pgsql: Silence Valgrind leakage complaints in more-or-less-hackish ways - Mailing list pgsql-committers

Silence Valgrind leakage complaints in more-or-less-hackish ways.

These changes don't actually fix any leaks.  They just make sure that
Valgrind will find pointers to data structures that remain allocated
at process exit, and thus not falsely complain about leaks.  In
particular, we are trying to avoid situations where there is no
pointer to the beginning of an allocated block (except possibly
within the block itself, which Valgrind won't count).

* Because dynahash.c never frees hashtable storage except by deleting
the whole hashtable context, it doesn't bother to track the individual
blocks of elements allocated by element_alloc().  This results in
"possibly lost" complaints from Valgrind except when the first element
of each block is actively in use.  (Otherwise it'll be on a freelist,
but very likely only reachable via "interior pointers" within element
blocks, which doesn't satisfy Valgrind.)

To fix, if we're building with USE_VALGRIND, expend an extra pointer's
worth of space in each element block so that we can chain them all
together from the HTAB header.  Skip this in shared hashtables though:
Valgrind doesn't track those, and we'd need additional locking to make
it safe to manipulate a shared chain.

While here, update a comment obsoleted by 9c911ec06.

* Put the dlist_node fields of catctup and catclist structs first.
This ensures that the dlist pointers point to the starts of these
palloc blocks, and thus that Valgrind won't consider them
"possibly lost".

* The postmaster's PMChild structs and the autovac launcher's
avl_dbase structs also have the dlist_node-is-not-first problem,
but putting it first still wouldn't silence the warning because we
bulk-allocate those structs in an array, so that Valgrind sees a
single allocation.  Commonly the first array element will be pointed
to only from some later element, so that the reference would be an
interior pointer even if it pointed to the array start.  (This is the
same issue as for dynahash elements.)  Since these are pretty simple
data structures, I don't feel too bad about faking out Valgrind by
just keeping a static pointer to the array start.

(This is all quite hacky, and it's not hard to imagine usages where
we'd need some other idea in order to have reasonable leak tracking of
structures that are only accessible via dlist_node lists.  But these
changes seem to be enough to silence this class of leakage complaints
for the moment.)

* Free a couple of data structures manually near the end of an
autovacuum worker's run when USE_VALGRIND, and ensure that the final
vac_update_datfrozenxid() call is done in a non-permanent context.
This doesn't have any real effect on the process's total memory
consumption, since we're going to exit as soon as that last
transaction is done.  But it does pacify Valgrind.

* Valgrind complains about the postmaster's socket-files and
lock-files lists being leaked, which we can silence by just
not nulling out the static pointers to them.

* Valgrind seems not to consider the global "environ" variable as
a valid root pointer; so when we allocate a new environment array,
it claims that data is leaked.  To fix that, keep our own
statically-allocated copy of the pointer, similarly to the previous
item.

Author: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/285483.1746756246@sss.pgh.pa.us

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/db01c90b2f024298b08dca8aed6b43a2347dee0e

Modified Files
--------------
src/backend/libpq/pqcomm.c          |  1 -
src/backend/postmaster/autovacuum.c | 26 ++++++++++++++++++-
src/backend/postmaster/pmchild.c    | 18 ++++++++++++-
src/backend/utils/hash/dynahash.c   | 52 ++++++++++++++++++++++++++++++++-----
src/backend/utils/init/miscinit.c   |  1 -
src/backend/utils/misc/ps_status.c  | 16 ++++++++++++
src/include/utils/catcache.h        | 23 +++++++++-------
7 files changed, 118 insertions(+), 19 deletions(-)