pgsql: Don't be so trusting that shm_toc_lookup() will always succeed.

Don't be so trusting that shm_toc_lookup() will always succeed.

Given the possibility of race conditions and so on, it seems entirely
unsafe to just assume that shm_toc_lookup() always finds the key it's
looking for --- but that was exactly what all but one call site were
doing. To fix, add a "bool noError" argument, similarly to what we
have in many other functions, and throw an error on an unexpected
lookup failure. Remove now-redundant Asserts that a rather random
subset of call sites had.

I doubt this will throw any light on buildfarm member lorikeet's
recent failures, because if an unnoticed lookup failure were involved,
you'd kind of expect a null-pointer-dereference crash rather than the
observed symptom. But you never know ... and this is better coding
practice even if it never catches anything.

Discussion: https://postgr.es/m/9697.1496675981@sss.pgh.pa.us

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/d4663350646ca0c069a36d906155a0f7e3372eb7

Modified Files
--------------
src/backend/access/transam/parallel.c | 31 ++++++++++++-------------------
src/backend/executor/execParallel.c | 16 ++++++++--------
src/backend/executor/nodeBitmapHeapscan.c | 2 +-
src/backend/executor/nodeCustom.c | 2 +-
src/backend/executor/nodeForeignscan.c | 2 +-
src/backend/executor/nodeIndexonlyscan.c | 2 +-
src/backend/executor/nodeIndexscan.c | 2 +-
src/backend/executor/nodeSeqscan.c | 2 +-
src/backend/storage/ipc/shm_toc.c | 10 +++++++++-
src/include/storage/shm_toc.h | 2 +-
src/test/modules/test_shm_mq/worker.c | 6 +++---
11 files changed, 39 insertions(+), 38 deletions(-)