[bomanaps]

Evaluate :missing=true in MongoDB transaction and batch conditional criteria (ifNoneExist, conditional update/delete) inside the transaction's session with the same semantics as search, instead of rejecting the bundle fixes #2003.

[smunini]

Thanks @bomanaps. The semantics are right: the routes match search, writes earlier in the same Bundle are visible, and the freshness, opt-out and index-build gates from #2181/#2209/#2001/#2211 all apply. Without this PR both of its tests fail with unsupported modifier 'missing=true'; with it they pass.

Benchmarking turned up a scaling problem, so I pushed a fix to this branch. Your rebased commit is kept, followed by a merge of current main (28cf9b3) and the fix (2563f80).

The problem

absent_ids_in_bundle_transaction built the absent-id set for the whole resource type before any other criterion narrowed it, then intersected it with the candidates. Search stopped working that way in #2004. As a result:

• Metadata route: a fetch plus a $lookup for every absent resource of the type, on every conditional entry.
• Value route (metadata off or stale): it lists every live id, so past 300k resources it fails with TooManyResults, even when identifier matches exactly one.
• No ceiling: the metadata route had no cap, so its $in / distinct would eventually hit MongoDB's 16 MB document limit.

The fix

• The :missing=true criteria are classified once per entry, after the same freshness probe search runs.
• They are applied to each candidate batch after the other criteria: the 128-id batches of the index pager.
• When nothing else can drive (only _id / _lastUpdated alongside :missing=true), an unsorted cursor is read in 512-id batches and stops at the second match, which is all ifNoneExist and conditional update/delete need. When metadata answers the parameter, the cursor itself is narrowed by the missing_params condition.
• Hint in a shared function: metadata_missing_ids now hints idx_resources_identity when it is given candidates. The profiler showed that, without the hint, checking a 512-id batch walked idx_resources_missing_params at about 113k keys per batch (the MongoDB: broad :missing=true on missing_params metadata scans every index key and runs a $lookup per match (slower than the value route) #2233 shape). Search calls the same function; the GET shapes below are unchanged.
• New test: mongodb_missing_conditional_criteria_cross_candidate_batches puts the absent resource after 600 present ones, past the first batch. It covers 1 match, 2+ matches (412) and 0 matches, with metadata on and off.
• Docs: the paragraph in docs/mongodb/search-indexes.md now describes per-batch evaluation.

Numbers

310k Patients, half without gender. Release build, mongo 7.0 replica set. Each row is a single-entry transaction whose ifNoneExist matches one existing resource; main and this branch were run alternately.

ifNoneExist criteria	as submitted	this branch	GET, same criteria
identifier=X&gender:missing=true, metadata on	6.4–13 s	3 ms	1.3–1.7 ms
identifier=X&gender:missing=true, metadata off	fails with found 300001, maximum is 300000	2 ms	0.7 ms
_id=X&gender:missing=true	11.3 s	2–3 ms	–
gender:missing=true alone, ~155k match (412)	11.9 s, or the 300k error with metadata off	23 ms, 4 ms with metadata off	–
birthdate:missing=true alone, 1 match	–	2 ms, 1.2 s with metadata off	–
criteria without :missing=true	–	same as main	–
GET active=true&gender:missing=true, _count=50	–	5 ms / 11 ms (main: 5 ms / 10 ms)	–

At 50k Patients the submitted version already took about 900 ms per entry.

The remaining slow case is a criterion with only :missing=true that matches almost nothing, with metadata off. It has to read the whole type, at about 1.2 s for 310k, which is roughly what the same GET costs. It now finishes instead of erroring.

Checks

• Clippy with the CI flags: clean. Lib tests (backends::mongodb): 505/505.
• Full mongodb_tests on the fix: 529 passed, 0 failed. Locally it had to be split in two because of the known createIndexes stall with ~320 databases in one container, which happens on main too.
• On the pushed tree: missing_index 25/25, and the transaction / conditional / ifNoneExist tests 78/78.