Give read-only transactions the same meaning on Mongo as on Hibernate

[codeconsole]

Problem

@ReadOnly means different things depending on which datastore is underneath.

On Hibernate it genuinely suppresses the flush:

// GrailsHibernateTransactionManager:55
if (definition.isReadOnly()) {
    holder.session.setHibernateFlushMode(FlushMode.MANUAL)
}

On the DatastoreTransactionManager path there is no equivalent. doBegin sets FlushModeType.COMMIT — already the Session default, and as strict as the JPA enum gets, since it has no MANUAL/NEVER — and doCommit's guard is then defeated by the transaction's own commit:

// DatastoreTransactionManager.doCommit
if (!status.isReadOnly()) {
    if (session != null) { ... session.flush(); }   // skipped when read-only
}
transaction.commit();                               // ...which flushes anyway

Both implementations flush unconditionally — MongoTransaction.commit() and SessionOnlyTransaction.commit(). So a read-only transaction writes whatever the surrounding session had queued.

That matters because a read-only transaction has no pending operations of its own. Anything it flushes belongs to the caller's session, and gets written at a moment the caller did not choose. The sharp version is a read taken during a flush — a referential check in a validator, or a beforeInsert hook — where the commit re-validates the entity being saved and the validator reads again, recursing until the stack is gone.

Change

Session.beginTransaction(TransactionDefinition) already existed for exactly this; AbstractSession discarded the argument:

public Transaction beginTransaction(TransactionDefinition definition) {
    transaction = beginTransactionInternal();   // definition dropped
    return transaction;
}

It now passes the definition to an overridable beginTransactionInternal(TransactionDefinition) whose default delegates to the existing no-arg method, so datastores that do not override it — Neo4j and the simple map datastore — keep exactly the path they had today. Mongo overrides it, and MongoTransaction / SessionOnlyTransaction decline to flush when the definition is read-only.

Read-write commits are untouched.

Tests

Three added, on both paths — server-side transactions (MongoTransactionSpec) and the session-only fallback (MongoTransactionDisabledSpec):

• a read-only transaction commits without flushing the surrounding session (both paths)
• a read-write transaction still flushes it (guards the scope of the change)

Both read-only tests fail on 8.0.x without the source change and pass with it; the read-write test passes either way. Full :grails-datastore-core:test, :grails-data-mongodb-core:test and :grails-data-simple:test are green.

Notes

Independent of #16212, which removes a @ReadOnly from GormService in scaffolding. Neither depends on the other; either can land alone. #16212 is where this surfaced.

This is a behaviour change: an application that writes inside an @ReadOnly method on a DatastoreTransactionManager datastore gets those writes at commit today, and would not after this. Hibernate already behaves that way, so this makes the two consistent rather than introducing a new rule. If it should wait for a major, or land with a warning when a read-only commit finds pending operations, I can rework it.

[codecov]

Codecov Report

❌ Patch coverage is 82.35294% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 54.7484%. Comparing base (c1b532a) to head (f25990c).
⚠️ Report is 198 commits behind head on 8.0.x.

Files with missing lines	Patch %	Lines
...e/mapping/transactions/SessionOnlyTransaction.java	66.6667%	2 Missing ⚠️
.../datastore/mapping/mongo/AbstractMongoSession.java	75.0000%	1 Missing ⚠️

Additional details and impacted files

@@                Coverage Diff                 @@
##                8.0.x     #16214        +/-   ##
==================================================
+ Coverage     54.7365%   54.7484%   +0.0119%     
- Complexity      20470      20479         +9     
==================================================
  Files            2103       2103                
  Lines          101077     101086         +9     
  Branches        17928      17930         +2     
==================================================
+ Hits            55326      55343        +17     
+ Misses          37876      37867         -9     
- Partials         7875       7876         +1     

Files with missing lines	Coverage Δ
...ails/datastore/mapping/mongo/MongoTransaction.java	53.0612% <100.0000%> (+1.9974%)	⬆️
...grails/datastore/mapping/core/AbstractSession.java	69.5279% <100.0000%> (-0.1495%)	⬇️
...ping/transactions/DatastoreTransactionManager.java	53.6842% <100.0000%> (ø)
.../datastore/mapping/mongo/AbstractMongoSession.java	64.8936% <75.0000%> (-0.3238%)	⬇️
...e/mapping/transactions/SessionOnlyTransaction.java	70.0000% <66.6667%> (-5.0000%)	⬇️

... and 2 files with indirect coverage changes

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[testlens-app]

✅ All tests passed ✅

🏷️ Commit: f25990c
▶️ Tests: 18463 executed
⚪️ Checks: 89/89 completed

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