I just opened my rails app and was immediately greeted with this error:

PG_ERROR: could not connect to server: No such file or directory Is the server running locally and accepting connections on Unix domain socket "/tmp/.s.PGSQL.5432"?

OK, so LaunchControl failed to start postgreSQL. Let’s start it manually:

/usr/local/Cellar/postgresql/9.0.4/bin/postgres -D /usr/local/var/postgres -r /usr/local/var/postgres/server.log

Now I get a more useful error:

FATAL: could not create shared memory segment: Cannot allocate memory DETAIL: Failed system call was shmget(key=5432001, size=1703936, 03600).

A quick Google search brings up the solution: http://willbryant.net/software/mac_os_x/postgres_initdb_fatal_shared_memory_error_on_leopard (I’m running Lion, of course, and the solution applies to Snow Leopard as well).

I had set shmall and shmax when I installed postgres (and homebrew does this for you, apparently), but sometimes Mac OS X updates will reset these values. In this case, it appeared to be the latest Safari update that was the culprit (bizarre, right?).