Commit 5b148706c5 exposed functionality that allows multiple processes to
use the same replication origin, enabling non-builtin logical replication
solutions to implement parallel apply for large transactions.
With this functionality, if two backends acquire the same replication
origin and one of them resets it first, the acquired_by flag is cleared
without acknowledging that another backend is still actively using the
origin. This can lead to the origin being unintentionally dropped. If the
shared memory for that dropped origin is later reused for a newly created
origin, the remaining backend that still holds a pointer to the old memory
may inadvertently advance the LSN of a completely different origin,
causing unpredictable behavior.
Although the underlying issue predates commit 5b148706c5, it did not
surface earlier because the internal parallel apply worker mechanism
correctly coordinated origin resets and drops.
This commit resolves the problem by introducing a reference counter for
replication origins. The reference count increases when a backend sets the
origin and decreases when it resets it. Additionally, the backend that
first acquires the origin will not release it until all other backends
using the origin have released it as well.
The patch also prevents dropping a replication origin when acquired_by is
zero but the reference counter is nonzero, covering the scenario where the
first session exits without properly releasing the origin.
Author: Hou Zhijie <houzj.fnst@fujitsu.com>
Author: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Reviewed-by: Shveta Malik <shveta.malik@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Discussion: https://postgr.es/m/TY4PR01MB169077EE72ABE9E55BAF162D494B5A@TY4PR01MB16907.jpnprd01.prod.outlook.com
Discussion: https://postgr.es/m/CAMPB6wfe4zLjJL8jiZV5kjjpwBM2=rTRme0UCL7Ra4L8MTVdOg@mail.gmail.com
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