save: Use dd with the notrunc & fsync option

Using notrunc  will stop the overall truncation of the target file done by sudo.
We need to do this because dd, like other coreutils, already truncates the file
on open(). In case we can't store the backup file afterwards we would end up in
a truncated file for which the user has no write permission by default.
Instead we use a second call of `dd` to perform the necessary truncation
on the command line.
With the fsync option we force the dd process to synchronize the written file
to the underlying device.

Author: JoeKar

internal/buffer/save.go

@@ -26,6 +26,7 @@ import (
 const LargeFileThreshold = 50000
 
 type wrappedFile struct {
+	name        string
 	writeCloser io.WriteCloser
 	withSudo    bool
 	screenb     bool
@@ -83,7 +84,13 @@ func openFile(name string, withSudo bool) (wrappedFile, error) {
 	var sigChan chan os.Signal
 
 	if withSudo {
-		cmd = exec.Command(config.GlobalSettings["sucmd"].(string), "dd", "bs=4k", "of="+name)
+		conv := "notrunc"
+		// TODO: both platforms do not support dd with conv=fsync yet
+		if !(runtime.GOOS == "illumos" || runtime.GOOS == "netbsd") {
+			conv += ",fsync"
+		}
+
+		cmd = exec.Command(config.GlobalSettings["sucmd"].(string), "dd", "bs=4k", "conv="+conv, "of="+name)
 		writeCloser, err = cmd.StdinPipe()
 		if err != nil {
 			return wrappedFile{}, err
@@ -113,7 +120,18 @@ func openFile(name string, withSudo bool) (wrappedFile, error) {
 		}
 	}
 
-	return wrappedFile{writeCloser, withSudo, screenb, cmd, sigChan}, nil
+	return wrappedFile{name, writeCloser, withSudo, screenb, cmd, sigChan}, nil
+}
+
+func (wf wrappedFile) Truncate() error {
+	if wf.withSudo {
+		// we don't need to stop the screen here, since it is still stopped
+		// by openFile()
+		// truncate might not be available on every platfom, so use dd instead
+		cmd := exec.Command(config.GlobalSettings["sucmd"].(string), "dd", "count=0", "of="+wf.name)
+		return cmd.Run()
+	}
+	return wf.writeCloser.(*os.File).Truncate(0)
 }
 
 func (wf wrappedFile) Write(b *Buffer) (int, error) {
@@ -134,12 +152,9 @@ func (wf wrappedFile) Write(b *Buffer) (int, error) {
 		eol = []byte{'\n'}
 	}
 
-	if !wf.withSudo {
-		f := wf.writeCloser.(*os.File)
-		err := f.Truncate(0)
-		if err != nil {
-			return 0, err
-		}
+	err := wf.Truncate()
+	if err != nil {
+		return 0, err
 	}
 
 	// write lines