mirror/postgresql
2
0
Fork 0
mirror of https://git.postgresql.org/git/postgresql.git synced 2025-03-07 19:47:50 +08:00
Code Issues Packages Projects Releases Wiki Activity
47,093 Commits 37 Branches 648 Tags 476 MiB
C 85.4%
PLpgSQL 6%
Perl 4.3%
Yacc 1.2%
Meson 0.7%
Other 2.1%
85ccb6899c
Go to file
Tom Lane 85ccb6899c Rearrange pgstat_bestart() to avoid failures within its critical section. 
We long ago decided to design the shared PgBackendStatus data structure to
minimize the cost of writing status updates, which means that writers just
have to increment the st_changecount field twice.  That isn't hooked into
any sort of resource management mechanism, which means that if something
were to throw error between the two increments, the st_changecount field
would be left odd indefinitely.  That would cause readers to lock up.
Now, since it's also a bad idea to leave the field odd for longer than
absolutely necessary (because readers will spin while we have it set),
the expectation was that we'd treat these segments like spinlock critical
sections, with only short, more or less straight-line, code in them.

That was fine as originally designed, but commit 9029f4b37 broke it
by inserting a significant amount of non-straight-line code into
pgstat_bestart(), code that is very capable of throwing errors, not to
mention taking a significant amount of time during which readers will spin.
We have a report from Neeraj Kumar of readers actually locking up, which
I suspect was due to an encoding conversion error in X509_NAME_to_cstring,
though conceivably it was just a garden-variety OOM failure.

Subsequent commits have loaded even more dubious code into pgstat_bestart's
critical section (and commit fc70a4b0d deserves some kind of booby prize
for managing to miss the critical section entirely, although the negative
consequences seem minimal given that the PgBackendStatus entry should be
seen by readers as inactive at that point).

The right way to fix this mess seems to be to compute all these values
into a local copy of the process' PgBackendStatus struct, and then just
copy the data back within the critical section proper.  This plan can't
be implemented completely cleanly because of the struct's heavy reliance
on out-of-line strings, which we must initialize separately within the
critical section.  But still, the critical section is far smaller and
safer than it was before.

In hopes of forestalling future errors of the same ilk, rename the
macros for st_changecount management to make it more apparent that
the writer-side macros create a critical section.  And to prevent
the worst consequences if we nonetheless manage to mess it up anyway,
adjust those macros so that they really are a critical section, ie
they now bump CritSectionCount.  That doesn't add much overhead, and
it guarantees that if we do somehow throw an error while the counter
is odd, it will lead to PANIC and a database restart to reset shared
memory.

Back-patch to 9.5 where the problem was introduced.

In HEAD, also fix an oversight in commit b0b39f72b: it failed to teach
pgstat_read_current_status to copy st_gssstatus data from shared memory to
local memory.  Hence, subsequent use of that data within the transaction
would potentially see changing data that it shouldn't see.

Discussion: https://postgr.es/m/CAPR3Wj5Z17=+eeyrn_ZDG3NQGYgMEOY6JV6Y-WRRhGgwc16U3Q@mail.gmail.com
2019-05-11 21:27:29 -04:00
config Update config.guess and config.sub 2019-04-27 14:25:00 +02:00
contrib postgres_fdw: Fix cost estimation for aggregate pushdown. 2019-05-09 18:39:23 +09:00
doc docs: remove second mention of btree max length reduction 2019-05-11 18:24:31 -04:00
src Rearrange pgstat_bestart() to avoid failures within its critical section. 2019-05-11 21:27:29 -04:00
.dir-locals.el Make Emacs perl-mode indent more like perltidy. 2019-01-13 11:32:31 -08:00
.gitattributes doc: Add some images 2019-03-27 23:10:23 +01:00
.gitignore Support for optimizing and emitting code in LLVM JIT provider. 2018-03-22 11:05:22 -07:00
aclocal.m4 Fix configure's AC_CHECK_DECLS tests to work correctly with clang. 2018-11-19 12:01:47 -05:00
configure Reorder configure tests for accept() in Windows 2019-03-04 14:42:12 -05:00
configure.in Fix PERMIT_DECLARATION_AFTER_STATEMENT initialization. 2019-02-16 13:12:28 -08:00
COPYRIGHT Update copyright for 2019 2019-01-02 12:44:25 -05:00
GNUmakefile.in Process EXTRA_INSTALL serially, during the first temp-install. 2018-12-31 13:54:38 -08:00
HISTORY Change documentation references to PG website to use https: not http: 2017-05-20 21:50:47 -04:00
Makefile Fix non-GNU makefiles for AIX make. 2017-11-30 00:57:22 -08:00
README Change documentation references to PG website to use https: not http: 2017-05-20 21:50:47 -04:00
README.git Change documentation references to PG website to use https: not http: 2017-05-20 21:50:47 -04:00

README 

PostgreSQL Database Management System
=====================================

This directory contains the source code distribution of the PostgreSQL
database management system.

PostgreSQL is an advanced object-relational database management system
that supports an extended subset of the SQL standard, including
transactions, foreign keys, subqueries, triggers, user-defined types
and functions.  This distribution also contains C language bindings.

PostgreSQL has many language interfaces, many of which are listed here:

	https://www.postgresql.org/download

See the file INSTALL for instructions on how to build and install
PostgreSQL.  That file also lists supported operating systems and
hardware platforms and contains information regarding any other
software packages that are required to build or run the PostgreSQL
system.  Copyright and license information can be found in the
file COPYRIGHT.  A comprehensive documentation set is included in this
distribution; it can be read as described in the installation
instructions.

The latest version of this software may be obtained at
https://www.postgresql.org/download/.  For more information look at our
web site located at https://www.postgresql.org/.