[bartlomieju]

Ports MemoryCache.read() to Rust JSG, behind a new RUST_MEMORY_CACHE autogate. This is the last PR of a stack: #7963 → #8030 → #8034 → this one.

Note

This branch includes a cherry-pick of #8064 (Drop Rust JSG promise reaction data once a reaction runs), which the port depends on. When #8064 lands, I'll rebase and that commit will drop out. Please review it in #8064.

Commits to review

1. Add Local::to_detail_string() to Rust JSG. Wraps v8::Value::ToDetailString(). It describes a value the way C++ kj::str() of a v8::Local does ("TypeError: message", "[object Object]", and so on) without running any JavaScript. The Rust MemoryCache uses it for a failed fallback's fallback_error tag.
2. Port MemoryCache.read() to Rust, behind an autogate.
   • src/workerd/api/memory-cache-api holds the Rust MemoryCache. It sits on top of the cache in src/rust/memory-cache and uses the io-context crate's await_io(), add_object() and trace spans.
     • lib.rs: read(), key validation, and cache hits.
     • fallback.rs: a read that waits as the leader (until it runs its fallback) or as a waiter (until the read it waits for finishes).
     • trace.rs: the same spans and tags that memory-cache.c++ records.
   • MemoryCacheProvider::bindForRust() gives the Rust implementation the same shared namespaces the C++ one uses.
   • workerd-api.c++ creates the Rust MemoryCache for memory cache bindings when the autogate is enabled.

Behaviour

The Rust implementation is meant to be indistinguishable from the C++ one, including its trace spans. A few details:

• Cache errors become internal errors, as in C++.
• The fallback gets the key decoded from the UTF-8 bytes the cache holds. A lone surrogate therefore reaches the fallback as U+FFFD for each of its three bytes, the same as in C++.
• A fallback whose request ends is abandoned with that request, so another read runs its own fallback. It is not left waiting until the garbage collector drops the promise.
• The read resumes inside a Durable Object's blockConcurrencyWhile() critical section, rather than waiting on the input gate.

Testing

• Span parity: a new test in memory-cache-test.c++ runs the same scenario against both implementations and checks they record the same spans.

  • The scenario covers misses and hits, coalesced fallbacks, a failed leader followed by a waiter running its own fallback, sync and async throws, a NaN expiration, a rejection with a non-Error, a value that is too large, an expiration in the past, and eviction.
  • SpanRecorder now records spans in the order they open, not the order they end. When each implementation releases its spans is an implementation detail.

• New JS tests in memory-cache-test.js:

  • a fallback abandoned when its request ends;
  • the async context a fallback runs in;
  • the key a fallback gets for a lone surrogate;
  • a NaN expiration;
  • a fallback inside blockConcurrencyWhile().

  The @all-autogates variant runs all the JS tests against the Rust implementation.

Known gap

Rust JSG doesn't throw DOMExceptions yet (#7951). For a value that can't be serialized, Rust throws a plain Error where C++ throws a DataCloneError, so the fallback_error tags differ. The parity scenario leaves that case out, with a TODO(#7951).

[codecov-commenter]

Codecov Report

❌ Patch coverage is 87.80919% with 69 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (biwanczuk/rust-io-context-trace-spans@73517e9). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
src/workerd/api/memory-cache-api/fallback.rs	85.90%	31 Missing ⚠️
src/rust/jsg/promise.rs	44.44%	15 Missing ⚠️
src/workerd/api/memory-cache-api/lib.rs	90.83%	11 Missing ⚠️
src/workerd/api/memory-cache-test.c++	86.36%	3 Missing and 6 partials ⚠️
src/workerd/api/memory-cache-api/trace.rs	96.42%	3 Missing ⚠️

Additional details and impacted files

@@                           Coverage Diff                            @@
##             biwanczuk/rust-io-context-trace-spans    #8066   +/-   ##
========================================================================
  Coverage                                         ?   70.04%           
========================================================================
  Files                                            ?      706           
  Lines                                            ?   182826           
  Branches                                         ?    29286           
========================================================================
  Hits                                             ?   128062           
  Misses                                           ?    37635           
  Partials                                         ?    17129           

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