pgsql: Refactor postmaster child process launching

Refactor postmaster child process launching

Introduce new postmaster_child_launch() function that deals with the
differences in EXEC_BACKEND mode.

Refactor the mechanism of passing information from the parent to child
process. Instead of using different command-line arguments when
launching the child process in EXEC_BACKEND mode, pass a
variable-length blob of startup data along with all the global
variables. The contents of that blob depend on the kind of child
process being launched. In !EXEC_BACKEND mode, we use the same blob,
but it's simply inherited from the parent to child process.

Reviewed-by: Tristan Partin, Andres Freund
Discussion: https://www.postgresql.org/message-id/7a59b073-5b5b-151e-7ed3-8b01ff7ce9ef@iki.fi

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/aafc05de1bf5c0324cb5e690c6742118c1ac4af6

Modified Files
--------------
src/backend/postmaster/autovacuum.c | 153 +-------
src/backend/postmaster/auxprocess.c | 53 +--
src/backend/postmaster/bgworker.c | 20 +-
src/backend/postmaster/bgwriter.c | 8 +-
src/backend/postmaster/checkpointer.c | 8 +-
src/backend/postmaster/launch_backend.c | 432 +++++++++++++++++------
src/backend/postmaster/pgarch.c | 8 +-
src/backend/postmaster/postmaster.c | 517 +++++-----------------------
src/backend/postmaster/startup.c | 8 +-
src/backend/postmaster/syslogger.c | 280 +++++++--------
src/backend/postmaster/walsummarizer.c | 8 +-
src/backend/postmaster/walwriter.c | 8 +-
src/backend/replication/logical/slotsync.c | 72 +---
src/backend/replication/walreceiver.c | 8 +-
src/backend/utils/init/globals.c | 1 +
src/include/postmaster/autovacuum.h | 10 +-
src/include/postmaster/auxprocess.h | 4 +-
src/include/postmaster/bgworker_internals.h | 2 +-
src/include/postmaster/bgwriter.h | 4 +-
src/include/postmaster/pgarch.h | 2 +-
src/include/postmaster/postmaster.h | 18 +-
src/include/postmaster/startup.h | 2 +-
src/include/postmaster/syslogger.h | 4 +-
src/include/postmaster/walsummarizer.h | 2 +-
src/include/postmaster/walwriter.h | 2 +-
src/include/replication/slotsync.h | 4 +-
src/include/replication/walreceiver.h | 2 +-
src/tools/pgindent/typedefs.list | 4 +
28 files changed, 656 insertions(+), 988 deletions(-)