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save: Use dd with the notrunc & fsync option

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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.
This commit is contained in:
Jöran Karl
2025-07-22 19:00:22 +02:00
parent 87ce5738e1
commit 165a5a4896
1 changed files with 23 additions and 8 deletions
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31
internal/buffer/save.go
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@@ -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