pgsql: Wait for transactions of an initial decoding snapshot to commit

Wait for transactions of an initial decoding snapshot to commit

SnapBuildInitialSnapshot() converts the snapshot builder's list of
committed transactions into a regular MVCC snapshot, which is then used
with HeapTupleSatisfiesMVCC(). However, a snapshot produced that way
and used as an MVCC snapshot can potentially cause data corruption, if
it captures a transaction after it writes its commit WAL record but
before it has updated its CLOG entry: it will be incorrectly used to set
hint bits as if that transaction had aborted.

Fix by having SnapBuildInitialSnapshot() wait until such transactions
have removed themselves from procarray, which guarantees correct
visibility.

Other uses of SnapBuildBuildSnapshot only use the snapshot as
historical, so they don't need the waits, but add commentary in that
function to alert possible future callers.

While at this, backpatch commit 504fe10d1d26, "Fix misplaced comment in
snapbuild.c." The comment it moved was nearby the place we modify.

This bug was diagnosed by stress-testing of REPACK (CONCURRENTLY) in
branch 19, so add a test case that tickles it using that feature in that
branch and master. However, the bug exists in all branches, and in
branch 18 we can add a test with regular logical decoding using
injection points that tickles it. Unfortunately that test cannot be
backpatched to earlier branches for lack of facilities.

Reported-by: Mihail Nikalayeu <mihailnikalayeu(at)gmail(dot)com>
Author: Antonin Houska <ah(at)cybertec(dot)at>
Author: Rui Zhao <zhaorui126(at)gmail(dot)com>
Backpatch-through: 14
Discussion: https://postgr.es/m/85833.1768840165@localhost
Discussion: https://postgr.es/m/CADzfLwU78as45To9a%3D-Qkr5jEg3tMxc5rUtdKy2MTv4r_SDGng%40mail.gmail.com

Branch
------
REL_17_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/b93df6eacb76fdb89a7bb4c0aac1b2e62814c242

Modified Files
--------------
src/backend/replication/logical/snapbuild.c | 46 +++++++++++++++++++++++++++--
1 file changed, 43 insertions(+), 3 deletions(-)