pgsql: Assign "backend" type earlier during process start-up

Assign "backend" type earlier during process start-up

Instead of assigning the backend type in the Main function of each
postmaster child, do it right after fork(), by which time it is already
known by postmaster_child_launch(). This reduces the time frame during
which MyBackendType is incorrect.

Before this commit, ProcessStartupPacket would overwrite MyBackendType
to B_BACKEND for dead-end backends, which is quite dubious. Stop that.

We may now see MyBackendType == B_BG_WORKER before setting up
MyBgworkerEntry. As far as I can see this is only a problem if we try
to log a message and %b is in log_line_prefix, so we now have a constant
string to cover that case. Previously, it would print "unrecognized",
which seems strictly worse.

Author: Euler Taveira <euler(at)eulerto(dot)com>
Discussion: https://postgr.es/m/e85c6671-1600-4112-8887-f97a8a5d07b2@app.fastmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0c8e082fba8d36434552d3d7800abda54acafd57

Modified Files
--------------
src/backend/postmaster/autovacuum.c | 2 --
src/backend/postmaster/bgworker.c | 1 -
src/backend/postmaster/bgwriter.c | 1 -
src/backend/postmaster/checkpointer.c | 1 -
src/backend/postmaster/launch_backend.c | 3 +++
src/backend/postmaster/pgarch.c | 1 -
src/backend/postmaster/startup.c | 1 -
src/backend/postmaster/syslogger.c | 1 -
src/backend/postmaster/walsummarizer.c | 1 -
src/backend/postmaster/walwriter.c | 1 -
src/backend/replication/logical/slotsync.c | 2 --
src/backend/replication/walreceiver.c | 1 -
src/backend/storage/aio/method_worker.c | 1 -
src/backend/tcop/backend_startup.c | 3 +--
src/backend/utils/error/elog.c | 7 ++++++-
15 files changed, 10 insertions(+), 17 deletions(-)