pgsql: Move named LWLock tranche requests to shared memory.

Move named LWLock tranche requests to shared memory.

In EXEC_BACKEND builds, GetNamedLWLockTranche() can segfault when
called outside of the postmaster process, as it might access
NamedLWLockTrancheRequestArray, which won't be initialized. Given
the lack of reports, this is apparently unusual, presumably because
it is usually called from a shmem_startup_hook like this:

mystruct = ShmemInitStruct(..., &found);
if (!found)
{
mystruct->locks = GetNamedLWLockTranche(...);
...
}

This genre of shmem_startup_hook evades the aforementioned
segfaults because the struct is initialized in the postmaster, so
all other callers skip the !found path.

We considered modifying the documentation or requiring
GetNamedLWLockTranche() to be called from the postmaster, but
ultimately we decided to simply move the request array to shared
memory (and add it to the BackendParameters struct), thereby
allowing calls outside postmaster on all platforms. Since the main
shared memory segment is initialized after accepting LWLock tranche
requests, postmaster builds the request array in local memory first
and then copies it to shared memory later.

Given the lack of reports, back-patching seems unnecessary.

Reported-by: Sami Imseih <samimseih(at)gmail(dot)com>
Reviewed-by: Sami Imseih <samimseih(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAA5RZ0v1_15QPg5Sqd2Qz5rh_qcsyCeHHmRDY89xVHcy2yt5BQ%40mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/ed1aad15e09d7d523f4ef413e3c4d410497c8065

Modified Files
--------------
src/backend/postmaster/launch_backend.c | 3 +++
src/backend/storage/lmgr/lwlock.c | 31 ++++++++++++++++++++++++++-----
src/include/storage/lwlock.h | 4 ++++
3 files changed, 33 insertions(+), 5 deletions(-)