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fix(syncthing): avoid writing panic log to nil fd (#10154)

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### Purpose

This change fixes a logical bug in the panic log writing where we could
end up writing to a uninitialized file descriptor.

On the very first iteration, `panicFd` is nil. We enter the if `panicFd
== nil { … }` block, check for “panic:” or “fatal error:”, and if
neither matches, we skip instantiating `panicFd` altogether. However,
immediately after, still within `if panicFd == nil { … }`, we call
`panicFd.WriteString("Panic at ...")`. But `panicFd` would in this case
be `nil`, which will cause a run‐time panic.

It's not clear to me why panicFd is only initialized if the lines start
with "panic:" or "fatal error:" so I've left that logic untouched. With
this change we at least avoid the risk of writing to a nil
filedescriptor.
## Authorship

Your name and email will be added automatically to the AUTHORS file
based on the commit metadata.

---------

Co-authored-by: Jakob Borg <jakob@kastelo.net>
This commit is contained in:
ardevd
2025-06-03 07:39:21 +02:00
committed by GitHub
parent f4abc71dcc
commit ebbe57d0ab
1 changed files with 23 additions and 25 deletions
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48
cmd/syncthing/monitor.go
View File

@@ -238,19 +238,18 @@ func copyStderr(stderr io.Reader, dst io.Writer) {
return
}
if panicFd == nil {
dst.Write([]byte(line))
dst.Write([]byte(line))
if strings.HasPrefix(line, "panic:") || strings.HasPrefix(line, "fatal error:") {
panicFd, err = os.Create(locations.GetTimestamped(locations.PanicLog))
if err != nil {
l.Warnln("Create panic log:", err)
continue
}
if panicFd == nil && (strings.HasPrefix(line, "panic:") || strings.HasPrefix(line, "fatal error:")) {
panicFd, err = os.Create(locations.GetTimestamped(locations.PanicLog))
if err != nil {
l.Warnln("Create panic log:", err)
continue
}
l.Warnf("Panic detected, writing to \"%s\"", panicFd.Name())
if strings.Contains(line, "leveldb") && strings.Contains(line, "corrupt") {
l.Warnln(`
l.Warnf("Panic detected, writing to \"%s\"", panicFd.Name())
if strings.Contains(line, "leveldb") && strings.Contains(line, "corrupt") {
l.Warnln(`
*********************************************************************************
* Crash due to corrupt database. *
* *
@@ -263,22 +262,21 @@ func copyStderr(stderr io.Reader, dst io.Writer) {
* https://docs.syncthing.net/users/faq.html#my-syncthing-database-is-corrupt *
*********************************************************************************
`)
} else {
l.Warnln("Please check for existing issues with similar panic message at https://github.com/syncthing/syncthing/issues/")
l.Warnln("If no issue with similar panic message exists, please create a new issue with the panic log attached")
}
stdoutMut.Lock()
for _, line := range stdoutFirstLines {
panicFd.WriteString(line)
}
panicFd.WriteString("...\n")
for _, line := range stdoutLastLines {
panicFd.WriteString(line)
}
stdoutMut.Unlock()
} else {
l.Warnln("Please check for existing issues with similar panic message at https://github.com/syncthing/syncthing/issues/")
l.Warnln("If no issue with similar panic message exists, please create a new issue with the panic log attached")
}
stdoutMut.Lock()
for _, line := range stdoutFirstLines {
panicFd.WriteString(line)
}
panicFd.WriteString("...\n")
for _, line := range stdoutLastLines {
panicFd.WriteString(line)
}
stdoutMut.Unlock()
panicFd.WriteString("Panic at " + time.Now().Format(time.RFC3339) + "\n")
}