dsm_create and dsm_attach assumed that a current resource owner was
always in place. Exploration with the API show that this is
inconvenient: sometimes one must create a dummy resowner, create/attach
the DSM, only to pin the mapping later, which is wasteful. Change
create/attach so that if there is no current resowner, the dsm is
effectively pinned right from the start.
Discussion: https://postgr.es/m/20170324232710.32acsfsvjqfgc6ud@alvherre.pgsql
Reviewed by Thomas Munro.
c.h #includes a number of core libc header files, such as <stdio.h>.
There's no point in re-including these after having read postgres.h,
postgres_fe.h, or c.h; so remove code that did so.
While at it, also fix some places that were ignoring our standard pattern
of "include postgres[_fe].h, then system header files, then other Postgres
header files". While there's not any great magic in doing it that way
rather than system headers last, it's silly to have just a few files
deviating from the general pattern. (But I didn't attempt to enforce this
globally, only in files I was touching anyway.)
I'd be the first to say that this is mostly compulsive neatnik-ism,
but over time it might save enough compile cycles to be useful.
Previously, the handle for the control segment could not be zero, but
some other DSM segment could potentially have a handle value of zero.
However, that means that if someone wanted to store a dsm_handle that
might or might not be valid, they would need a separate boolean to
keep track of whether the associated value is legal. That's annoying,
so change things so that no DSM segment can ever have a handle of 0 -
or as we call it here, DSM_HANDLE_INVALID.
Thomas Munro. This was submitted as part of a much larger patch to
add an malloc-like allocator for dynamic shared memory, but this part
seems like a good idea independently of the rest of the patch.
Commits 470d886c3 et al intended to fix the problem that the postmaster
selected the same "random" DSM control segment ID on every start. But
using PostmasterRandom() for that destroys the intended property that the
delay between random_start_time and random_stop_time will be unpredictable.
(Said delay is probably already more predictable than we could wish, but
that doesn't mean that reducing it by a couple orders of magnitude is OK.)
Revert the previous patch and add a comment warning against misuse of
PostmasterRandom. Fix the original problem by calling srandom() early in
PostmasterMain, using a low-security seed that will later be overwritten
by PostmasterRandom.
Discussion: <20789.1474390434@sss.pgh.pa.us>
If you have previously pinned a segment and decide that you don't
actually want to keep it around until shutdown, this new API lets you
remove the pin. This is pretty trivial except on Windows, where it
requires closing the duplicate handle that was used to implement the
pin.
Thomas Munro and Amit Kapila, reviewed by Amit Kapila and by me.
dsm_control->nitems never decreases, so this is testing whether the
server has *ever* run out of DSM segments, not whether it is
*currently* out of DSM segments.
Reported off-list by Amit Kapila.
Right now, there's only one flag, DSM_CREATE_NULL_IF_MAXSEGMENTS,
which suppresses the error that would normally be thrown when the
maximum number of segments already exists, instead returning NULL.
It might be useful to add more flags in the future, such as one to
ignore allocation errors, but I haven't done that here.
This reassociates a dynamic shared memory handle previous passed to
dsm_pin_mapping with the current resource owner, so that it will be
cleaned up at the end of the current query.
Patch by me. Review of the function name by Andres Freund, Amit
Kapila, Jim Nasby, Petr Jelinek, and Álvaro Herrera.
Nobody seemed concerned about this naming when it originally went in,
but there's a pending patch that implements the opposite of
dsm_keep_mapping, and the term "unkeep" was judged unpalatable.
"unpin" has existing precedent in the PostgreSQL code base, and the
English language, so use this terminology instead.
Per discussion, back-patch to 9.4.
The previous coding would potentially cause attaching to segment A to
fail if segment B was at the same time in the process of going away.
Andres Freund, with a comment tweak by me
Instead of storing the ID of the dynamic shared memory control
segment in a file within the data directory, store it in the main
control segment. This avoids a number of nasty corner cases,
most seriously that doing an online backup and then using it on
the same machine (e.g. to fire up a standby) would result in the
standby clobbering all of the master's dynamic shared memory
segments.
Per complaints from Heikki Linnakangas, Fujii Masao, and Tom
Lane.
The new function dsm_detach_all() can be used either by postmaster
children that don't wish to take any risk of accidentally corrupting
shared memory; or by forked children of regular backends with
the same need. This patch also updates the postmaster children that
already do PGSharedMemoryDetach() to do dsm_detach_all() as well.
Per discussion with Tom Lane.
If a postmaster child invokes fork() and then calls on_exit_reset, that
should be sufficient to let it exit() without breaking anything, but
dynamic shared memory broke that by not updating on_exit_reset() to
discard callbacks registered with dynamic shared memory segments.
Per investigation of a complaint from Tom Lane.
Since C99, it's been standard for printf and friends to accept a "z" size
modifier, meaning "whatever size size_t has". Up to now we've generally
dealt with printing size_t values by explicitly casting them to unsigned
long and using the "l" modifier; but this is really the wrong thing on
platforms where pointers are wider than longs (such as Win64). So let's
start using "z" instead. To ensure we can do that on all platforms, teach
src/port/snprintf.c to understand "z", and add a configure test to force
use of that implementation when the platform's version doesn't handle "z".
Having done that, modify a bunch of places that were using the
unsigned-long hack to use "z" instead. This patch doesn't pretend to have
gotten everyplace that could benefit, but it catches many of them. I made
an effort in particular to ensure that all uses of the same error message
text were updated together, so as not to increase the number of
translatable strings.
It's possible that this change will result in format-string warnings from
pre-C99 compilers. We might have to reconsider if there are any popular
compilers that will warn about this; but let's start by seeing what the
buildfarm thinks.
Andres Freund, with a little additional work by me
Just as backends must clean up their shared memory state (releasing
lwlocks, buffer pins, etc.) before exiting, they must also perform
any similar cleanups related to dynamic shared memory segments they
have mapped before unmapping those segments. So add a mechanism to
ensure that.
Existing on_shmem_exit hooks include both "user level" cleanup such
as transaction abort and removal of leftover temporary relations and
also "low level" cleanup that forcibly released leftover shared
memory resources. On-detach callbacks should run after the first
group but before the second group, so create a new before_shmem_exit
function for registering the early callbacks and keep on_shmem_exit
for the regular callbacks. (An earlier draft of this patch added an
additional argument to on_shmem_exit, but that had a much larger
footprint and probably a substantially higher risk of breaking third
party code for no real gain.)
Patch by me, reviewed by KaiGai Kohei and Andres Freund.
Alvaro Herrera
Tom Lane
Bruce Momjian
Robert Haas
Simon Riggs
Heikki Linnakangas
Peter Eisentraut