Add cDAC no-fallback test leg to runtime-diagnostics pipeline by max-charlamb · Pull Request #126752 · dotnet/runtime

max-charlamb · 2026-04-10T16:15:48Z

Note

This PR description was generated with the assistance of GitHub Copilot.

Summary

Add a granular, per-method allowlist (LegacyFallbackHelper) that controls which delegation-only APIs may fall back to the legacy DAC when CDAC_NO_FALLBACK=1 is set. This enables selective no-fallback testing — blocking fallback for most APIs while allowing specific APIs that are known to not yet be implemented in the cDAC.

Wire this into the runtime-diagnostics CI pipeline using the -noFallback flag from dotnet/diagnostics#5806.

All fallback attempts (both allowed and blocked) are logged to stderr with method name, file, and line number for capture by the diagnostics test infrastructure.

Changes

LegacyFallbackHelper.cs — Granular fallback control

New static helper that every delegation-only call site invokes via CanFallback(). Uses [CallerMemberName], [CallerFilePath], and [CallerLineNumber] to identify the call site.

Normal mode (CDAC_NO_FALLBACK unset): Always returns true (single bool check, [AggressiveInlining])
No-fallback mode (CDAC_NO_FALLBACK=1): Checks method name against a HashSet<string> allowlist and file name against a file-level allowlist

Per-method allowlist:

Method	Reason
`EnumMemoryRegions`	Dump creation — cDAC has no memory enumeration implementation
`GetInterface`	IMetaDataImport QI (PR #127028)
`GetMethodDefinitionByToken`	IXCLRDataModule — not yet implemented in cDAC
`IsTrackedType`	GC heap analysis (PR #125895)
`TraverseLoaderHeap`	Loader heap traversal (PR #125129)

File-level allowlist:

File	Reason
`DacDbiImpl.cs`	Entire DBI/ICorDebug interface (122 methods) — deferred

Entrypoints.cs — Simplified creation

Both CreateSosInterface and CreateDacDbiInterface now follow the same pattern: the legacy implementation is always passed through, and LegacyFallbackHelper.CanFallback() at each call site decides whether to delegate. Removed prevent_release, noFallback env var check, and null-legacy-ref logic.

13 Legacy wrapper files — Instrumented delegation sites

All 296 delegation-only methods across all legacy wrapper files now call LegacyFallbackHelper.CanFallback():

SOSDacImpl.cs (12 methods)
SOSDacImpl.IXCLRDataProcess.cs (38 methods, Flush() intentionally excluded — cache management)
ClrDataModule.cs (29 methods + IMetaDataImport QI)
DacDbiImpl.cs (122 methods)
Other wrappers: ClrDataTask.cs, ClrDataExceptionState.cs, ClrDataFrame.cs, ClrDataValue.cs, ClrDataTypeInstance.cs, ClrDataMethodInstance.cs, ClrDataStackWalk.cs, ClrDataProcess.cs

CI Pipeline — `-noFallback` flag

Updated runtime-diag-job.yml to accept a noFallback parameter that passes -noFallback to the diagnostics build script. The cDAC_no_fallback leg in runtime-diagnostics.yml now uses noFallback: true instead of setting CDAC_NO_FALLBACK as a pipeline-level environment variable. The -noFallback flag (from dotnet/diagnostics#5806) properly:

Sets DOTNET_ENABLE_CDAC=1 and CDAC_NO_FALLBACK=1 on the debugger process
Defines CDAC_NO_FALLBACK_TESTING to skip ClrStack -i tests (ICorDebug not implemented in cDAC)

Stderr logging

Every fallback attempt is logged to stderr in the format:

[cDAC] Allowed fallback: CreateStackWalk at DacDbiImpl.cs:590
[cDAC] Blocked fallback: SomeMethod at SOSDacImpl.cs:123

The diagnostics test infrastructure (ProcessRunner) captures stderr and routes it to xunit test output with STDERROR: prefix, making fallback usage visible in test results.

Test Results

With CDAC_NO_FALLBACK=1 and the current allowlist, running the full SOS test suite against a private runtime build:

24 passed, 2 failed (flaky/pre-existing), 2 skipped (Linux-only)
0 blocked fallbacks

Motivation

The existing cDAC test leg always has the legacy DAC as a fallback, so unimplemented APIs are silently handled. The granular no-fallback mode makes gaps visible per-method, helping track progress toward full cDAC coverage while keeping tests green for known-deferred APIs.

Copilot

Pull request overview

Adds a new “cDAC no-fallback” diagnostics test leg and a corresponding runtime switch so diagnostics tests can run with cDAC without delegating unimplemented APIs to the legacy DAC.

Changes:

Entrypoints.cs: honors CDAC_NO_FALLBACK=1 by passing null as the legacy DAC implementation to SOSDacImpl.
runtime-diagnostics.yml: adds a cDAC_no_fallback job that runs diagnostics tests with useCdac: true and CDAC_NO_FALLBACK=1.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
`src/native/managed/cdac/mscordaccore_universal/Entrypoints.cs`	Adds an env-var controlled path to disable legacy DAC fallback in `CreateSosInterface`.
`eng/pipelines/runtime-diagnostics.yml`	Introduces a third diagnostics test leg to run cDAC tests with fallback disabled.

Copilot

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

Copilot

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 5 comments.

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Copilot

Pull request overview

Copilot reviewed 16 out of 16 changed files in this pull request and generated 2 comments.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

- Revert AuxiliarySymbolInfo.Address type change (unrelated to this PR) - Restore CanFallback() before _legacyModulePointer check so fallback attempts are always logged Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot

Pull request overview

Copilot reviewed 15 out of 15 changed files in this pull request and generated 2 comments.

The managed cDAC reads this field with ReadCodePointerField() which asserts the declared type is CodePointer. Using T_POINTER causes Debug.Assert crashes in checked builds and incorrect address comparisons on ARM32 (missing THUMB_CODE tag bit). Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

max-charlamb · 2026-04-17T20:38:23Z

/ba-g no logical changes since last clean test run

Copilot

Pull request overview

Copilot reviewed 16 out of 16 changed files in this pull request and generated 2 comments.

Comments suppressed due to low confidence (1)

src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Legacy/ClrDataModule.cs:64

LegacyFallbackHelper.CanFallback() is called unconditionally at the start of GetInterface(). In no-fallback mode this will log an “Allowed/Blocked fallback” even when _legacyModulePointer == 0, or when the requested iid is not IID_IMetaDataImport (i.e., no legacy QI/delegation is attempted). Consider moving the CanFallback() call so it only runs when _legacyModulePointer != 0 and iid == IID_IMetaDataImport (right before Marshal.QueryInterface).

        if (!LegacyFallbackHelper.CanFallback() || _legacyModulePointer == 0)
            return CustomQueryInterfaceResult.NotHandled;

        // Legacy DAC implementation of IXCLRDataModule handles QIs for IMetaDataImport by creating and
        // passing out an implementation of IMetaDataImport. Note that it does not do COM aggregation.
        // It simply returns a completely separate object. See ClrDataModule::QueryInterface in task.cpp
        if (iid == IID_IMetaDataImport && Marshal.QueryInterface(_legacyModulePointer, iid, out ppv) >= 0)
            return CustomQueryInterfaceResult.Handled;

Copilot · 2026-04-17T20:41:55Z

    }
    int ISOSDacInterface.GetAssemblyLocation(ClrDataAddress assembly, int count, char* location, uint* pNeeded)
-        => _legacyImpl is not null ? _legacyImpl.GetAssemblyLocation(assembly, count, location, pNeeded) : HResults.E_NOTIMPL;
+        => LegacyFallbackHelper.CanFallback() && _legacyImpl is not null ? _legacyImpl.GetAssemblyLocation(assembly, count, location, pNeeded) : HResults.E_NOTIMPL;


LegacyFallbackHelper.CanFallback() is evaluated before checking _legacyImpl is not null, so in CDAC_NO_FALLBACK=1 mode this will log an “Allowed/Blocked fallback” even when there is no legacy implementation to actually delegate to (and adds overhead for the null case). Consider reordering the condition to check _legacyImpl first and only call CanFallback() when delegation is possible.

Suggested change

=> LegacyFallbackHelper.CanFallback() && _legacyImpl is not null ? _legacyImpl.GetAssemblyLocation(assembly, count, location, pNeeded) : HResults.E_NOTIMPL;

=> _legacyImpl is not null && LegacyFallbackHelper.CanFallback() ? _legacyImpl.GetAssemblyLocation(assembly, count, location, pNeeded) : HResults.E_NOTIMPL;

Copilot · 2026-04-17T20:41:56Z

+    internal static bool CanFallback(
+        [CallerMemberName] string name = "",
+        [CallerFilePath] string file = "",
+        [CallerLineNumber] int line = 0)
+    {
+        if (!s_noFallback)
+            return true;
+


The PR description mentions CanFallback() is [AggressiveInlining] in normal mode, but the method currently has no MethodImplOptions.AggressiveInlining attribute. If the intent is to keep the fast-path overhead minimal (this helper is called at many delegation sites), consider adding the attribute (or update the PR description if this is no longer intended).

> [!NOTE] > This PR was generated with the assistance of GitHub Copilot. ## Summary Implement `IXCLRDataModule.GetMethodDefinitionByToken` in the cDAC `ClrDataModule` wrapper instead of delegating entirely to the legacy DAC. Remove it from the no-fallback allowlist and add newly-exposed downstream methods. ## Changes **`ClrDataModule.cs`** — Replaced the one-line legacy delegation stub with a full cDAC implementation that: 1. Calls the legacy implementation first (if available) for DEBUG parity validation 2. Validates the token is an `mdtMethodDef` (throws `ArgumentException` → `E_INVALIDARG`) 3. Creates a `ClrDataMethodDefinition` with the token, module address, and legacy method definition 4. Includes `#if DEBUG` `ValidateHResult` check against the legacy result **`LegacyFallbackHelper.cs`** — Removed `GetMethodDefinitionByToken` from the no-fallback allowlist. Added four `IXCLRDataMethodDefinition` methods that are now exposed as blocked fallbacks because `GetMethodDefinitionByToken` returns a cDAC-managed wrapper instead of a legacy object: - `StartEnumInstances` - `GetName` - `SetCodeNotification` - `HasClassOrMethodInstantiation` These methods are not yet implemented in the cDAC and must remain on the allowlist until they are. ## Pattern Follows the same pattern used by `EnumMethodDefinitionByName` (line 302) which already creates `ClrDataMethodDefinition` objects in the same way. The C++ legacy implementation (`task.cpp:2215-2251`) validates the token type, then calls `ClrDataMethodDefinition::NewFromModule` which simply creates a `ClrDataMethodDefinition` — the cDAC version mirrors this behavior. ## Motivation This enables removing `GetMethodDefinitionByToken` from the cDAC no-fallback allowlist (#126752). --------- Co-authored-by: Max Charlamb <maxcharlamb@microsoft.com> Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot AI review requested due to automatic review settings April 10, 2026 16:15

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Comment thread src/native/managed/cdac/mscordaccore_universal/Entrypoints.cs Outdated

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[cDAC] Allow testing without fallback #119078

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[cDAC] Track no-fallback allowlist — remaining unimplemented APIs #126760

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Comment thread .../managed/cdac/Microsoft.Diagnostics.DataContractReader.Legacy/SOSDacImpl.IXCLRDataProcess.cs

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System.IO.Tests.Directory_Delete_MountVolume.RunTest failure #125295

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Multiple Helix work items fail on maccatalyst/tvos CoreCLR Release #126460

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Comment thread src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Legacy/Dbi/DacDbiImpl.cs Outdated

Comment thread ...cdac/Microsoft.Diagnostics.DataContractReader.Legacy/SOSDacImpl.ICLRDataEnumMemoryRegions.cs

Remove unused _prevent_release field from DacDbiImpl

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Comment thread src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Legacy/LegacyFallbackHelper.cs

Comment thread src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Legacy/LegacyFallbackHelper.cs

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[cDAC] Implement GetMethodDefinitionByToken in ClrDataModule #127091

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Remove AggressiveInlining from CanFallback

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max-charlamb merged commit 41cf879 into main Apr 17, 2026
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[main] Source code updates from dotnet/runtime dotnet/dotnet#6098

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	=> LegacyFallbackHelper.CanFallback() && _legacyImpl is not null ? _legacyImpl.GetAssemblyLocation(assembly, count, location, pNeeded) : HResults.E_NOTIMPL;
	=> _legacyImpl is not null && LegacyFallbackHelper.CanFallback() ? _legacyImpl.GetAssemblyLocation(assembly, count, location, pNeeded) : HResults.E_NOTIMPL;

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Summary

Changes

LegacyFallbackHelper.cs — Granular fallback control

Entrypoints.cs — Simplified creation

13 Legacy wrapper files — Instrumented delegation sites

CI Pipeline — -noFallback flag

Stderr logging

Test Results

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max-charlamb commented Apr 17, 2026

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max-charlamb commented Apr 10, 2026 •

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CI Pipeline — `-noFallback` flag