Port StreamWriter leaveOpen:true dispose fix (runtime#125189) — Impact Assessment

[ViveliDuCh]

Goal

Assess whether dotnet/runtime#125189 — which fixes StreamWriter.Dispose() not setting _disposed = true when leaveOpen: true — should be merged into dotnet/runtime, by porting the changes into this VMR fork and measuring the blast radius across the entire .NET codebase.

Upstream issue: dotnet/runtime#89646

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What this branch contains

All 5 file changes from runtime#125189, ported into src/runtime/:

File	Change
StreamWriter.cs	Core fix — _closable moved from outer guard to inner if, so _disposed = true executes unconditionally
Console.cs	Reactive fix — new NonClosableStreamWriter subclass prevents Console.Out/Console.Error from breaking
StreamWriter.CloseTests.cs	Tests expanded to cover leaveOpen: true disposal
StreamWriter.DisposeAsync.cs	New DisposeAsync tests for disposed state
StreamReaderTests.cs	Parallel test coverage for StreamReader consistency

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Investigation Summary

More details in: https://gist.github.com/ViveliDuCh/e6e796a88f14d8a9918b67b06e3fe431

VMR-wide codebase search (25+ repos, 178,000+ files)

• 46 files use StreamWriter with leaveOpen: true (9 production, 37 test)
• 0 files contain the breaking pattern (writing to a disposed StreamWriter)
• 5 StreamWriter subclasses found — none affected
• Every usage follows the intended pattern: dispose writer → reuse stream

Test results

Core test suites (directly exercise the fix):

Test Suite	Total	Passed	Failed	Regressions
System.IO.Tests	1,713	1,712	1 (pre-existing, unrelated)	0
System.Console.Tests	4,266	4,266	0	0

Extended test suites (exercise leaveOpen consumers found in codebase search):

Test Suite	Consumer(s) Tested	Total	Passed	Failed	Regressions
System.IO.Tests (TestLeaveOpen)	StreamWriter/Reader leaveOpen contract	4	4	0	0
System.Formats.Tar.Tests	TarReader/TarWriter with leaveOpen streams	6,384	6,363	0	0
System.Xml.Linq.SDMSample.Tests	BridgeHelpers (29 StreamWriter leaveOpen usages)	110	110	0	0
NuGet.Packaging.Test	KeyPairFileWriter (net10.0 TFM)	2,944	2,905	0	0
Microsoft.NET.Sdk.Razor.Tool.Tests	ServerCommand.cs (leaveOpen StreamWriter)	50	37	13 (infra)	0

Previously blocked — now unblocked (after disk resize + repo bootstrapping):

Test Suite	Consumer(s) Tested	Total	Passed	Failed	Regressions
GetDocumentInsider.Tests	GetDocumentCommandWorker.cs (aspnetcore)	14	14	0	0
Microsoft.Build.Tasks.UnitTests	ManifestWriter.cs (msbuild)	3,389	3,318	0	0

Combined: 18,874 total tests | 18,729 passed | 0 regressions

The 13 Razor Tool failures are SdkTestContext initialization errors (missing redist build artifact) — the 2 ServerCommand tests that directly exercise StreamWriter + leaveOpen both passed.

Key findings

1. Zero internal breaking patterns. The only .NET code that depended on this bug was Console.Out/Console.Error (for 12+ years), which the PR addresses with NonClosableStreamWriter.
2. Correctness alignment. StreamReader has always set _disposed = true regardless of leaveOpen. This fix makes StreamWriter consistent.
3. Safe failure mode. Any external code that breaks will get a clear ObjectDisposedException at the exact call site — no silent corruption.
4. Bounded external risk. The affected pattern (use-after-dispose) is documented as erroneous. Shipping in Preview 3 provides early signal before GA.

Reviewer concerns addressed

• "Is there value in fixing behavior that has existed forever?" This is not a recent regression — the behavior dates back to when leaveOpen was introduced. Our finding: the measured blast radius is zero across the VMR, the correctness value is clear (IDisposable contract consistency with StreamReader), and there is no evidence of problems.
• "The reactive Console fix hints at considerable noise." The concern is that needing to fix Console.Out/Console.Error suggests widespread dependents. Our finding: the VMR-wide search shows zero other code depending on this bug. The Console dependency was the sole exception, not the rule.

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Recommendation

✅ Merge the fix into dotnet/runtime.

The change is correct, the measured impact is zero within .NET's own codebase (18,874 tests across 9 suites, 0 regressions — including all previously blocked suites now unblocked and passing), and the risk to external consumers is bounded by Preview 3 feedback and a trivial workaround. Not fixing it preserves a 12-year inconsistency between StreamWriter and StreamReader that undermines the IDisposable contract.

Suggested follow-up actions

• Document as a breaking change in dotnet/docs
• Monitor Preview 3 feedback for external reports of breakage
• Consider adding an analyzer rule to detect use-after-dispose on StreamWriter
• If significant external breakage emerges during preview, consider adding an AppContext switch as a temporary compat bridge