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Tom Lane 0c8b4e9cfc Detect pfree or repalloc of a previously-freed memory chunk. 
Before the major rewrite in commit c6e0fe1f2, AllocSetFree() would
typically crash when asked to free an already-free chunk.  That was
an ugly but serviceable way of detecting coding errors that led to
double pfrees.  But since that rewrite, double pfrees went through
just fine, because the "hdrmask" of a freed chunk isn't changed at all
when putting it on the freelist.  We'd end with a corrupt freelist
that circularly links back to the doubly-freed chunk, which would
usually result in trouble later, far removed from the actual bug.

This situation is no good at all for debugging purposes.  Fortunately,
we can fix it at low cost in MEMORY_CONTEXT_CHECKING builds by making
AllocSetFree() check for chunk->requested_size == InvalidAllocSize,
relying on the pre-existing code that sets it that way just below.

I investigated the alternative of changing a freed chunk's methodid
field, which would allow detection in non-MEMORY_CONTEXT_CHECKING
builds too.  But that adds measurable overhead.  Seeing that we didn't
notice this oversight for more than three years, it's hard to argue
that detecting this type of bug is worth any extra overhead in
production builds.

Likewise fix AllocSetRealloc() to detect repalloc() on a freed chunk,
and apply similar changes in generation.c and slab.c.  (generation.c
would hit an Assert failure anyway, but it seems best to make it act
like aset.c.)  bump.c doesn't need changes since it doesn't support
pfree in the first place.  Ideally alignedalloc.c would receive
similar changes, but in debugging builds it's impossible to reach
AlignedAllocFree() or AlignedAllocRealloc() on a pfreed chunk, because
the underlying context's pfree would have wiped the chunk header of
the aligned chunk.  But that means we should get an error of some
sort, so let's be content with that.

Per investigation of why the test case for bug #19438 didn't appear to
fail in v16 and up, even though the underlying bug was still present.
(This doesn't fix the underlying double-free bug, just cause it to
get detected.)

Bug: #19438
Reported-by: Dmitriy Kuzmin <kuzmin.db4@gmail.com>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: David Rowley <dgrowleyml@gmail.com>
Discussion: https://postgr.es/m/19438-9d37b179c56d43aa@postgresql.org
Backpatch-through: 16
2026-03-30 12:02:08 -04:00
config Allow PG_PRINTF_ATTRIBUTE to be different in C and C++ code. 2026-02-25 11:57:26 -05:00
contrib Avoid memory leak on error while parsing pg_stat_statements dump file 2026-03-27 12:21:38 +02:00
doc doc: Document IF NOT EXISTS option for ALTER FOREIGN TABLE ADD COLUMN. 2026-03-09 18:25:05 +09:00
src Detect pfree or repalloc of a previously-freed memory chunk. 2026-03-30 12:02:08 -04:00
.abi-compliance-history Placate ABI checker. 2026-02-07 11:48:14 +13:00
.cirrus.star ci: Simplify ci-os-only handling 2025-08-14 12:02:42 -04:00
.cirrus.tasks.yml Fix typo 2026-01-07 15:48:54 +01:00
.cirrus.yml ci: Per-repo configuration for manually trigger tasks 2025-08-14 11:33:47 -04:00
.dir-locals.el Make Emacs perl-mode indent more like perltidy. 2019-01-13 11:32:31 -08:00
.editorconfig Add .editorconfig 2019-12-18 09:13:13 +01:00
.git-blame-ignore-revs Add previous commit to .git-blame-ignore-revs. 2025-10-21 10:02:19 -05:00
.gitattributes Fix git whitespace warning 2025-08-15 10:32:07 +02:00
.gitignore Update top-level .gitignore. 2022-12-04 15:23:00 -05:00
aclocal.m4 autoconf: Move export_dynamic determination to configure 2022-12-06 18:55:28 -08:00
configure Allow PG_PRINTF_ATTRIBUTE to be different in C and C++ code. 2026-02-25 11:57:26 -05:00
configure.ac Allow PG_PRINTF_ATTRIBUTE to be different in C and C++ code. 2026-02-25 11:57:26 -05:00
COPYRIGHT Update copyright for 2026 2026-01-01 13:24:10 -05:00
GNUmakefile.in Allow selecting the git revision to be packaged by "make dist". 2024-05-03 11:08:50 -04:00
HISTORY Canonicalize some URLs 2020-02-10 20:47:50 +01:00
Makefile Adapt REL_17_STABLE to its new status as a stable branch 2024-07-01 08:05:35 +09:00
meson.build Allow PG_PRINTF_ATTRIBUTE to be different in C and C++ code. 2026-02-25 11:57:26 -05:00
meson_options.txt Allow selecting the git revision to be packaged by "make dist". 2024-05-03 11:08:50 -04:00
README.md Adapt REL_17_STABLE to its new status as a stable branch 2024-07-01 08:05:35 +09:00

README.md

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.

Copyright and license information can be found in the file COPYRIGHT.

General documentation about this version of PostgreSQL can be found at https://www.postgresql.org/docs/17/. In particular, information about building PostgreSQL from the source code can be found at https://www.postgresql.org/docs/17/installation.html.

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