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Remove support for background workers without BGWORKER_SHMEM_ACCESS.

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Background workers without shared memory access have been broken on
EXEC_BACKEND / windows builds since shortly after background workers have been
introduced, without that being reported. Clearly they are not commonly used.

The problem is that bgworker startup requires to be attached to shared memory
in EXEC_BACKEND child processes. StartBackgroundWorker() detaches from shared
memory for unconnected workers, but at that point we already have initialized
subsystems referencing shared memory.

Fixing this problem is not entirely trivial, so removing the option to not be
connected to shared memory seems the best way forward. In most use cases the
advantages of being connected to shared memory far outweigh the disadvantages.

As there have been no reports about this issue so far, we have decided that it
is not worth trying to address the problem in the back branches.

Per discussion with Alvaro Herrera, Robert Haas and Tom Lane.

Author: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/20210802065116.j763tz3vz4egqy3w@alap3.anarazel.de
This commit is contained in:
Andres Freund
2021-08-13 05:49:26 -07:00
parent 1d5135f004
commit 80a8f95b3b
4 changed files with 38 additions and 63 deletions
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10
doc/src/sgml/bgworker.sgml
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@ -11,8 +11,8 @@
PostgreSQL can be extended to run user-supplied code in separate processes.
Such processes are started, stopped and monitored by <command>postgres</command>,
which permits them to have a lifetime closely linked to the server's status.
These processes have the option to attach to <productname>PostgreSQL</productname>'s
shared memory area and to connect to databases internally; they can also run
These processes are attached to <productname>PostgreSQL</productname>'s
shared memory area and have the option to connect to databases internally; they can also run
multiple transactions serially, just like a regular client-connected server
process. Also, by linking to <application>libpq</application> they can connect to the
server and behave like a regular client application.
@ -89,11 +89,7 @@ typedef struct BackgroundWorker
<listitem>
<para>
<indexterm><primary>BGWORKER_SHMEM_ACCESS</primary></indexterm>
Requests shared memory access. Workers without shared memory access
cannot access any of <productname>PostgreSQL's</productname> shared
data structures, such as heavyweight or lightweight locks, shared
buffers, or any custom data structures which the worker itself may
wish to create and use.
Requests shared memory access. This flag is required.
</para>
</listitem>
</varlistentry>