Stop the server gracefully when SIGTERM is sent (as docker stop does)

`docker stop` sends the `SIGTERM` signal to the process to stop it, but the
Minecraft server process handles this by quitting immediately, not saving the
world first. This differs from when the `stop` Minecraft command is used, as
that will save the world first before quitting.

This change adds a script (`start.sh`) which catches the `SIGTERM` signal and
handles it by sending the `stop` command to the Minecraft server. As a bonus,
`sendcommand` is now able to send commands to the server directly (by using a
named pipe) rather than using RCON.

Dockerfile

@@ -10,12 +10,14 @@ RUN mkdir /mc
 
 ADD https://s3.amazonaws.com/Minecraft.Download/versions/${minecraft_version}/minecraft_server.${minecraft_version}.jar /mc/minecraft_server.jar
 ADD "https://search.maven.org/remotecontent?filepath=com/github/t9t/minecraft-rcon-client/minecraft-rcon-client/${rcon_client_version}/minecraft-rcon-client-${rcon_client_version}.jar" /mc/minecraft-rcon-client.jar
-ADD sendcommand.sh /mc/sendcommand.sh
-RUN ln -s /mc/sendcommand.sh /usr/local/bin/sendcommand
 
-WORKDIR /world
+ADD rcon.sh sendcommand.sh start.sh /mc/
 
-EXPOSE 25565
+RUN ln -s /mc/sendcommand.sh /usr/local/bin/sendcommand
+RUN ln -s /mc/rcon.sh /usr/local/bin/rcon
 
-CMD java -Xms${JAVA_MIN_MEMORY} -Xmx${JAVA_MAX_MEMORY} -jar /mc/minecraft_server.jar nogui
+WORKDIR /world
 VOLUME /world
+EXPOSE 25565 25575
+
+CMD /mc/start.sh

README.md

@@ -6,28 +6,55 @@
 - Source: https://github.com/t9t/minecraft-server-docker-image
 - Report issues: https://github.com/t9t/minecraft-server-docker-image/issues
 
-This image uses a volume to store the Minecraft world data, which also contains configuration files such as `server.properties`, `banned-ips.json`, `banned-players.json`, `ops.json`, and `whitelist.json`.
+This image uses a volume to store the Minecraft world data, which also contains configuration files such as
+`server.properties`, `banned-ips.json`, `banned-players.json`, `ops.json`, and `whitelist.json`.
 
 # Start an instance
 To start an instance, use something like the following:
 
 ```
-docker run -d --name minecraft-server -v /var/lib/minecraft/world/:/world -p 25565:25565 t9t9t/minecraft-server
+docker run \
+    --init --detach \
+    --name=minecraft-server \
+    --publish=25565:25565 \
+    --restart=on-failure:5 \
+    --stop-timeout=30 \
+    --volume=/var/lib/minecraft/world/:/world \
+    t9t9t/minecraft-server
 ```
 
-
 This will use the directory `/var/lib/minecraft/world` as the world directory.
 
-If it does not contain a valid `eula.txt` yet, the server will be shut down immediately. Make sure you agree to the [EULA](https://account.mojang.com/documents/minecraft_eula), then add `eula=true` to `eula.txt`.
+If it does not contain a valid `eula.txt` yet, the server will be shut down immediately. Make sure you agree to the
+[EULA](https://account.mojang.com/documents/minecraft_eula), then add `eula=true` to `eula.txt`.
+
+If it does not contain a `server.properties`, the server will create a default one. It is recommended that you tweak all
+settings though.
 
-If it does not contain a `server.properties`, the server will create a default one. It is recommended that you tweak all settings though.
+You can configure the Java minimum and maximum memory usage using the environment variables `JAVA_MIN_MEMORY` and
+`JAVA_MAX_MEMORY` environment variables respectively. The defaults are `128M` and `2048M` respectively.
 
 # Sending commands
 
-To send commands to the server, first ensure that the RCON console is enabled (by adding `enable-rcon=true` to `server.properties` and setting `rcon.password` to some password). Then use the following command:
+To send commands to the server, use the `sendcommand` script:
+
+```
+docker exec minecraft-server sendcommand 'say Teleporting skankhunt42 to (0,200,0)'
+docker exec minecraft-server sendcommand teleport skankhunt42 0 200 0
+```
+
+Note that any output will be sent to the standard output of the Minecraft server, which will end up in the docker logs
+and the Minecraft logs in `logs` directory.
+
+Alternatively, you may enable RCON (by adding `enable-rcon=true` to `server.properties` and setting `rcon.password`
+to some password), and use the `rcon` command:
 
 ```
 docker exec minecraft-server sendcommand rconpassword 'say Teleporting skankhunt42 to (0,200,0)' 'teleport skankhunt42 0 200 0'
 ```
 
-This assumes that the RCON port is set to the default of `25575` (you may of course expose it using any other port by using the `-p` option of `docker run`).
+Quotes are mandatory in this case, because the `rcon` can accept multiple commands. This will return the output to the
+standard output of `docker exec`.
+
+This assumes that the RCON port is set to the default of `25575` (you may of course expose it using any other port by
+using the `-p` option of `docker run`).

rcon.sh

@@ -0,0 +1,8 @@
+#!/bin/sh
+
+if [ $# -lt 2 ]; then
+  echo "Usage: rcon <password> <commands>"
+  exit 1
+fi
+
+java -jar /mc/minecraft-rcon-client.jar 127.0.0.1:25575 "$@"

sendcommand.sh

@@ -1,8 +1,8 @@
 #!/bin/sh
 
-if [ $# -lt 2 ]; then
-  echo "Usage: sendcommand <password> <commands>"
+if [ $# -lt 1 ]; then
+  echo "Usage: sendcommand <command>"
   exit 1
 fi
 
-java -jar /mc/minecraft-rcon-client.jar 127.0.0.1:25575 "$@"
+echo $* > /minecraft_stdin

start.sh

@@ -0,0 +1,26 @@
+#!/bin/sh
+
+_stop_server() {
+    echo Received SIGTERM, stopping
+    echo stop > /minecraft_stdin
+    wait ${JAVA_PID}
+    exit
+}
+
+mkfifo /minecraft_stdin
+trap "_stop_server" TERM
+
+cd /world
+
+java -Xms${JAVA_MIN_MEMORY} -Xmx${JAVA_MAX_MEMORY} -jar /mc/minecraft_server.jar nogui < /minecraft_stdin &
+JAVA_PID=$!
+
+# See: https://unix.stackexchange.com/questions/72962/getting-stdin-from-a-named-pipe
+exec 3> /minecraft_stdin
+
+# See: https://veithen.github.io/2014/11/16/sigterm-propagation.html
+wait ${JAVA_PID}
+trap - TERM INT
+wait ${JAVA_PID}
+
+exec 3>&-