[BUG] Race in online checksums launcher_exit()

Hi hackers,

While using the pg_enable_data_checksums() feature, I found a likely bug, a
race condition in datachecksum_state.c's launcher_exit().

When pg_enable_data_checksums() is called twice before the first launcher
starts, two bg workers are registered (the code expects this). The
redundant launcher exits early, but it's launcher_exit() callback
unconditionally clears the shared launcher_running flag and may call
SetDataChecksumsOff() -- even though it never owned the flag.

This allows a third pg_enable_data_checksums() call to launch another
launcher concurrently with the first (duplicate work, doubled I/O, spurious
warnings). Worse, if the redundant launcher initialized after the winner
transitioned to inprogress-on, its exit handler calls
SetDataChecksumsOff(), silently aborting the enable operation. (I have
not triggered the SetDataChecksumsOff part though calling out ad it can be
a likely scenario based on timing of workers)

Reproduced by firing three calls in quick succession:

psql -c "SELECT pg_enable_data_checksums();" &
psql -c "SELECT pg_enable_data_checksums();" &
sleep 0.5
psql -c "SELECT pg_enable_data_checksums();" &

Log shows two launchers processing databases concurrently:

[2093292]: LOG: processing database "postgres"
[2093293]: LOG: already running, exiting
[2093299]: WARNING: cannot set data checksums to "on", current state is not "inprogress-on"
admitted
[2093292]: LOG: processing database "postgres"
[2093299]: WARNING: cannot set data checksums to "on", current state is not "inprogress-on"
concurrently
[2093299]: WARNING: cannot set data checksums to "on", current state is not "inprogress-on"
not "inprogress-on"

I think the process-local launcher_running flag exists for this purpose and
is already used for the worker-kill block, but the flag-clear and
state-revert blocks do not use it.

The attached patch returns early from launcher_exit() when the local flag
is false. Thoughts?

Regards,
Ayush