Fix log accumulation across DynamicData test invocations

[Evangelink]

Summary

Fixes #7846

When running DynamicData tests with multiple invocations, the TestContextImplementation output buffers (_stdOutStringBuilder, _stdErrStringBuilder, _traceStringBuilder) were never cleared between invocations. Since DynamicData tests reuse the same TestContext instance across all invocations, logs accumulated exponentially:

• Test 1 result: logs from test 1
• Test 2 result: logs from test 1 + test 2
• Test 3 result: logs from test 1 + test 2 + test 3
• ...

This caused >20 GB RAM usage for simple projects and truncated/broken log output in test summaries.

Root Cause

GetOut(), GetErr(), and GetTrace() on TestContextImplementation returned the StringBuilder contents via ToString() but never cleared them. In contrast, GetAndClearDiagnosticMessages() and GetResultFiles() already followed a get-then-clear pattern.

Fix

Replace GetOut(), GetErr(), GetTrace() with GetAndClearOutput(), GetAndClearError(), GetAndClearTrace() that clear the StringBuilder after extracting the value — matching the existing GetAndClearDiagnosticMessages() pattern.

Changes

• TestContextImplementation.cs — Replace non-clearing getters with GetAndClearOutput/Error/Trace methods
• TestMethodInfo.cs — Update call site in InvokeAsync finally block
• UnitTestRunner.cs — Update call sites in assembly init/cleanup
• TestClassInfo.cs — Update call sites in class init/cleanup
• TestMethodInfoTests.cs — Add 3 regression tests verifying buffers don't accumulate across invocations
• TestContextImplementationTests.cs — Update existing thread-safety test to use renamed methods

[Copilot]

Pull request overview

Fixes MSTest adapter log buffer accumulation across multiple DynamicData invocations by switching stdout/stderr/trace retrieval to a get-then-clear pattern, preventing exponential growth in captured output and memory usage.

Changes:

• Replace GetOut/GetErr/GetTrace with GetAndClearOutput/GetAndClearError/GetAndClearTrace in TestContextImplementation.
• Update execution pipeline call sites (test method invoke; assembly/class init & cleanup) to use the new draining APIs.
• Add regression unit tests to ensure output/error/trace don’t accumulate across multiple invocations.

Show a summary per file

File	Description
test/UnitTests/MSTestAdapter.PlatformServices.UnitTests/Services/TestContextImplementationTests.cs	Updates background-thread write test to use new get-and-clear APIs.
test/UnitTests/MSTestAdapter.PlatformServices.UnitTests/Execution/TestMethodInfoTests.cs	Adds regression tests validating buffers don’t accumulate across repeated InvokeAsync calls.
src/Adapter/MSTestAdapter.PlatformServices/Services/TestContextImplementation.cs	Introduces get-and-clear methods for stdout/stderr/trace buffers.
src/Adapter/MSTestAdapter.PlatformServices/Execution/UnitTestRunner.cs	Updates assembly init/cleanup log capture to drain buffers.
src/Adapter/MSTestAdapter.PlatformServices/Execution/TestMethodInfo.cs	Updates per-test invocation log capture to drain buffers.
src/Adapter/MSTestAdapter.PlatformServices/Execution/TestClassInfo.cs	Updates class init/cleanup log capture to drain buffers.

Copilot's findings

• Files reviewed: 6/6 changed files
• Comments generated: 1

[Evangelink]

Summary

Workflow: Test Expert Reviewer 🧪
Date: 2026-04-29
Repository: microsoft/testfx

Key Findings

1. [Coverage — Minor] WritesFromBackgroundThreadShouldNotThrow in TestContextImplementationTests.cs was updated for Output and Error but not for Trace. GetAndClearTrace() also calls Clear() under concurrency and should be included in the thread-safety test.

2. [Coverage — Important] No test in TestContextImplementationTests.cs directly verifies the clearing contract of the new GetAndClear* methods. The only test for these methods is the thread-safety check (which only asserts no exception). A dedicated read-then-clear test would document and protect the core invariant introduced by this fix.

3. [Coverage — Minor] Regression tests for log accumulation are scoped to TestMethodInfo.InvokeAsync. The TestClassInfo (class init/cleanup) and UnitTestRunner (assembly init/cleanup) call sites that were also changed have no equivalent accumulation tests.

Recommendations

• Add a GetAndClearOutput_ShouldReturnContentThenClearBuffer-style test (and equivalents for Error/Trace) in TestContextImplementationTests.cs to directly validate the clearing behavior.
• Extend WritesFromBackgroundThreadShouldNotThrow to also call GetAndClearTrace() for completeness.
• Consider adding accumulation regression tests for the class/assembly init-cleanup paths, or add a comment noting they're exercised by acceptance tests.

The existing accumulation tests in TestMethodInfoTests.cs are well-structured and correctly detect the regression: result2.LogOutput.Should().Be("invocation_output") would fail with the old non-clearing code (it would produce "invocation_outputinvocation_output"), so the core fix is properly verified.

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Generated by Test Expert Reviewer 🧪

🧪 Test quality reviewed by Test Expert Reviewer 🧪

[Evangelink]

Summary

Workflow: Expert Code Reviewer
Date: 2026-04-29
Repository: microsoft/testfx

Key Findings

[Threading — Minor/Accepted] GetAndClearOutput/Error/Trace each perform two individually-synchronized calls (ToString() + Clear()). Content written by a background thread in the TOCTOU window is silently discarded. This matches the pre-existing GetAndClearDiagnosticMessages() pattern and is an accepted design trade-off in this codebase. See inline comment.

Positive Observations

• Root cause addressed correctly: The TestContextImplementation output buffers now clear after each read, consistent with the established GetAndClearDiagnosticMessages pattern. The exponential memory growth scenario is fully closed.
• Bonus fix in multi-assembly cleanup: In UnitTestRunner.RunAssemblyCleanupAsync, the foreach over multiple TestAssemblyInfo entries previously accumulated logs across iterations (each GetOut() returned all prior logs); the new GetAndClearOutput() correctly scopes each iteration to only its own cleanup logs.
• Regression tests are valid: The three new public async Task tests in TestMethodInfoTests.cs are properly discoverable by TestFramework.ForTestingMSTest (which discovers all public parameterless methods returning void or Task in TestContainer subclasses — no [TestMethod] attribute required).
• Exclusive control flow: In both TestClassInfo.DoRunAsync and RunAssemblyCleanupAsync, the failure path (creates a new TestResult and returns) and the success path (lastResult +=) are mutually exclusive, so there is no double-clear or double-append risk.
• No public API surface changed: All three renamed methods remain internal.

Recommendations

• Consider adding a brief comment to the three new methods noting the intentional TOCTOU (consistent with GetAndClearDiagnosticMessages), to signal future maintainers that this is deliberate rather than an oversight.

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Generated by Expert Code Reviewer

🧠 Reviewed by Expert Code Reviewer 🧠

[Copilot]

Copilot's findings

• Files reviewed: 6/6 changed files
• Comments generated: 0 new

[Evangelink]

/backport to rel/4.2

[github-actions]

Started backporting to rel/4.2: https://github.com/microsoft/testfx/actions/runs/25104582574