Reuse an embedded MongoDB only when it is running the backend being asked for

[codeconsole]

Follow-up to #16192.

Problem

EmbeddedMongoInitializer hands a restart the server this JVM already started when the settings match the ones it was started with:

StartedServer started = discard(STARTED.get(port), settings);

EmbeddedMongoSettings.equals compares port, version, databaseDir and replicaSet. The backend is not among them, and StartedServer does not carry it either, so nothing in the reuse path knows which backend is running.

Switching embedded.mongodb.backend between in-memory and flapdoodle across a devtools restart, changing nothing else, therefore keeps the old server. An application that asked for a real mongod carries on against the in-memory reimplementation, which implements neither transactions, change streams nor $text — so the failure surfaces later and somewhere else, as a missing feature rather than a backend that was never started.

Worth noting the shape of the omission: the check does compare version, which InMemoryMongoBackend ignores entirely. The one setting that decides what the server actually is was the one left out.

Fix

Keep the backend the server was started with alongside its settings, and compare it:

private record StartedServer(RunningEmbeddedMongo running, EmbeddedMongoSettings settings, String backend) { }

if (started == null || (started.settings().equals(settings) && started.backend().equals(backend))) {
    return started;
}

The backend is resolved in initialize before the reuse question rather than inside start, since it is part of that question, and passed to start so it is selected once.

EmbeddedMongoSettings is public API and its constructor is unchanged; the backend lives on the private StartedServer record instead.

One behaviour change worth calling out: asking for a backend whose library is absent now fails even when a server is already running on that port, because selectBackend runs before the reuse check. Previously such a restart was silently handed the running server. The new behaviour matches what the same configuration does when nothing is running.

Tests

A restart that switches backend while holding version, database and replica set unchanged now gets a new server. The assertion is behavioural — a document written to the first server, then counted after the switch — because object identity does not distinguish the two cases here: initialize constructs a fresh EmbeddedMongoLifecycle on every call, reused server or not.

That last point applies to the existing test beside it. a restart that asks for a different server is not handed the one already running was vacuous: I removed version from EmbeddedMongoSettings.equals and it still passed, so it was not verifying replacement at all. It is rewritten here with the same behavioural assertion, and now fails when version is removed from the comparison.

Both tests fail on unmodified 8.0.x and pass with the fix; :grails-data-mongodb-embedded:test is green.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 54.1139%. Comparing base (e40cb27) to head (1e74b1a).

Additional details and impacted files

@@                Coverage Diff                 @@
##                8.0.x     #16216        +/-   ##
==================================================
- Coverage     54.1238%   54.1139%   -0.0099%     
+ Complexity      20307      20301         -6     
==================================================
  Files            2107       2107                
  Lines          101144     101144                
  Branches        17921      17921                
==================================================
- Hits            54743      54733        -10     
- Misses          38595      38603         +8     
- Partials         7806       7808         +2     

Files with missing lines	Coverage Δ
...orm/mongodb/embedded/EmbeddedMongoInitializer.java	95.9016% <100.0000%> (ø)

... and 1 file with indirect coverage changes

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[testlens-app]

✅ All tests passed ✅

🏷️ Commit: 1e74b1a
▶️ Tests: 69325 executed
⚪️ Checks: 83/83 completed

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