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#!/bin/bash |
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# Lockfiles are a feature in Linux which allows multiple processes to use a file as a mutex. |
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# This script contains example code based on this blog post, but with a bugfix: https://dmorgan.info/posts/linux-lock-files/ |
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# Wrapper function for easy reuse. |
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# I've heavily annotated it, but without the comments or arguments it's only 13 lines long. |
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# Parameters (you can hard-code them instead if you want) |
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# - LOCK_FILE: Which file to use for the lock. |
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# Needs to be a writable location and unique to the "thing" that needs to be locked on. |
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# The file doesn't need to exist yet. |
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# - LOCK_TIMEOUT: (Optional) How many SECONDS to wait before aborting |
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# ...remaining args: Command to run |
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function lock_cmd { |
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LOCK_FILE="$1"; shift |
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LOCK_TIMEOUT="$1"; shift; |
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( |
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# Remove the lockfile automatically when the function finishes |
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trap "rm -f $LOCK_FILE" 0 |
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# "flock", or "file lock", takes a file descriptor ID as a parameter instead of a filename. |
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# That means you either have to open a file yourself with a specific handle (Here, I've arbitrarily used 200) |
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# or let the library handle it for you and somehow get the ID of the opened file. |
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flock -x -w $LOCK_TIMEOUT 200 |
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RETVAL=$? |
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# "flock" returns 1 if the timeout has occurred. |
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if [ $RETVAL -ne 0 ]; then |
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echo -e "Failed to aquire lock on $LOCK_FILE within $LOCK_TIMEOUT seconds. Is a similar script hung?" |
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exit $RETVAL |
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fi |
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# "flock" can run a command automatically with the "-c" flag, but if the command fails then it returns the command's exit code. |
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# That makes it impossible between the command failing and hitting the timeout. |
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echo -e "Running command: $@" |
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$@ |
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) 200>"$LOCK_FILE" |
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} |
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# Example usage: |
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# The motivation for this demo was that "docker network create" contains a race condition |
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# which allows multiple networks to be created with the same name if run twice simultaneously |
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# (https://github.com/moby/moby/issues/20648) |
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netname=my_network |
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lock_file=/tmp/docker-network-create-lock |
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lock_timeout=5 |
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function assert_one { |
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docker network ls --filter 'name=foo' --format '{{.ID}}' |
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if [[ $(docker network ls --filter "name=$1" --format '{{.ID}}' | wc -l) -eq 1 ]]; then |
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echo -e "\033[1;32m:) Pass\033[0m" |
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else |
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echo -e "\033[1;34m:( Fail\033[0m" |
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exit 1 |
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fi |
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} |
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function cleanup { |
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docker network ls --filter "name=$1" --format '{{.ID}}' | xargs --no-run-if-empty -n1 docker network rm |
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} |
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cleanup $netname |
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echo -e "Testing with timeout $lock_timeout" |
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lock_cmd $lock_file $lock_timeout docker network create $netname & |
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first_pid=$! |
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lock_cmd $lock_file $lock_timeout docker network create $netname & |
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second_pid=$! |
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tail --pid $first_pid -f /dev/null # Wait for processes to die |
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tail --pid $second_pid -f /dev/null |
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echo -e "There should be only one network:" |
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assert_one $netname |
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cleanup $netname |
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# The `flock` command has an optional timeout parameter. If we decrease it to zero, we see what happens when a command takes too long |
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echo -e "Testing with timeout 0" |
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lock_cmd $lock_file 0 docker network create $netname & |
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first_pid=$! |
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lock_cmd $lock_file 0 docker network create $netname & |
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second_pid=$! |
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tail --pid $first_pid -f /dev/null |
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tail --pid $second_pid -f /dev/null |
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assert_one $netname |
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cleanup $netname |
karlrwjohnson revised this gist