Use no-op MD Importer for DIA-backed stacktraces

[hoyosjs]

When resolving line numbers and sequence points for classic PDB-backed modules, Module::GetISymUnmanagedReader on main passes a real metadata importer into DIA. Materializing that importer can switch metadata access to the RW-backed variant, and later metadata reads can contend on its lock-heavy path.

This change avoids that transition by passing a process-wide no-op importer (NoopMetadataImport) that only satisfies the binder's non-null COM contract. Since it contains no module-linked state, it is a singleton and has no collectible-lifetime coupling.

NoopMetadataImport behavior:

• QI supports IUnknown, IMetaDataImport, IMetaDataImport2 and does not expose IID_IGetIMDInternalImport.
• AddRef/Release are process-lifetime no-ops.
• Metadata APIs assert in Checked/Debug and return E_NOTIMPL in Release.

The binder requires a non-null metadata import interface, but the StackTrace/source-line in-proc path does not need metadata signatures from that importer. A no-op importer keeps the binder contract while avoiding the RW swap.

Performance tests

Same repro from issue: https://gist.github.com/jkotas/617109b64d9b0ad52b491cdf021b2f8a

Machine:

• 6 physical cores / 12 logical processors

Experiment:

• 2x2x2 matrix: baseline vs change, full vs portable PDB, PDB present vs deleted
• 5 repetitions per cell
• 20 seconds per repetition
• Throughput metric: exceptions/sec in the harness

Results (avg exceptions/sec):

• full + PDB present: 865 -> 855 (0.99x)
• portable + PDB present: 14,115 -> 14,264 (1.01x)
• portable + no PDB: 35,843 -> 36,545 (1.02x)
• full + no PDB: 10,848 -> 26,381 (2.43x)

This matches the target scenario (full + no PDB) and keeps non-target cells effectively unchanged.

Fixes #90563

[Copilot]

Pull request overview

This PR changes CoreCLR’s symbol-reader setup in Module::GetISymUnmanagedReader to avoid materializing a module’s real public metadata importer when constructing DIA-backed symbol readers, by instead passing a process-wide inert IMetaDataImport2 implementation.

Changes:

• Introduces NoopMetadataImport, a singleton IMetaDataImport2 implementation intended only to satisfy the binder’s non-null metadata-import parameter.
• Updates Module::GetISymUnmanagedReader to pass the noop importer to ISymUnmanagedBinder::GetReaderFromStream / GetReaderForFile instead of using the module’s importer.
• Wires the new files into the VM build via src/coreclr/vm/CMakeLists.txt.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/coreclr/vm/noopmetadataimport.h	Declares NoopMetadataImport singleton implementing IMetaDataImport2 as an inert binder placeholder.
src/coreclr/vm/noopmetadataimport.cpp	Implements singleton creation, COM QI/AddRef/Release behavior, and E_NOTIMPL stubs for metadata APIs.
src/coreclr/vm/CMakeLists.txt	Adds the new noop importer source/header to VM build lists.
src/coreclr/vm/ceeload.cpp	Switches symbol-reader binder calls to use NoopMetadataImport::GetInstance() instead of module metadata importers.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 15 out of 15 changed files in this pull request and generated 4 comments.

[jkotas]

fullpdbstacktrace is failing on CoreCLR, and the tests need to be disabled on Mono

[Copilot]

Pull request overview

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

[Copilot]

Pull request overview

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

[Copilot]

Pull request overview

Copilot reviewed 15 out of 15 changed files in this pull request and generated 4 comments.

[hoyosjs]

/ba-g fix for wasm is merged in main