Add truncate() and sync() on resize.

This commit fixes an issue with ext3/ext4 filesystems where metadata file size is not synced
when resizing a file. It also resizes the entire resize instead of updating the size during
individual page writes.

Thanks to @tv42 for the fix.

bolt_unix.go

@@ -3,6 +3,7 @@
 package bolt
 
 import (
+	"fmt"
 	"os"
 	"syscall"
 	"time"
@@ -41,6 +42,16 @@ func funlock(f *os.File) error {
 
 // mmap memory maps a DB's data file.
 func mmap(db *DB, sz int) error {
+	// Truncate and fsync to ensure file size metadata is flushed.
+	// https://github.com/boltdb/bolt/issues/284
+	if err := db.file.Truncate(int64(sz)); err != nil {
+		return fmt.Errorf("file resize error: %s", err)
+	}
+	if err := db.file.Sync(); err != nil {
+		return fmt.Errorf("file sync error: %s", err)
+	}
+
+	// Map the data file to memory.
 	b, err := syscall.Mmap(int(db.file.Fd()), 0, sz, syscall.PROT_READ, syscall.MAP_SHARED)
 	if err != nil {
 		return err