WIP: command_tag_format — protocol-level command tag negotiation via _pq_

[NikolayS]

(live hacking session: https://www.youtube.com/watch?v=VKuxQZlvd8E)

Adds a new protocol-level feature _pq_.command_tag_format that allows clients to negotiate richer command completion tags at connection time.

The problem

Every INSERT returns INSERT 0 N — the 0 is a vestigial OID field, hardcoded to zero since PG12 dropped table OIDs. It wastes bytes and confuses users, but changing the wire format breaks old clients.

Solution: protocol negotiation

New clients send _pq_.command_tag_format in the startup packet. Old clients never send it, so they always get legacy format. Zero breakage.

Formats

Format	Output	Example
legacy	INSERT 0 N (default)	INSERT 0 1
verbose	INSERT tablename N	INSERT users 1
fqn	INSERT schema.tablename N	INSERT public.users 1

verbose and fqn also work for UPDATE, DELETE, and MERGE.

Safety guarantees

Scenario	Result
Old client + new server (no _pq_)	INSERT 0 1 — always safe
New client + old server (_pq_ sent)	silently ignored
SET command_tag_format = 'verbose'	ERROR: cannot be changed
options=-ccommand_tag_format=verbose	FATAL: cannot be changed

The GUC is PGC_INTERNAL — only the _pq_ startup packet path can set it. SET, options=-c, ALTER SYSTEM, and postgresql.conf are all blocked.

Implementation (8 files, ~115 insertions)

• cmdtag.h / cmdtag.c — format constants, format-aware BuildQueryCompletionString
• pquery.c — populates relation name from executor for verbose/fqn
• backend_startup.c — _pq_ handler, stores in Port->pq_command_tag_format
• libpq-be.h — new Port field for deferred protocol option
• postinit.c — deferred application via SetConfigOption(PGC_INTERNAL, PGC_S_OVERRIDE)
• guc_parameters.dat / guc_tables.c — GUC registration with GUC_REPORT

NOT changed: fe-exec.c (libpq)

Old protocol is completely untouched. New-protocol-aware clients handle the new format themselves.

Evolution

1. v1: command_tag_omit_oid — simple bool GUC (PGC_USERSET) + libpq patch
2. v2: command_tag_format — enum with 4 modes, _pq_ negotiation, GUC_REPORT
3. v3 (current): protocol-only — PGC_INTERNAL, removed modern mode, reverted libpq, separate Port field + PGC_S_OVERRIDE deferred application

Test results

See comment with full test evidence — 8/8 scenarios pass with both stock PG17 psql and raw _pq_ startup packet test.