[CaffeinatedCoder]

Fixes #312.

Problem

In collection-per-file mode (PageFileConfig.Server), every read, write and free of a collection page resolves the page's slot to a collection name without taking _collectionSlotLock (GetPageFile → GetCollectionNameBySlot). The slot → name map was a plain Dictionary<int, string>. It is written under that lock whenever a collection gets its slot.

A lookup that runs while the dictionary resizes can miss. The resizes happen at the 4th, 8th, 18th and 38th collection. On a miss, GetPageFile falls through to the main file, and the operation fails with InvalidDataException: Page … is outside the page file.

Change

• StorageEngine.cs: _collectionSlotToName is now a ConcurrentDictionary<int, string>. The comment on the registry says why both maps are read without the lock.
• StorageEngine.Memory.cs: a new slot is published slot → name before name → slot, in GetOrAssignCollectionSlot and in LoadCollectionSlots. A thread that finds the slot by name, such as GetCollectionPageIds from another session's collection instance, then also finds the file by the slot.
• Test: MultiFileStorageTests.CollectionPageRouting_WhileCollectionsAreCreated_FindsTheCollectionFile. Reader threads call the private slot lookup through a delegate, and one of them reads a page of collection c0 through ReadPage. Meanwhile the test assigns the other 63 slots of the engine through the private GetOrAssignCollectionSlot, which skips creating the files so that a round stays cheap. It runs 100 rounds.

	Old code	This change
Test result	10 / 10 runs fail (also with DOTNET_PROCESSOR_COUNT=2), first miss between rounds 14 and 63, about 1 s	10 / 10 pass, about 2 s

An earlier version of the test, which created the 63 collection files and used only ReadPage readers, also failed on the old code with the issue's exact error (Page 3221225536 is outside the page file (file holds 65 pages)). It did so in only 2 of 6 runs, which is why the test calls the slot lookup directly.

Tests

• dotnet test tests/BLite.Tests -c Release: 3093 passed, 4 skipped, 0 failed.
• BLite.Core builds for net10.0 and netstandard2.1. BLite.NetStandard21.Tests builds, but I could not run it here because it needs the .NET 6 runtime.

Independent of #313 (the checkpoint drain fix for #311): different files, no conflicts.

🤖 Generated with Claude Code

[mrdevrobot]

Verified: the new test fails 3 of 3 on 0450b97 and passes here; full suite passes. Minimal change (ConcurrentDictionary plus reversed publication order).