Optimize binary logger metadata serialization

[AlesProkop]

Related to #10490

Context

Binary log profiling identified task-item metadata enumeration and hashing in BuildEventArgsWriter.WriteTaskItemList as the dominant serialization cost. Copy-on-write task items frequently share the same immutable metadata dictionary, but the binary logger previously enumerated, unescaped, and hashed that metadata for every item.

Changes Made

• Added an identity cache from immutable backing metadata dictionaries to previously emitted NameValueList record IDs.
• Reused cached record IDs for items sharing copy-on-write metadata.
• Enumerated immutable backing metadata directly on cache misses while preserving existing key ordering and value unescaping.
• Preserved the existing fallback for TaskItemData, custom ITaskItem implementations, and .NET Framework transparent proxies.
• Short-circuited empty backing metadata to the existing zero-record encoding.
• Documented the escaped-value and ordering contract of IMetadataContainer.BackingMetadata.

Testing

• Built the full repository — 0 warnings, 0 errors.
• Built and replayed a binary log for src/Samples/Dependency/Dependency.csproj with the bootstrap MSBuild.
• Added and ran focused tests in BuildEventArgsSerializationTests:
  • TaskParameterSerializationReusesSharedBackingMetadata — verifies reuse for shared Utilities.TaskItem metadata.
  • TaskParameterSerializationCachesEmptyMetadataAndFallsBackForTaskItemData — verifies empty metadata and the TaskItemData fallback.
  • TaskParameterSerializationReusesItemDefinitionMetadata — verifies reuse for shared item-definition metadata.
  • TaskParameterBackingMetadataFastPathPreservesSerializedBytes — verifies byte-for-byte equivalence with the legacy fallback path.
  • Result: 16 passed, 0 failed on net11.0; 16 passed, 0 failed on net472.

Notes

• The binary log format is unchanged and does not require a format-version increment.
• Matched OrchardCore warm-build measurements with ProjectImports=None reduced median wall time from 27.20 seconds to 23.20 seconds under MT/server execution.
• Default out-of-process /m measurements reduced median wall time from 41.44 seconds to 39.79 seconds.
• A 20-round small-project control showed no fixed-cost regression.

[Copilot]

🟢 Approval recommended

The optimization is localized, preserves the existing on-wire encoding, and is covered by targeted tests including byte-for-byte equivalence checks.

Pull request overview

This PR optimizes binary logger serialization of task item metadata by memoizing previously-emitted NameValueList record IDs for shared copy-on-write immutable metadata dictionaries, reducing repeated enumeration, unescaping, and hashing work in BuildEventArgsWriter.WriteTaskItemList.

Changes:

• Added a ConditionalWeakTable<ImmutableDictionary<string,string>, StrongBox<int>> cache to reuse NameValueList record IDs for items sharing the same immutable backing metadata dictionary.
• Introduced a fast path that enumerates immutable backing metadata directly on cache misses while preserving the existing unescape behavior and metadata ordering contract.
• Added focused unit tests covering shared-metadata reuse, empty-metadata short-circuiting, TaskItemData fallback behavior, and byte-for-byte equivalence with the legacy path.

File summaries

File	Description
src/Framework/IMetadataContainer.cs	Documents the escaped-value + ordering contract required for BackingMetadata to be safely consumable for serialization fast paths.
src/Build/Logging/BinaryLogger/BuildEventArgsWriter.cs	Implements metadata record-id identity caching and updates WriteNameValueList to return the emitted record id for reuse.
src/Build.UnitTests/BuildEventArgsSerialization_Tests.cs	Adds regression/compat tests validating reuse, fallbacks, and byte-for-byte serialization equivalence.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 0
• Review effort level: Lite

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