Merge lp:~chipaca/snappy/fsync into lp:~snappy-dev/snappy/snappy-moved-to-github

[John Lenton]

do fsync after "atomic" file writes

This might make mounting boot "sync" unnecessary.

[lp:~chipaca/snappy/fsync]

432. By John Lenton on 2015-04-29

oops

[Michael Vogt (mvo)]

Thanks for doing this work, its really great! One comment/question inline.

[Michael Vogt (mvo)]

One more comment from Steve.

[Michael Vogt (mvo)]

review: Approve

=== modified file 'helpers/helpers.go'
--- helpers/helpers.go	2015-04-23 11:19:42 +0000
+++ helpers/helpers.go	2015-04-29 09:10:20 +0000
@@ -23,7 +23,6 @@
 	"encoding/hex"
 	"fmt"
 	"io"
-	"io/ioutil"
 	"math/rand"
 	"os"
 	"os/exec"
@@ -247,15 +246,45 @@
 
 // AtomicWriteFile updates the filename atomically and works otherwise
 // exactly like io/ioutil.WriteFile()
-func AtomicWriteFile(filename string, data []byte, perm os.FileMode) error {
+func AtomicWriteFile(filename string, data []byte, perm os.FileMode) (err error) {
 	tmp := filename + ".new"
 
-	if err := ioutil.WriteFile(tmp, data, perm); err != nil {
-		os.Remove(tmp)
-		return err
-	}
-
-	return os.Rename(tmp, filename)
+	// XXX: if go switches to use aio_fsync, we need to open the dir for writing
+	dir, err := os.Open(filepath.Dir(filename))
+	if err != nil {
+		return err
+	}
+	defer dir.Close()
+
+	fd, err := os.OpenFile(tmp, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, perm)
+	if err != nil {
+		return err
+	}
+	defer func() {
+		e := fd.Close()
+		if err == nil {
+			err = e
+		}
+		if err != nil {
+			os.Remove(tmp)
+		}
+	}()
+
+	// according to the docs, Write returns a non-nil error when n !=
+	// len(b), so don't worry about short writes.
+	if _, err := fd.Write(data); err != nil {
+		return err
+	}
+
+	if err := fd.Sync(); err != nil {
+		return err
+	}
+
+	if err := os.Rename(tmp, filename); err != nil {
+		return err
+	}
+
+	return dir.Sync()
 }
 
 // CurrentHomeDir returns the homedir of the current user. It looks at

=== modified file 'partition/bootloader_uboot.go'
--- partition/bootloader_uboot.go	2015-03-26 09:05:27 +0000
+++ partition/bootloader_uboot.go	2015-04-29 09:10:20 +0000
@@ -175,7 +175,12 @@
 		return err
 	}
 
-	defer file.Close()
+	defer func() {
+		e := file.Close()
+		if err == nil {
+			err = e
+		}
+	}()
 
 	writer := bufio.NewWriter(file)
 
@@ -184,7 +189,12 @@
 			return err
 		}
 	}
-	return writer.Flush()
+
+	if err := writer.Flush(); err != nil {
+		return err
+	}
+
+	return file.Sync()
 }
 
 func (u *uboot) MarkCurrentBootSuccessful() (err error) {
@@ -310,6 +320,13 @@
 func atomicFileUpdate(file string, lines []string) (err error) {
 	tmpFile := fmt.Sprintf("%s.NEW", file)
 
+	// XXX: if go switches to use aio_fsync, we need to open the dir for writing
+	dir, err := os.Open(filepath.Dir(file))
+	if err != nil {
+		return err
+	}
+	defer dir.Close()
+
 	if err := writeLines(lines, tmpFile); err != nil {
 		return err
 	}
@@ -319,7 +336,7 @@
 		return err
 	}
 
-	return nil
+	return dir.Sync()
 }
 
 // Rewrite the specified file, applying the specified set of changes.