Add a nil check before accessing sandbox.osSbox

[sanimej]

Following panic was reported by one of the docker users..

Nov 14 18:43:17 docker-demo9-d dockerd[1732]: panic: runtime error: invalid memory address or nil pointer dereference
Nov 14 18:43:17 docker-demo9-d dockerd[1732]: [signal 0xb code=0x1 addr=0x58 pc=0x8c1b44]
Nov 14 18:43:17 docker-demo9-d dockerd[1732]: goroutine 52749 [running]:
Nov 14 18:43:17 docker-demo9-d dockerd[1732]: panic(0x1a9cb40, 0xc82000a080)
Nov 14 18:43:17 docker-demo9-d dockerd[1732]: /usr/local/go/src/runtime/panic.go:481 +0x3e6
Nov 14 18:43:17 docker-demo9-d dockerd[1732]: github.com/docker/libnetwork.(*sandbox).execFunc(0xc82026c000, 0xc820fae450)
Nov 14 18:43:17 docker-demo9-d dockerd[1732]: /root/rpmbuild/BUILD/docker-engine/vendor/src/github.com/docker/libnetwork/sandbox.go:440 +0x44
Nov 14 18:43:17 docker-demo9-d dockerd[1732]: github.com/docker/libnetwork.(*resolver).ServeDNS(0xc820427a70, 0x7fd91c0d5120, 0xc820e19810, 0xc820294120)
Nov 14 18:43:17 docker-demo9-d dockerd[1732]: /root/rpmbuild/BUILD/docker-engine/vendor/src/github.com/docker/libnetwork/resolver.go:357 +0x665
Nov 14 18:43:17 docker-demo9-d dockerd[1732]: github.com/miekg/dns.(*Server).serve(0xc821492820, 0x7fd9218f7a08, 0xc82158a120, 0x7fd91c0d4378, 0xc820427a70, 0xc8210d7200, 0x26, 0x200, 0xc82015e390, 0xc821b862a0, ...)

Its not clear in what can situation sb.osSbox can be nil for a running container. One possibility is the container going down while the query is still being processed. Added a error message which should help in identifying the specific case.

Signed-off-by: Santhosh Manohar santhosh@docker.com

[aboch]

You are still accessing sb.osSbox outside lock.
That could have been set to nil before sb.osSbox.InvokeFunc(f) gets a chance to run

[sanimej]

that was a typo.. corrected it. thanks.

[aboch]

LGTM

[mavenugo]

LGTM