Re: gcov coverage data not full with immediate stop

(Strangely, I was just thinking about these branches of mine as I
closed my week last Friday...)

On 2020-May-10, Alexander Lakhin wrote:

So if we want to make the coverage reports more precise, I see the three
ways:
1. Change the stop mode in teardown_node to fast (probably only when
configured with --enable-coverage);
2. Explicitly stop nodes in TAP tests (where it's important) -- seems
too tedious and troublesome;
3. Explicitly call __gcov_flush in SIGQUIT handler (quickdie)?

I tried your idea 3 a long time ago and my experiments didn't show an
increase in coverage [1].  But I like this idea the best, and maybe I
did something wrong.  Attached is the patch I had (on top of
fc115d0f9fc6), but I don't know if it still applies.

Thanks for the reference to that discussion and your patch.
As I see the issue with that patch is that quickdie() is not the only SIGQUIT handler. When a backend is interrupted with SIGQUIT, it's exiting in SignalHandlerForCrashExit().
In fact if I only add __gcov_flush() in SignalHandlerForCrashExit(), it raises test coverage for `make check -C src/test/recovery/` from
106198 lines/6319 functions
to
106420 lines/6328 functions

It's not yet clear to me what happens when __gcov_flush() called inside __gcov_flush().
The test coverage changes to:
108432 lines/5417 functions
(number of function calls decreased)
And for example in coverage/src/backend/utils/cache/catcache.c.gcov.html I see
     147           8 : int2eqfast(Datum a, Datum b)
...
     153           0 : int2hashfast(Datum datum)
but without __gcov_flush in quickdie() we have:
     147       78038 : int2eqfast(Datum a, Datum b)
...
     153      255470 : int2hashfast(Datum datum)
So it needs more investigation.

But I can confirm that calling __gcov_flush() in SignalHandlerForCrashExit() really improves a code coverage report.
I tried to develop a test to elevate a coverage for gist:
https://coverage.postgresql.org/src/backend/access/gist/gistxlog.c.gcov.html
(Please look at the attached test if it could be interesting.)
and came to this issue with a coverage. I tried to play with GCOV_PREFIX, but without luck.
Yesterday I found the more recent discussion:
https://www.postgresql.org/message-id/flat/44ecae53-9861-71b7-1d43-4658acc52519%402ndquadrant.com#d02e2e61212831fbceadf290637913a0
(where probably the same problem came out).

Finally I've managed to get an expected coverage when I performed $node_standby->stop() (but __gcov_flush() in SignalHandlerForCrashExit() helps too).

Best regards,
Alexander