From 80cf87d1f02acd000b409ccfb5277cffb61959dd Mon Sep 17 00:00:00 2001
From: Sebastiaan van Stijn <github@gone.nl>
Date: Fri, 29 Jan 2021 14:55:08 +0100
Subject: [PATCH] fifo.Close(): prevent possible panic if fifo is nil

I'm not sure if this is the right approach, and synchronisation should probably
be added elsewhere to fix the underlying issue.

Trying to prevent a panic that was seen on container restore in th docker daemon:

    panic: runtime error: invalid memory address or nil pointer dereference
    [signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0x5586c892a7a4]

    goroutine 420 [running]:
    github.com/docker/docker/vendor/github.com/containerd/fifo.(*fifo).Close(0x0, 0x0, 0x0)
            /go/src/github.com/docker/docker/vendor/github.com/containerd/fifo/fifo.go:208 +0x44
    github.com/docker/docker/vendor/github.com/containerd/containerd/cio.(*cio).Close(0xc000d06f60, 0x5586cb5654d0, 0xc000d8e9e8)
            /go/src/github.com/docker/docker/vendor/github.com/containerd/containerd/cio/io.go:203 +0x90
    github.com/docker/docker/libcontainerd/remote.(*client).Restore.func1(0xc0008bf820, 0xc0008a2040)
            /go/src/github.com/docker/docker/libcontainerd/remote/client.go:86 +0x5a
    github.com/docker/docker/libcontainerd/remote.(*client).Restore(0xc00098e5b0, 0x5586cb61c7c0, 0xc000052088, 0xc0011b6500, 0x40, 0xc0008bf810, 0x5586cb05cf00, 0xffffffffffffffff, 0x0, 0x0, ...)
            /go/src/github.com/docker/docker/libcontainerd/remote/client.go:107 +0x923
    github.com/docker/docker/daemon.(*Daemon).restore.func3(0xc00079d9e0, 0xc000a38230, 0xc00000c1e0, 0xc00079d9a8, 0xc000d84f00, 0xc000d84ed0, 0xc000d84ea0, 0xc00128a280)
            /go/src/github.com/docker/docker/daemon/daemon.go:351 +0x48a
    created by github.com/docker/docker/daemon.(*Daemon).restore
            /go/src/github.com/docker/docker/daemon/daemon.go:319 +0x4b3

If the fifo is nil, there's nothing to be done in Close(), so returning early
in that situation.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
---
 fifo.go | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fifo.go b/fifo.go
index 96c214f..c3eec29 100644
--- a/fifo.go
+++ b/fifo.go
@@ -204,6 +204,10 @@ func (f *fifo) Write(b []byte) (int, error) {
 // before open(2) has returned and fifo was never opened.
 func (f *fifo) Close() (retErr error) {
 	for {
+		if f == nil {
+			return
+		}
+
 		select {
 		case <-f.closed:
 			f.handle.Close()