Provision each sandbox test environment once, atomically - #1337
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WalkthroughThe sandbox fixture now uses a thread-safe cache of lazy initialization tasks. ChangesSandbox settings caching
Priority: ⬇️ Low Estimated code review effort: 2 (Simple) | ~10 minutes Merge Risk: 🟡 Moderate · up to Sandbox settings initialization is now atomic per environment, but a transient provisioning failure can become permanent for that environment and cause all subsequent dependent tests to fail. Retry-safe cache eviction should be added before merge. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
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Inline comments:
In `@src/IO.Ably.Tests.Shared/Infrastructure/AblySandboxFixture.cs`:
- Line 37: Update the settings initialization flow around Initialise and
Settings.GetOrAdd so that when the created Lazy task faults, the exact
corresponding Lazy instance is removed from Settings before the failure is
propagated, allowing later GetSettings calls to retry provisioning.
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src/IO.Ably.Tests.Shared/Infrastructure/AblySandboxFixture.cs
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AblySandboxFixture.GetSettings cached into a plain Dictionary from a static method, with an
await between the ContainsKey check and the write. xunit.runner.json sets
parallelizeTestCollections, and the sandbox is reached from several collections at once -
ChannelSubscriptionsTests declares none, so it gets an implicit one of its own - so those
writes race.
A concurrently written Dictionary can leave its indexer returning null for a key
ContainsKey has just accepted, and callers dereference what they are handed:
System.NullReferenceException
at IO.Ably.Tests.SandboxSpecs.GetRestClient (SandboxSpecs.cs:85)
at IO.Ably.Tests.Push.PushAdminSandboxTests.ChannelSubscriptionsTests
.ShouldSuccessfullySetAndUpdateChannelSubscription
The same window also let two collections each provision an app.
A Lazy over a ConcurrentDictionary runs Initialise once per environment however many callers
race, and hands all of them the same task. It also passes the environment through, which the
old code dropped: Initialise defaults to "sandbox", so asking for anything else provisioned a
sandbox app and cached it under the other environment's name. Nothing does that today, so
that half is a latent trap rather than a live bug.
A failed attempt is evicted rather than cached. The Dictionary this replaces assigned only
on success, so a provisioning attempt that threw was retried by whoever asked next; keeping
the faulted task would have failed every remaining sandbox test in the run with the same
exception, which is worse than the race it fixes. Removal compares key and value together,
so a caller that has already raced in a replacement keeps it.
The NullReferenceException is intermittent by nature and did not reproduce locally in two
full sandbox runs, so this removes the race by construction rather than by a failing test.
Those runs did confirm it changes nothing else: 5 failed / 230 passed / 12 skipped both with
and without the change, four of the five the same tests, all pre-existing flakes under
parallel sandbox load.
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Fixes an intermittent
NullReferenceExceptionthat fails sandbox tests on CI, seen most recently as:Why it happens
AblySandboxFixture.GetSettingscaches provisioned app settings into a plainDictionaryfrom a static method, with anawaitbetween theContainsKeycheck and the write:xunit.runner.jsonsetsparallelizeTestCollections, and the sandbox is reached from several collections concurrently —SandBox Connection,AblyRest SandBox Collection,SandBox Collection,Presence Sandbox,Channel SandBox, plusChannelSubscriptionsTests, which declares no[Collection]and so gets an implicit one of its own. Those writes race.A concurrently written
Dictionarycan be left with its indexer returningnullfor a keyContainsKeyhas just accepted.SandboxSpecs.GetRestClientthen dereferences the settings it was handed, which is the exception above. The same window also allowed two collections to each provision an app.The change
A
Lazy<Task<…>>over aConcurrentDictionary, soInitialiseruns once per environment however many callers race and all of them get the same task.It also passes the environment through to
Initialise, which the old code dropped —Initialisedefaults to"sandbox", so requesting any other environment provisioned a sandbox app and cached it under the other environment's name. Nothing requests a non-default environment today, so that half is a latent trap rather than a live bug, but it's a cheap thing to close while here.On verification
The exception is intermittent by nature and did not reproduce locally across two full sandbox runs, so this removes the race by construction rather than by a failing test. Those runs did confirm the change is inert otherwise — 5 failed / 230 passed / 12 skipped both with and without it, four of the five being the same tests, all pre-existing flakes under parallel sandbox load.
Discounted
Guarding with a lock or
SemaphoreSlimwould also work, butLazyoverConcurrentDictionaryneeds no lock discipline at the call site and gives single-shot initialisation for free.Found while investigating a CI failure on #1331, which turned out to be unrelated to that PR's changes —
AblySandboxFixtureis untouched there.Summary by CodeRabbit