Re: Leader backend hang on IPC/ParallelFinish when LWLock held at parallel query start

I wrote:
> Here's a proposed patch along that line. I left the test case from
> ac04aa84a alone, since it works perfectly well to test this way too.

I'd modeled that on the existing recovery code for DSM segment creation
failure, just below. But a look at the code coverage report shows
(unsurprisingly) that that path is never exercised in our regression
tests, so I wondered if it actually works ... and it doesn't work
very well. To test, I lobotomized InitializeParallelDSM to always
force pcxt->nworkers = 0. That results in a bunch of unsurprising
regression test diffs, plus a couple of

+ERROR: could not find key 4 in shm TOC at 0x229f138

which turns out to be the fault of ExecHashJoinReInitializeDSM:
it's not accounting for the possibility that we didn't really
start a parallel hash join.

I'm also not happy about ReinitializeParallelWorkers'

Assert(pcxt->nworkers >= nworkers_to_launch);

The one existing caller manages not to trigger that because it's
careful to reduce its request based on pcxt->nworkers, but it
doesn't seem to me that callers should be expected to have to.

So I end with the attached. There might still be some more issues
that the regression tests don't reach, but I think this is the
best we can do for today.

regards, tom lane