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When a stats entry is reinitialized and dropped again while a backend still holds a reference from an older generation, the backend releasing the last reference found a generation mismatch and left behind an entry marked as dropped with a refcount of 0. Nothing would free it, and the checkpointer would trigger an assertion at shutdown when flushing the stats. Looking more at the issue, using some hardcoded sleeps before the dshash_find() that takes an exclusive lock when releasing an entry with some concurrent activity, there was more going on: - A missing entry could be reported as an error at backend exit, error that should not be hit even if the entry was reinitialized and dropped concurrently. - The generation check was not correct for the case of 0 due to the first issue. A generation of 0 refers to an entry that has been freshly created. This commit reworks the entry release to be safer concurrent-wise when an entry targetted for release is found as already existing: - If marked as dropped without refount at 0, free it. - In other cases, ignore it. The entry has been reinitialized or created again with the same key, and is still in use. Sami has posted a basic test case (equivalent added in this commit) that gives a way to reproduce the assertion failure on backend exit, which is a consequence of the incorrect handling of entry releases. Reported-by: Sami Imseih <samimseih.pg@gmail.com> Reviewed-by: Sami Imseih <samimseih.pg@gmail.com> Discussion: https://postgr.es/m/CAN12+YJXE+rEOVo74uM5Ofbj_t_m0DHvd_Mo4ppeHV=aDvvXoQ@mail.gmail.com Backpatch-through: 15
The PostgreSQL contrib tree
---------------------------
This subtree contains porting tools, analysis utilities, and plug-in
features that are not part of the core PostgreSQL system, mainly
because they address a limited audience or are too experimental to be
part of the main source tree. This does not preclude their
usefulness.
User documentation for each module appears in the main SGML
documentation.
When building from the source distribution, these modules are not
built automatically, unless you build the "world" target. You can
also build and install them all by running "make all" and "make
install" in this directory; or to build and install just one selected
module, do the same in that module's subdirectory.
Some directories supply new user-defined functions, operators, or
types. To make use of one of these modules, after you have installed
the code you need to register the new SQL objects in the database
system by executing a CREATE EXTENSION command. In a fresh database,
you can simply do
CREATE EXTENSION module_name;
See the PostgreSQL documentation for more information about this
procedure.