cmd/stgenfiles: Add utility for generating test data

I use this to generate ~40 gigs of random test data in 200k files:

    ~/stgenfiles -dir /data/benchdata -files 200000 -maxexp 22 -src /dev/urandom

cmd/stgenfiles/main.go

@@ -0,0 +1,124 @@
+// Copyright (C) 2016 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at http://mozilla.org/MPL/2.0/.
+
+package main
+
+import (
+	"flag"
+	"fmt"
+	"io"
+	"log"
+	"math/rand"
+	"os"
+	"path/filepath"
+	"time"
+)
+
+func main() {
+	dir := flag.String("dir", "~/files", "Directory to generate into")
+	files := flag.Int("files", 1000, "Number of files to create")
+	maxExp := flag.Int("maxexp", 20, "Max size exponent")
+	src := flag.String("src", "/dev/urandom", "Source of file data")
+	flag.Parse()
+	if err := generateFiles(*dir, *files, *maxExp, *src); err != nil {
+		log.Println(err)
+	}
+}
+
+func generateFiles(dir string, files, maxexp int, srcname string) error {
+	fd, err := os.Open(srcname)
+	if err != nil {
+		return err
+	}
+
+	for i := 0; i < files; i++ {
+		n := randomName()
+
+		if rand.Float64() < 0.05 {
+			// Some files and directories are dotfiles
+			n = "." + n
+		}
+
+		p0 := filepath.Join(dir, string(n[0]), n[0:2])
+		err = os.MkdirAll(p0, 0755)
+		if err != nil {
+			log.Fatal(err)
+		}
+
+		p1 := filepath.Join(p0, n)
+
+		s := int64(1 << uint(rand.Intn(maxexp)))
+		a := int64(128 * 1024)
+		if a > s {
+			a = s
+		}
+		s += rand.Int63n(a)
+
+		if err := generateOneFile(fd, p1, s); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+func generateOneFile(fd io.ReadSeeker, p1 string, s int64) error {
+	src := io.LimitReader(&inifiteReader{fd}, int64(s))
+	dst, err := os.Create(p1)
+	if err != nil {
+		return err
+	}
+
+	_, err = io.Copy(dst, src)
+	if err != nil {
+		return err
+	}
+
+	err = dst.Close()
+	if err != nil {
+		return err
+	}
+
+	_ = os.Chmod(p1, os.FileMode(rand.Intn(0777)|0400))
+
+	t := time.Now().Add(-time.Duration(rand.Intn(30*86400)) * time.Second)
+	err = os.Chtimes(p1, t, t)
+	if err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func randomName() string {
+	var b [16]byte
+	readRand(b[:])
+	return fmt.Sprintf("%x", b[:])
+}
+
+func readRand(bs []byte) (int, error) {
+	var r uint32
+	for i := range bs {
+		if i%4 == 0 {
+			r = uint32(rand.Int63())
+		}
+		bs[i] = byte(r >> uint((i%4)*8))
+	}
+	return len(bs), nil
+}
+
+type inifiteReader struct {
+	rd io.ReadSeeker
+}
+
+func (i *inifiteReader) Read(bs []byte) (int, error) {
+	n, err := i.rd.Read(bs)
+	if err == io.EOF {
+		err = nil
+		i.rd.Seek(0, 0)
+	}
+	return n, err
+}