heimdall

This is an experimental program that wraps an executable program inside of an exclusive lock provided by PostgreSQL's pg_try_advisory_lock.

pg_try_advisory_lock is a sibling of pg_advisory_lock, which is a locking function with two signatures. One accepts a 64-bit integer, and the other accepts 2 32-bit integers. The main difference between pg_advisory_lock and pg_try_advisory_lock is that pg_try_advisory_lock does not wait until a lock becomes available. Instead, pg_try_advisory_lock returns immediately with a t if it successfully acquired the lock, and f if it was unable to acquire it.

One last behavior that is important to note: if the connection to PostgreSQL is severed, any lock acquired via pg_try_advisory_lock is automatically released.

Usage

The heimdall program has four required arguments:

• A database URL (postgres://postgres@localhost/postgres?sslmode=disable)
• A lock namespace
• A lock name
• A command to wrap

An example wrapping the ls command looks like:

$ heimdall \
    --database "postgres://postgres@localhost/postgres?sslmode=disable" \
    --namespace joker \
    --name test \
    ls

Testing

A quick way to test an exclusive lock is to use sleep as the command.Executing heimdall once with sleep 10 and then immediately with another command in another shell should prevent the second command from executing.

Quick

Induce a deep sleep:

$ heimdall \
    --database "postgres://postgres@localhost/postgres?sslmode=disable" \
    --namespace joker \
    --name test \
    sleep 10

Try to get the date:

$ heimdall \
    --database "postgres://postgres@localhost/postgres?sslmode=disable" \
    --namespace joker \
    --name test \
    date

Local

The local tests require a PostgreSQL database connection fed to the test suite as a connection string via the DATABASE_URL environment variable:

$ DATABASE_URL="postgres://postgres@localhost/postgres?sslmode=disable" make test

Attribution

I was made aware of pg_try_advisory_lock through @ryandotsmith's work in lock-smith.