Docker Signal Tester

Docker does not proxy through signals to running containers in tty mode.

Setup

$ docker build -t docker-signal-test .

Test 1: docker run (no -t) with kill

Shell 1:

$ docker run --rm=true docker-signal-test ./signal_printer.sh
Waiting for a signal...

Shell 2:

$ docker ps
CONTAINER ID        IMAGE                       COMMAND                CREATED             STATUS              PORTS               NAMES
9650d1f6535d        docker-signal-test:latest   "./signal_printer.sh   31 seconds ago      Up 30 seconds                           romantic_pasteur    
$ ps | grep 'docker run'
45470 ttys003    0:00.17 docker run --rm=true docker-signal-test ./signal_printer.sh
$ kill -s TERM 45470

Shell 1 outputs...

Waiting for a signal...
Received SIGTERM
Exiting...
$

👍

Test 2: docker run -t with docker kill

Shell 1:

$ docker run -t --rm=true docker-signal-test ./signal_printer.sh
Waiting for a signal...

Shell 2:

$ docker ps
CONTAINER ID        IMAGE                       COMMAND                CREATED             STATUS              PORTS               NAMES
e8f5c658052b        docker-signal-test:latest   "./signal_printer.sh   41 seconds ago      Up 40 seconds                           romantic_shockley   
$ docker kill -s SIGTERM e8f5c658052b

Shell 1 outputs...

Waiting for a signal...
Received SIGTERM
Exiting...
$

👍

Test 3: docker run -t with kill

Shell 1:

$ docker run -t --rm=true docker-signal-test ./signal_printer.sh
Waiting for a signal...

Shell 2:

$ docker ps
CONTAINER ID        IMAGE                       COMMAND                CREATED              STATUS              PORTS               NAMES
17ccb9eeeb36        docker-signal-test:latest   "./signal_printer.sh   About a minute ago   Up About a minute                       sick_banach         
$ ps | grep 'docker run'
44420 ttys003    0:00.17 docker run -t --rm=true docker-signal-test ./signal_printer.sh
$ kill -s TERM 44420

Shell 1 outputs...

Waiting for a signal...
$

Shell 2 shows Docker is still running:

$ docker ps
CONTAINER ID        IMAGE                       COMMAND                CREATED              STATUS              PORTS               NAMES
17ccb9eeeb36        docker-signal-test:latest   "./signal_printer.sh   About a minute ago   Up About a minute                       sick_banach         

👎

This is no surprise really, because docker run --help says this:

  --sig-proxy=true           Proxy received signals to the process (non-TTY mode only). SIGCHLD, SIGSTOP, and SIGKILL are not proxied.

Test 4: docker-compose run with kill

Shell 1:

$ docker-compose run --rm test ./signal_printer.sh
Waiting for a signal...

Shell 2:

$ ps | grep 'docker-compose'
58623 ttys002    0:00.05 docker-compose run --rm test ./signal_printer.sh
58624 ttys002    0:00.21 docker-compose run --rm test ./signal_printer.sh
$ kill -s TERM 58623
(no output)

Shell 1:

Waiting for a signal...
Terminated: 15
$ docker ps
CONTAINER ID        IMAGE                          COMMAND                CREATED              STATUS              PORTS               NAMES
8e8c8fd3fdb7        dockersignaltest_test:latest   "./signal_printer.sh   About a minute ago   Up About a minute                       dockersignaltest_test_run_11   
$ docker logs 8e8c8fd3fdb7
Waiting for a signal...
Waiting for a signal...
$ docker kill -s TERM 8e8c8fd3fdb7
8e8c8fd3fdb7
$ docker logs 8e8c8fd3fdb7
Waiting for a signal...
Waiting for a signal...
Received SIGTERM
Exiting...

👎

Test 5: docker-compose run -T with kill

Shell 1:

$ docker-compose run -T --rm test ./signal_printer.sh
(no output)

Shell 2:

$ docker ps
CONTAINER ID        IMAGE                          COMMAND                CREATED             STATUS              PORTS               NAMES
37397bad0c61        dockersignaltest_test:latest   "./signal_printer.sh   4 minutes ago       Up 4 seconds                            dockersignaltest_test_run_8   
$ docker logs 37397bad0c61
Waiting for a signal...
Waiting for a signal...
$ ps | grep docker
55273 ttys002    0:00.06 docker-compose run -T test ./signal_printer.sh
55274 ttys002    0:00.24 docker-compose run -T test ./signal_printer.sh
$ kill -s TERM 55273

Shell 1:

$ docker-compose run -T test ./signal_printer.sh
Terminated: 15

Shell 2:

$ docker ps
CONTAINER ID        IMAGE                          COMMAND                CREATED             STATUS              PORTS               NAMES
37397bad0c61        dockersignaltest_test:latest   "./signal_printer.sh   7 minutes ago       Up 3 minutes                            dockersignaltest_test_run_8
$ docker logs 37397bad0c61
Waiting for a signal...
Waiting for a signal...
$ ps | grep docker
(no output)
$ docker kill -s SIGTERM 37397bad0c61
37397bad0c61
$ docker logs 37397bad0c61
Waiting for a signal...
Waiting for a signal...
Received SIGTERM
Exiting...

👎

In Summary

With docker run you can only send signals to the docker process (and have them passed to the inner process) if the container is running in non-tty mode (-T with docker-compose run, or without the -t for docker run). If you run in tty mode you need to send signals using docker kill -s SIGNAME.

With docker-compose you can't send any process signals to it directly, as it doesn't proxy them (whether started -T or not). Also when it does receive a TERM signal it echoes Terminated and terminates, but it fails to shut down the running Docker container. The only way to kill the running container is to run docker kill.