Fix npe in exec resize when exec errored

[cpuguy83]

- What I did

In cases where an exec start failed the exec process will be nil even though the channel to signal that the exec started was closed.

Ideally ExecConfig would get a nice refactor to handle this case better (ie. it's not started so don't close that channel). This is a minimal fix to prevent NPE. Luckilly this would only get called by a client and only the http request goroutine gets the panic (http lib recovers the panic).

- How I did it

Add nil check to exec process

- How to verify it

id="$(docker run -d busybox top)"
docker exec -it ${id} command_doesnt_exist

Watch the daemon logs there will be (or potentially will be) a nil pointer exception.

May 27 15:52:50 dev2 dockerd[1023]: 2023/05/27 15:52:50 http: panic serving @: runtime error: invalid memory address or nil pointer dereference
May 27 15:52:50 dev2 dockerd[1023]: goroutine 297 [running]:
May 27 15:52:50 dev2 dockerd[1023]: net/http.(*conn).serve.func1()
May 27 15:52:50 dev2 dockerd[1023]:         /usr/local/go/src/net/http/server.go:1854 +0xbf
May 27 15:52:50 dev2 dockerd[1023]: panic({0x55769d1eb920, 0x55769e9653d0})
May 27 15:52:50 dev2 dockerd[1023]:         /usr/local/go/src/runtime/panic.go:890 +0x263
May 27 15:52:50 dev2 dockerd[1023]: github.com/docker/docker/daemon.(*Daemon).ContainerExecResize(0x55769d1bd080?, {0xc000d75201?, 0x55769cb48910?}, 0x31, 0x118)
May 27 15:52:50 dev2 dockerd[1023]:         /home/cpuguy83/go/src/github.com/docker/docker/daemon/resize.go:51 +0x159
May 27 15:52:50 dev2 dockerd[1023]: github.com/docker/docker/api/server/router/container.(*containerRouter).postContainerExecResize(0xc0011dfb00, {0x0?, 0xc0008e7410?}, {0x55769af8a3e7?, 0x10?}, 0xc000b95500, 0xc000cd5080?)
May 27 15:52:50 dev2 dockerd[1023]:         /home/cpuguy83/go/src/github.com/docker/docker/api/server/router/container/exec.go:175 +0x14e
May 27 15:52:50 dev2 dockerd[1023]: github.com/docker/docker/api/server/middleware.ExperimentalMiddleware.WrapHandler.func1({0x55769d5c9180, 0xc001102b40}, {0x55769d5c7940?, 0xc001232540?}, 0x55769d0b3e20?, 0xc0019dac70?)
May 27 15:52:50 dev2 dockerd[1023]:         /home/cpuguy83/go/src/github.com/docker/docker/api/server/middleware/experimental.go:26 +0x15b
May 27 15:52:50 dev2 dockerd[1023]: github.com/docker/docker/api/server/middleware.VersionMiddleware.WrapHandler.func1({0x55769d5c9180, 0xc001102a20}, {0x55769d5c7940, 0xc001232540}, 0x55769d3c2dc0?, 0x769af86ded?)
May 27 15:52:50 dev2 dockerd[1023]:         /home/cpuguy83/go/src/github.com/docker/docker/api/server/middleware/version.go:62 +0x4d7
May 27 15:52:50 dev2 dockerd[1023]: github.com/docker/docker/pkg/authorization.(*Middleware).WrapHandler.func1({0x55769d5c9180, 0xc001102a20}, {0x55769d5c7940?, 0xc001232540?}, 0xc000b95500, 0x2?)
May 27 15:52:50 dev2 dockerd[1023]:         /home/cpuguy83/go/src/github.com/docker/docker/pkg/authorization/middleware.go:59 +0x649
May 27 15:52:50 dev2 dockerd[1023]: github.com/docker/docker/api/server/middleware.DebugRequestMiddleware.func1({0x55769d5c9180, 0xc001102a20}, {0x55769d5c7940, 0xc001232540}, 0xc000b95500, 0xc0019dab60?)
May 27 15:52:50 dev2 dockerd[1023]:         /home/cpuguy83/go/src/github.com/docker/docker/api/server/middleware/debug.go:25 +0x653
May 27 15:52:50 dev2 dockerd[1023]: github.com/docker/docker/api/server.(*Server).makeHTTPHandler.func1({0x55769d5c7940, 0xc001232540}, 0xc000b95400)
May 27 15:52:50 dev2 dockerd[1023]:         /home/cpuguy83/go/src/github.com/docker/docker/api/server/server.go:123 +0x1ce
May 27 15:52:50 dev2 dockerd[1023]: net/http.HandlerFunc.ServeHTTP(0xc000b95300?, {0x55769d5c7940?, 0xc001232540?}, 0xc0008e79e8?)
May 27 15:52:50 dev2 dockerd[1023]:         /usr/local/go/src/net/http/server.go:2122 +0x2f
May 27 15:52:50 dev2 dockerd[1023]: github.com/docker/docker/vendor/github.com/gorilla/mux.(*Router).ServeHTTP(0xc000a3d740, {0x55769d5c7940, 0xc001232540}, 0xc000b95200)
May 27 15:52:50 dev2 dockerd[1023]:         /home/cpuguy83/go/src/github.com/docker/docker/vendor/github.com/gorilla/mux/mux.go:210 +0x1cf
May 27 15:52:50 dev2 dockerd[1023]: net/http.serverHandler.ServeHTTP({0xc0010ec5a0?}, {0x55769d5c7940, 0xc001232540}, 0xc000b95200)
May 27 15:52:50 dev2 dockerd[1023]:         /usr/local/go/src/net/http/server.go:2936 +0x316
May 27 15:52:50 dev2 dockerd[1023]: net/http.(*conn).serve(0xc001490990, {0x55769d5c9180, 0xc000dd2b10})
May 27 15:52:50 dev2 dockerd[1023]:         /usr/local/go/src/net/http/server.go:1995 +0x612
May 27 15:52:50 dev2 dockerd[1023]: created by net/http.(*Server).Serve
May 27 15:52:50 dev2 dockerd[1023]:         /usr/local/go/src/net/http/server.go:3089 +0x5ed

- Description for the changelog

Fix nil pointer exception in exec resize when exec command errored

- A picture of a cute animal (not mandatory but encouraged)

[thaJeztah]

LGTM, but left one suggestion

[thaJeztah]

LGTM ☺️