background info.
+`, fileCount)
+	os.Exit(1)
+}
+
+func main() {
+	flag.Usage = usage
+	flag.Parse()
+
+	hello := []byte("hello world")
+	srcFiles := make(map[string]struct{})
+
+	// prepare source files
+	fmt.Print("creating files")
+	for i := 0; i < fileCount; i++ {
+		srcName := fmt.Sprintf("src.atomicrename.%d", i)
+		srcFiles[srcName] = struct{}{}
+		buf := bytes.Repeat([]byte("_"), i)
+		buf = append(buf, hello...)
+		if err := ioutil.WriteFile(srcName, buf, 0600); err != nil {
+			panic(err)
+		}
+		fmt.Print(".")
+	}
+	fmt.Print("\n")
+
+	// prepare destination file
+	const dstName = "dst.atomicrename"
+	if err := ioutil.WriteFile(dstName, hello, 0600); err != nil {
+		panic(err)
+	}
+
+	var running int32 = 1
+
+	stats := stats{}
+
+	// read thread
+	go func() {
+		for atomic.LoadInt32(&running) == 1 {
+			have, err := ioutil.ReadFile(dstName)
+			if err != nil {
+				fmt.Println(err)
+				stats.readError++
+				continue
+			}
+			if !strings.HasSuffix(string(have), string(hello)) {
+				fmt.Printf("content mismatch: have %q\n", have)
+				stats.readContentMismatch++
+				continue
+			}
+			fmt.Printf("content ok len=%d\n", len(have))
+			stats.readOk++
+		}
+	}()
+
+	// rename thread = main thread
+	for srcName := range srcFiles {
+		if err := os.Rename(srcName, dstName); err != nil {
+			fmt.Println(err)
+			stats.renameError++
+		}
+		stats.renameOk++
+	}
+	// Signal the Read goroutine to stop when loop is done
+	atomic.StoreInt32(&running, 0)
+
+	syscall.Unlink(dstName)
+	fmt.Printf("stats: %#v\n", stats)
+}