While back-patching e0e569e1, I noted that there were some other
places where we ought to be applying DH_free(); namely, where we
load some DH parameters from a file and then reject them as not
being sufficiently secure.  While it seems really unlikely that
anybody would hit these code paths in production, let alone do
so repeatedly, let's fix it for consistency.

Back-patch to v10 where this code was introduced.

Discussion: https://postgr.es/m/16160-18367e56e9a28264@postgresql.org

RestoreGUCState applied InitializeOneGUCOption to already-live
GUC entries, causing any malloc'd subsidiary data to be forgotten.
We do want the effect of resetting the GUC to its compiled-in
default, and InitializeOneGUCOption seems like the best way to do
that, so add code to free any existing subsidiary data beforehand.

The interaction between can_skip_gucvar, SerializeGUCState, and
RestoreGUCState is way more subtle than their opaque comments
would suggest to an unwary reader.  Rewrite and enlarge the
comments to try to make it clearer what's happening.

Remove a long-obsolete assertion in read_nondefault_variables: the
behavior of set_config_option hasn't depended on IsInitProcessingMode
since f5d9698a installed a better way of controlling it.

Although this is fixing a clear memory leak, the leak is quite unlikely
to involve any large amount of data, and it can only happen once in the
lifetime of a worker process.  So it seems unnecessary to take any
risk of back-patching.

Discussion: https://postgr.es/m/4105247.1616174862@sss.pgh.pa.us

Since commit 2ce439f3 we have opened every file in the data directory
and called fsync() at the start of crash recovery.  This can be very
slow if there are many files, leading to field complaints of systems
taking minutes or even hours to begin crash recovery.

Provide an alternative method, for Linux only, where we call syncfs() on
every possibly different filesystem under the data directory.  This is
equivalent, but avoids faulting in potentially many inodes from
potentially slow storage.

The new mode comes with some caveats, described in the documentation, so
the default value for the new setting is "fsync", preserving the older
behavior.
Reported-by: Michael Brown <michael.brown@discourse.org>
Reviewed-by: Fujii Masao <masao.fujii@oss.nttdata.com>
Reviewed-by: Paul Guo <guopa@vmware.com>
Reviewed-by: Bruce Momjian <bruce@momjian.us>
Reviewed-by: Justin Pryzby <pryzby@telsasoft.com>
Reviewed-by: David Steele <david@pgmasters.net>
Discussion: https://postgr.es/m/11bc2bb7-ecb5-3ad0-b39f-df632734cd81%40discourse.org
Discussion: https://postgr.es/m/CAEET0ZHGnbXmi8yF3ywsDZvb3m9CbdsGZgfTXscQ6agcbzcZAw%40mail.gmail.com

The function added in be45be9c is comparing integer lists (IntList) by
length and contents, but there were two bugs.  Firstly, it used intVal()
to extract the value, but that's for Value nodes, not for extracting int
values from IntList.  Secondly, it called it directly on the ListCell,
without doing lfirst().  So just do lfirst_int() instead.

Interestingly enough, this did not cause any crashes on the buildfarm,
but valgrind rightfully complained about it.

Discussion: https://postgr.es/m/bf3805a8-d7d1-ae61-fece-761b7ff41ecc@postgresfriends.org

Introduced by commit bbe0a81d.

Per buildfarm member prion.

There is now a per-column COMPRESSION option which can be set to pglz
(the default, and the only option in up until now) or lz4. Or, if you
like, you can set the new default_toast_compression GUC to lz4, and
then that will be the default for new table columns for which no value
is specified. We don't have lz4 support in the PostgreSQL code, so
to use lz4 compression, PostgreSQL must be built --with-lz4.

In general, TOAST compression means compression of individual column
values, not the whole tuple, and those values can either be compressed
inline within the tuple or compressed and then stored externally in
the TOAST table, so those properties also apply to this feature.

Prior to this commit, a TOAST pointer has two unused bits as part of
the va_extsize field, and a compessed datum has two unused bits as
part of the va_rawsize field. These bits are unused because the length
of a varlena is limited to 1GB; we now use them to indicate the
compression type that was used. This means we only have bit space for
2 more built-in compresison types, but we could work around that
problem, if necessary, by introducing a new vartag_external value for
any further types we end up wanting to add. Hopefully, it won't be
too important to offer a wide selection of algorithms here, since
each one we add not only takes more coding but also adds a build
dependency for every packager. Nevertheless, it seems worth doing
at least this much, because LZ4 gets better compression than PGLZ
with less CPU usage.

It's possible for LZ4-compressed datums to leak into composite type
values stored on disk, just as it is for PGLZ. It's also possible for
LZ4-compressed attributes to be copied into a different table via SQL
commands such as CREATE TABLE AS or INSERT .. SELECT.  It would be
expensive to force such values to be decompressed, so PostgreSQL has
never done so. For the same reasons, we also don't force recompression
of already-compressed values even if the target table prefers a
different compression method than was used for the source data.  These
architectural decisions are perhaps arguable but revisiting them is
well beyond the scope of what seemed possible to do as part of this
project.  However, it's relatively cheap to recompress as part of
VACUUM FULL or CLUSTER, so this commit adjusts those commands to do
so, if the configured compression method of the table happens not to
match what was used for some column value stored therein.

Dilip Kumar. The original patches on which this work was based were
written by Ildus Kurbangaliev, and those were patches were based on
even earlier work by Nikita Glukhov, but the design has since changed
very substantially, since allow a potentially large number of
compression methods that could be added and dropped on a running
system proved too problematic given some of the architectural issues
mentioned above; the choice of which specific compression method to
add first is now different; and a lot of the code has been heavily
refactored.  More recently, Justin Przyby helped quite a bit with
testing and reviewing and this version also includes some code
contributions from him. Other design input and review from Tomas
Vondra, Álvaro Herrera, Andres Freund, Oleg Bartunov, Alexander
Korotkov, and me.

Discussion: http://postgr.es/m/20170907194236.4cefce96%40wp.localdomain
Discussion: http://postgr.es/m/CAFiTN-uUpX3ck%3DK0mLEk-G_kUQY%3DSNOTeqdaNRR9FMdQrHKebw%40mail.gmail.com

The TAP test was written so that it was not waiting for the correct SQL
command, but for output from the preceding one.  This resulted in race
conditions, allowing the commands to run in a different order, not block
as expected and so on.  This fixes it by inverting the order of commands
where possible, so that observing the output guarantees the data was
inserted properly, and waiting for a lock to appear in pg_locks.

Discussion: https://postgr.es/m/CAH503wDKdYzyq7U-QJqGn%3DGm6XmoK%2B6_6xTJ-Yn5WSvoHLY1Ww%40mail.gmail.com

Buildfarm member fairywren doesn't like the test case I added
in commit 081876d7.  I'm guessing the reason is that I shouldn't
be using a perl2host-ified path in the tar command line.

Commit 86c23a6e changed the option to specify that postgres will
stop all other server processes by sending the signal SIGSTOP,
from -s to -T. But previously there were comments incorrectly
explaining that SIGSTOP behavior is set by -s option. This commit
fixes them.

Author: Kyotaro Horiguchi
Reviewed-by: Fujii Masao
Discussion: https://postgr.es/m/20210316.165141.1400441966284654043.horikyota.ntt@gmail.com

We leaked the error report from PQconninfoParse, when there was
one.  It seems unlikely that real usage patterns would repeat
the failure often enough to create serious bloat, but let's
back-patch anyway to keep the code similar in all branches.

Found via valgrind testing.
Back-patch to v10 where this code was added.

Discussion: https://postgr.es/m/3816764.1616104288@sss.pgh.pa.us

Because guc.c prefers to keep all its string values in malloc'd
not palloc'd storage, it has to be more careful than usual to
avoid leaks.  Error exits out of string GUC hook checks failed
to clear the proposed value string, and error exits out of
ProcessGUCArray() failed to clear the malloc'd results of
ParseLongOption().

Found via valgrind testing.
This problem is ancient, so back-patch to all supported branches.

Discussion: https://postgr.es/m/3816764.1616104288@sss.pgh.pa.us

The text search cache mechanisms assume that we can clean up
an invalidated dictionary cache entry simply by resetting the
associated long-lived memory context.  However, that does not work
for ispell affixes that make use of regular expressions, because
the regex library deals in plain old malloc.  Hence, we leaked
compiled regex(es) any time we dropped such a cache entry.  That
could quickly add up, since even a fairly trivial regex can use up
tens of kB, and a large one can eat megabytes.  Add a memory context
callback to ensure that a regex gets freed when its owning cache
entry is cleared.

Found via valgrind testing.
This problem is ancient, so back-patch to all supported branches.

Discussion: https://postgr.es/m/3816764.1616104288@sss.pgh.pa.us

Some code paths in this function perform syscache lookups, which
can lead to table accesses and possibly leakage of cruft into
the caller's context.  If said context is CacheMemoryContext,
we eventually will have visible bloat.  But fixing this is no
harder than moving one memory context switch step.  (The other
callers don't have a problem.)

Andres Freund and I independently found this via valgrind testing.
Back-patch to v12 where this code was added.

Discussion: https://postgr.es/m/20210317023101.anvejcfotwka6gaa@alap3.anarazel.de
Discussion: https://postgr.es/m/3816764.1616104288@sss.pgh.pa.us

Although these lists are usually NIL, and even when not empty
are unlikely to be large, constant relcache update traffic could
eventually result in visible bloat of CacheMemoryContext.

Found via valgrind testing.
Back-patch to v10 where this field was added.

Discussion: https://postgr.es/m/3816764.1616104288@sss.pgh.pa.us

The test included in cd91de0d had two simple flaws.

Firstly, the number of rows was low and on some platforms (e.g. 32-bit)
the sort did not require on-disk sort, so on those machines it was not
testing the automatic removal.  The test was however failing, because
without any temporary files the base/pgsql_tmp directory was not even
created.  Fixed by increasing the rowcount to 5000, which should be high
engough on any platform.

Secondly, the test used a simple sleep to wait for the temporary file to
be created.  This is obviously problematic, because on slow machines (or
with valgrind, CLOBBER_CACHE_ALWAYS etc.) it may take a while to create
the temporary file.  But we also want the tests run reasonably fast.
Fixed by instead relying on a UNIQUE constraint, blocking the query that
created the temporary file.

Author: Euler Taveira
Reviewed-by: Tomas Vondra
Discussion: https://postgr.es/m/CAH503wDKdYzyq7U-QJqGn%3DGm6XmoK%2B6_6xTJ-Yn5WSvoHLY1Ww%40mail.gmail.com

Only "IMPORT" was showing as result of the completion, while IMPORT
FOREIGN SCHEMA is the only command using this keyword in first
position.  This changes the completion to show the full command name
instead of just "IMPORT".

Reviewed-by: Georgios Kokolatos, Julien Rouhaud
Discussion: https://postgr.es/m/YFL6JneBiuMWYyoh@paquier.xyz

Our coding convention requires this macro's result to be assigned
back to the original List variable.  In this usage, since the
List could not become empty, there was no actual bug --- but
some compilers warned about it.  Oversight in be45be9c.

Discussion: https://postgr.es/m/35077b31-2d62-1e31-0e2e-ddb52d590b73@enterprisedb.com

With grouping sets, it's possible that some of the grouping sets are
duplicate.  This is especially common with CUBE and ROLLUP clauses. For
example GROUP BY CUBE (a,b), CUBE (b,c) is equivalent to

  GROUP BY GROUPING SETS (
    (a, b, c),
    (a, b, c),
    (a, b, c),
    (a, b),
    (a, b),
    (a, b),
    (a),
    (a),
    (a),
    (c, a),
    (c, a),
    (c, a),
    (c),
    (b, c),
    (b),
    ()
  )

Some of the grouping sets are calculated multiple times, which is mostly
unnecessary.  This commit implements a new GROUP BY DISTINCT feature, as
defined in the SQL standard, which eliminates the duplicate sets.

Author: Vik Fearing
Reviewed-by: Erik Rijkers, Georgios Kokolatos, Tomas Vondra
Discussion: https://postgr.es/m/bf3805a8-d7d1-ae61-fece-761b7ff41ecc@postgresfriends.org

After a crash of a backend using temporary files, the files used to be
left behind, on the basis that it might be useful for debugging. But we
don't have any reports of anyone actually doing that, and it means the
disk usage may grow over time due to repeated backend failures (possibly
even hitting ENOSPC). So this behavior is a bit unfortunate, and fixing
it required either manual cleanup (deleting files, which is error-prone)
or restart of the instance (i.e. service disruption).

This implements automatic cleanup of temporary files, controled by a new
GUC remove_temp_files_after_crash. By default the files are removed, but
it can be disabled to restore the old behavior if needed.

Author: Euler Taveira
Reviewed-by: Tomas Vondra, Michael Paquier, Anastasia Lubennikova, Thomas Munro
Discussion: https://postgr.es/m/CAH503wDKdYzyq7U-QJqGn%3DGm6XmoK%2B6_6xTJ-Yn5WSvoHLY1Ww%40mail.gmail.com

pg_read_file() is the function that's in core, pg_file_read() is in
adminpack. But when using pg_file_read() in adminpack it calls the *C*
level function pg_read_file() in core, which probably threw the original
author off. But the error hint should be about the SQL function.

Reported-By: Sergei Kornilov
Backpatch-through: 11
Discussion: https://postgr.es/m/373021616060475@mail.yandex.ru

Commit c8f78b61 added a new reloption to enable inserts in parallel-mode
but forgot to update at one of the places about the same in docs. In
passing, fix a typo in the same commit.

Reported-by: Justin Pryzby
Author: Justin Pryzby
Reviewed-by: "Hou, Zhijie", Amit Kapila
Discussion: https://postgr.es/m/20210318025228.GE11765@telsasoft.com

Commit 05c8482f added the implementation of parallel SELECT for
"INSERT INTO ... SELECT ..." which may incur non-negligible overhead in
the additional parallel-safety checks that it performs, even when, in the
end, those checks determine that parallelism can't be used. This is
normally only ever a problem in the case of when the target table has a
large number of partitions.

A new GUC option "enable_parallel_insert" is added, to allow insert in
parallel-mode. The default is on.

In addition to the GUC option, the user may want a mechanism to allow
inserts in parallel-mode with finer granularity at table level. The new
table option "parallel_insert_enabled" allows this. The default is true.

Author: "Hou, Zhijie"
Reviewed-by: Greg Nancarrow, Amit Langote, Takayuki Tsunakawa, Amit Kapila
Discussion: https://postgr.es/m/CAA4eK1K-cW7svLC2D7DHoGHxdAdg3P37BLgebqBOC2ZLc9a6QQ%40mail.gmail.com
Discussion: https://postgr.es/m/CAJcOf-cXnB5cnMKqWEp2E2z7Mvcd04iLVmV=qpFJrR3AcrTS3g@mail.gmail.com

When accessing replication slot stats, introduced in 98681675,
pgstat_read_statsfiles() reads the data into newly allocated
memory. Unfortunately the current memory context at that point is the
callers, leading to leaks and use-after-free dangers.

The fix is trivial, explicitly use pgStatLocalContext. There's some
potential for further improvements, but that's outside of the scope of
this bugfix.

No backpatch necessary, feature is only in HEAD.

Author: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/20210317230447.c7uc4g3vbs4wi32i@alap3.anarazel.de

Seems to have been a copy-and-paste mistake in 093129c9.
Per report from max1@inbox.ru.

Discussion: https://postgr.es/m/161591740692.24273.4202054598867879464@wrigleys.postgresql.org

While looking at Robert Foggia's report, I noticed a passel of
other issues in the same area:

* The scheme for backslash-quoting newlines in pathnames is just
wrong; it will misbehave if the last ordinary character in a pathname
is a backslash.  I'm not sure why we're bothering to allow newlines
in tablespace paths, but if we're going to do it we should do it
without introducing other problems.  Hence, backslashes themselves
have to be backslashed too.

* The author hadn't read the sscanf man page very carefully, because
this code would drop any leading whitespace from the path.  (I doubt
that a tablespace path with leading whitespace could happen in
practice; but if we're bothering to allow newlines in the path, it
sure seems like leading whitespace is little less implausible.)  Using
sscanf for the task of finding the first space is overkill anyway.

* While I'm not 100% sure what the rationale for escaping both \r and
\n is, if the idea is to allow Windows newlines in the file then this
code failed, because it'd throw an error if it saw \r followed by \n.

* There's no cross-check for an incomplete final line in the map file,
which would be a likely apparent symptom of the improper-escaping
bug.

On the generation end, aside from the escaping issue we have:

* If needtblspcmapfile is true then do_pg_start_backup will pass back
escaped strings in tablespaceinfo->path values, which no caller wants
or is prepared to deal with.  I'm not sure if there's a live bug from
that, but it looks like there might be (given the dubious assumption
that anyone actually has newlines in their tablespace paths).

* It's not being very paranoid about the possibility of random stuff
in the pg_tblspc directory.  IMO we should ignore anything without an
OID-like name.

The escaping rule change doesn't seem back-patchable: it'll require
doubling of backslashes in the tablespace_map file, which is basically
a basebackup format change.  The odds of that causing trouble are
considerably more than the odds of the existing bug causing trouble.
The rest of this seems somewhat unlikely to cause problems too,
so no back-patch.

Robert Foggia of Trustwave reported that read_tablespace_map()
fails to prevent an overrun of its on-stack input buffer.
Since the tablespace map file is presumed trustworthy, this does
not seem like an interesting security vulnerability, but still
we should fix it just in the name of robustness.

While here, document that pg_basebackup's --tablespace-mapping option
doesn't work with tar-format output, because it doesn't.  To make it
work, we'd have to modify the tablespace_map file within the tarball
sent by the server, which might be possible but I'm not volunteering.
(Less-painful solutions would require changing the basebackup protocol
so that the source server could adjust the map.  That's not very
appetizing either.)

The existing test script does run pg_basebackup with the -Ft option,
but it makes no real attempt to verify the sanity of the results.
We wouldn't know if the output is incompatible with standard "tar"
programs, nor if the server fails to start from the restored output.
Notably, this means that xlog.c's read_tablespace_map() is not being
meaningfully tested, since that code is used only in the tar-format
case.  (We do have reasonable coverage of restoring from plain-format
output, though it's over in src/test/recovery not here.)

Hence, attempt to untar the output and start a server from it,
rather just hoping it's OK.

This test assumes that the local "tar" has the "-C directory"
switch.  Although that's not promised by POSIX, my research
suggests that all non-extinct tar implementations have it.
Should the buildfarm's opinion differ, we can complicate the
test a bit to avoid requiring that.

Possibly this should be back-patched, but I'm unsure about
whether it could work on Windows before d66b23b0.

This was a bit disjointed, partly because of a not-well-considered
decision to document SQL commands that don't return result rows as
though they had nothing in common with commands that do.  Rearrange
so that we have one discussion of variable substitution that clearly
applies to all types of SQL commands, and then handle the question
of processing command output separately.  Clarify that EXPLAIN,
CREATE TABLE AS SELECT, and similar commands that incorporate an
optimizable statement will act like optimizable statements for the
purposes of variable substitution.  Do a bunch of minor wordsmithing
in the same area.

David Johnston and Tom Lane, reviewed by Pavel Stehule and David
Steele

Discussion: https://postgr.es/m/CAKFQuwYvMKucM5fnZvHSo-ah4S=_n9gmKeu6EAo=_fTrohunqQ@mail.gmail.com

This reverts commit 378802e3.
This reverts commit 3b8981b6.

Discussion: https://postgr.es/m/CA%2BhUKGJmcqAE3MZeDCLLXa62cWM0AJbKmp2JrJYaJ86bz36LFA%40mail.gmail.com

578b2297, that removed support for WITH OIDS, has changed
CatalogTupleInsert() to not return an Oid, but one comment was still
mentioning that.

Author: Vik Fearing
Discussion: https://postgr.es/m/fef01975-ed10-3601-7b9e-80ecef72d00b@postgresfriends.org

Commit 3048898e dropped -ING from some wait event names that correspond
to barrier phases.  Update the phases' names to match.

While we're here making cosmetic changes, also rename "DONE" to "FREE".
That pairs better with "ALLOCATE", and describes the activity that
actually happens in that phase (as we do for the other phases) rather
than describing a state.  The distinction is clearer after bugfix commit
3b8981b6 split the phase into two.  As for the growth barriers, rename
their "ALLOCATE" phase to "REALLOCATE", which is probably a better
description of what happens then.  Also improve the comments about
the phases a bit.

Discussion: https://postgr.es/m/CA%2BhUKG%2BMDpwF2Eo2LAvzd%3DpOh81wUTsrwU1uAwR-v6OGBB6%2B7g%40mail.gmail.com

With very unlucky timing and parallel_leader_participation off, PHJ
could attempt to access per-batch state just as it was being freed.
There was code intended to prevent that by checking for a cleared
pointer, but it was buggy.

Fix, by introducing an extra barrier phase.  The new phase
PHJ_BUILD_RUNNING means that it's safe to access the per-batch state to
find a batch to help with, and PHJ_BUILD_DONE means that it is too late.
The last to detach will free the array of per-batch state as before, but
now it will also atomically advance the phase at the same time, so that
late attachers can avoid the hazard, without the data race.  This
mirrors the way per-batch hash tables are freed (see phases
PHJ_BATCH_PROBING and PHJ_BATCH_DONE).

Revealed by a one-off build farm failure, where BarrierAttach() failed a
sanity check assertion, because the memory had been clobbered by
dsa_free().

Back-patch to 11, where the code arrived.
Reported-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/20200929061142.GA29096%40paquier.xyz

It seems like a useful sanity check to be able to run "installcheck"
against a cluster running with default_transaction_level set to
serializable or repeatable read.  Only one thing currently fails in
those configurations, so let's fix that.

No back-patch for now, because it fails in many other places in some of
the stable branches.  We'd have to go back and fix those too if we
included this configuration in automated testing.
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CA%2BhUKGJUaHeK%3DHLATxF1JOKDjKJVrBKA-zmbPAebOM0Se2FQRg%40mail.gmail.com

Commit ce0fdbfe made tablesync slots permanent and allow Drop
Subscription to drop such slots. However, it is possible that before
tablesync worker could get the acknowledgment of slot creation, drop
subscription stops it and that can lead to a dangling slot on the
publisher. Prevent cancel/die interrupts while creating a slot in the
tablesync worker.

Reported-by: Thomas Munro as per buildfarm
Author: Amit Kapila
Reviewed-by: Vignesh C, Takamichi Osumi
Discussion: https://postgr.es/m/CA+hUKGJG9dWpw1cOQ2nzWU8PHjm=PTraB+KgE5648K9nTfwvxg@mail.gmail.com

Commit 46482432 added a new column substream in pg_subscription but
forgot to update the docs.

Reported-by: Peter Smith
Author: Amit Kapila
Reviewed-by: Peter Smith
Discussion: https://postgr.es/m/CAHut+PuPGGASnh2Dy37VYODKULVQo-5oE=Shc6gwtRizDt==cA@mail.gmail.com

Pass CURSOR_OPT_PARALLEL_OK to pg_plan_query() so that parallel plans
are considered when running the underlying SELECT query.  This wasn't
done in commit e9baa5e9, which did this for CREATE MATERIALIZED VIEW,
because it wasn't yet known to be safe.

Since REFRESH always inserts into a freshly created table before later
merging or swapping the data into place with separate operations, we can
enable such plans here too.

Author: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Reviewed-by: Hou, Zhijie <houzj.fnst@cn.fujitsu.com>
Reviewed-by: Luc Vlaming <luc@swarm64.com>
Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://postgr.es/m/CALj2ACXg-4hNKJC6nFnepRHYT4t5jJVstYvri%2BtKQHy7ydcr8A%40mail.gmail.com

Oversight in commit d168b666, which added bottom-up index deletion.

Empty sibling pages can occasionally be much more common than any other
event that we report on at elevel DEBUG1.  Increase the elevel for
relevant cases to DEBUG2 to avoid overwhelming the user with relatively
insignificant details.

After reading the root cert list from the ssl_ca_file, immediately
install it as client CA list of the new SSL context.  That gives the
SSL context ownership of the list, so that SSL_CTX_free will free it.
This avoids a permanent memory leak if we fail further down in
be_tls_init(), which could happen if bogus CRL data is offered.

The leak could only amount to something if the CRL parameters get
broken after server start (else we'd just quit) and then the server
is SIGHUP'd many times without fixing the CRL data.  That's rather
unlikely perhaps, but it seems worth fixing, if only because the
code is clearer this way.

While we're here, add some comments about the memory management
aspects of this logic.

Noted by Jelte Fennema and independently by Andres Freund.
Back-patch to v10; before commit de41869b it doesn't matter,
since we'd not re-execute this code during SIGHUP.

Discussion: https://postgr.es/m/16160-18367e56e9a28264@postgresql.org