BUG #19764: ManualBug3 Assert failure in pgstat_setup_shared_refs crashes initdb post-bootstrap subprocess

The following bug has been logged on the website:

Bug reference: 19764
Logged by: Yuelin Wang
Email address: 1217816127(at)qq(dot)com
PostgreSQL version: 19rc1
Operating system: Linux (Ubuntu 24.04, x86_64)
Description:

### What happens?

During `initdb`, the "performing post-bootstrap initialization" subprocess
calls `pgstat_initialize()` (which sets `pgStatLocal.shmem->gc_request_count
= 1` via `StatsShmemInit`), then calls `StartupXLOG()`.
Inside `StartupXLOG()`, `pgstat_restore_stats()` is invoked.
After `pgstat_restore_stats()` completes, `gc_request_count` has been zeroed
out to 0 (verified via debug instrumentation in the working-tree build).
Later, during catalog initialization (`InitPostgres` calls
`table_beginscan_catalog`, which calls `heap_beginscan`, which calls
`pgstat_assoc_relation`), `pgstat_get_entry_ref` is invoked for the first
time in this subprocess.
This triggers `pgstat_setup_shared_refs()`
(src/backend/utils/activity/pgstat_shmem.c), which reads `pgStatSharedRefAge
= pg_atomic_read_u64(&pgStatLocal.shmem->gc_request_count)` and gets 0.
Immediately after, the function asserts `Assert(pgStatSharedRefAge != 0)` at
pgstat_shmem.c:939, which fires and aborts the subprocess, causing initdb to
fail and clean up the data directory.
The violated invariant is: `gc_request_count` must remain at its initialized
value (1) and never fall to 0, since `pgstat_setup_shared_refs` relies on a
non-zero value to guarantee `pgStatSharedRefAge != 0`.
The `own_hash` feature introduced in commit 44539786b8 added `entry_counts`,
`kind_hash_handles`, and `raw_kind_dsa_area` arrays to
`PgStat_ShmemControl`, altering the struct layout and the initialization
sequence in `pgstat_restore_stats`/`pgstat_read_statsfile`.
The debug trace confirms that `gc_request_count` transitions from 1 to 0
precisely during `pgstat_restore_stats()`.
This matches the Entry91 root-cause pattern: a field that must be
initialized to a sentinel non-zero value before use is inadvertently reset
to the uninitialized zero value by a code path that was modified by the same
recent commit.

### To Reproduce

Start a fresh PostgreSQL ASAN instance with initdb:

```sh
TMPDIR=$(mktemp -d /tmp/pg_scan_XXXX)
DATADIR=$TMPDIR/pgdata
PGUSER=$(whoami)

ASAN_OPTIONS=detect_leaks=0:abort_on_error=0:log_path=$TMPDIR/asan \
/data/ylwang/Projects/DirectedDB/latesttarget/postgre/build_asan/install/bin/initdb
\
-D $DATADIR -U $PGUSER --no-locale --encoding=UTF8 2>&1
```

Trigger output:

```
The files belonging to this database system will be owned by user
"yuelinwang".
This user must also own the server process.

The database cluster will be initialized with locale "C".
The default text search configuration will be set to "english".

Data page checksums are enabled.

creating directory .../pgdata ... ok
creating subdirectories ... ok
selecting dynamic shared memory implementation ... posix
selecting default "max_connections" ... 100
selecting default "shared_buffers" ... 128MB
selecting default time zone ... Asia/Singapore
creating configuration files ... ok
running bootstrap script ... ok
performing post-bootstrap initialization ... TRAP: failed
Assert("pgStatSharedRefAge != 0"), File: "pgstat_shmem.c", Line: 939, PID:
3146498
/lib/x86_64-linux-gnu/libasan.so.8(+0x831e0)[...]
.../postgres(ExceptionalCondition+0x165)[...]
.../postgres(+0x14c282a)[...]
.../postgres(pgstat_get_entry_ref+0x33b)[...]
.../postgres(pgstat_prep_pending_entry+0x37)[...]
.../postgres(+0x14b9ee9)[...]
.../postgres(pgstat_assoc_relation+0x1f7)[...]
.../postgres(+0x5a3a24)[...]
.../postgres(heap_beginscan+0x8c0)[...]
.../postgres(+0x6e832b)[...]
.../postgres(table_beginscan_catalog+0x9a)[...]
.../postgres(+0x1963b82)[...]
.../postgres(InitPostgres+0x63a)[...]
.../postgres(PostgresMain+0x553)[...]
.../postgres(main+0x5d5)[...]
Aborted (core dumped)
child process exited with exit code 134
initdb: removing data directory ".../pgdata"
```

The assertion failure prevents initdb from completing and the data directory
is cleaned up automatically.

### Environment

- OS: Ubuntu 24.04.2 LTS, x86_64 (Linux 6.8.0-55-generic)
- PostgreSQL Version: PostgreSQL 20devel (from
build_asan/install/bin/postgres --version)
- Client: initdb, ASAN instrumented build (built from source with
-fsanitize=address)
- Hardware: N/A

### Identity Disclosure

- Full Name: Yuelin
- Affiliation: Individual