Decrement active contexts count after a pooled DbContext instance is torn-down

[AndriySvyryd]

Fixes #38538

[Copilot]

Pull request overview

This PR addresses an instrumentation bug in EF Core’s microsoft.entityframeworkcore.active_dbcontexts metric when using DbContextPool: contexts created during pool overflow were not decrementing the active count after they were fully torn down, causing the metric to monotonically increase (Fixes #38538).

Changes:

• Add metric-focused tests to validate that active DbContext counts decrement when an overflowed pooled context is torn down.
• Add a pooling regression test asserting that re-leasing a fully torn-down pooled context throws rather than dereferencing a cleared configuration snapshot.
• Update DbContext pooling/disposal internals to handle “fully torn down” pooled contexts more explicitly.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
test/EFCore.Tests/Infrastructure/EntityFrameworkMetricsTest.cs	Adds tests covering active_dbcontexts behavior for pooled overflow tear-down scenarios (factory + scoped pooling).
test/EFCore.SqlServer.FunctionalTests/DbContextPoolingTest.cs	Adds a regression test for attempting to set a lease on a fully torn-down pooled context.
src/EFCore/DbContext.cs	Adjusts lease-setting and disposal logic to account for fully torn-down pooled contexts and metrics.

Comments suppressed due to low confidence (1)

src/EFCore/DbContext.cs:1136

• The new dispose guard else if (!_disposed || _configurationSnapshot != null) will run the full teardown path (including clearing _configurationSnapshot) even when the context has already been disposed but is just sitting idle in the pool (it retains a non-null _configurationSnapshot). That makes a second Dispose() on a pooled context (which is expected to be an idempotent no-op) fully tear the context down while it remains enqueued in the pool, corrupting the pool and causing subsequent rentals (which call SetLease) to start throwing ObjectDisposedException.

Consider keeping the idempotency guard here (only run teardown when transitioning to disposed), and instead make the pool’s teardown path explicitly trigger teardown+metrics (e.g., have IDbContextPoolable.ClearLease() also mark the context as not-disposed so the following Dispose() performs teardown, or introduce a dedicated pool-only teardown method).

        else if (!_disposed || _configurationSnapshot != null)
        {
            EntityFrameworkMetricsData.ReportDbContextDisposing();

            _dbContextDependencies?.InfrastructureLogger.ContextDisposed(this);