perf(grpc-js): add synchronous fast path for FilterStack and CompressionFilter

[olavloite]

Previously, FilterStack and CompressionFilter always executed sendMetadata, sendMessage, and receiveMessage through async functions and Promise chains—even when no compression was used and no asynchronous filters were present in the stack. As a result, every uncompressed RPC had to allocate multiple intermediate Promises and closures and hop through the microtask queue for metadata filtering, request framing, and response deframing.

This change adds an optional synchronous fast path (sendMetadataMaybeSync, sendMessageMaybeSync, and receiveMessageMaybeSync) to Filter, FilterStack, CompressionFilter, and ResolvingCall.

• Synchronous metadata and uncompressed message framing/deframing: CompressionHandler.writeMessage and readMessage now return a Buffer synchronously when no compression or decompression is required (such as identity encoding or when WriteFlags.NoCompress is set), only returning a Promise when zlib compression or decompression is actually needed.

• Zero-allocation pass-through for BaseFilter methods: FilterStack skips methods inherited unchanged from BaseFilter (such as pass-through methods on RouterFilter) without allocating a Promise.

• Shared IDENTITY_HANDLER singleton: IdentityHandler has no instance state, so CompressionFilter now reuses a module-level singleton instead of allocating two new IdentityHandler instances per call.

• Synchronous child call start on warm channels: When channel configuration is already resolved and metadata filters are synchronous, ResolvingCall now starts the child RetryingCall and forwards the initial request message and halfClose in the same turn, avoiding the intermediate pendingMessage queuing in ResolvingCall and allowing LoadBalancingCall to flush the initial message and half-close together when call credentials resolve.

• Full backward compatibility for existing filters and callers: The existing sendMetadata, sendMessage, and receiveMessage methods on Filter, BaseFilter, and FilterStack are preserved. When a legacy filter is present in the stack, FilterStack seamlessly transitions to Promise chaining for that filter and any subsequent filters in the pipeline.

• Unchanged ordering and async guarantees: When any filter in the stack returns a Promise (for example, gzip or deflate compression, or a custom async filter), ResolvingCall continues to set writeFilterPending and readFilterPending to defer halfClose and onReceiveStatus until the pending filter finishes. Status delivery in outputStatus continues to use process.nextTick.

• Minor housekeeping:

  • Clear this.pendingMessage = null in ResolvingCall and LoadBalancingCall once the queued message has been forwarded to the child call so the request buffer is not retained for the rest of the call.
  • Check if (this.ended) return; when asynchronous metadata or message filters resolve so cancelled calls do not start a child call or forward in-flight messages after cancellation.