Implement EventSource (Server-Sent Events) #188

Goal#

Implement the EventSource interface for consuming text/event-stream server pushes, including automatic reconnection and event ID tracking.

Depends on: "Integrate Streams API with fetch: Response.body and streaming Request bodies" (uses streaming HTTP responses).

Scope#

• text/event-stream parser (WHATWG HTML §9.2.6)
  • Fields: data:, event:, id:, retry:, comments (: prefix)
  • Multi-line data: accumulates with a newline separator
  • Event dispatch on blank line; reset buffers between events
  • id: field updates the connection's lastEventId; an explicit id: with no value clears it; an id: field containing a NUL byte is ignored
  • retry: field updates the reconnection time (must be a non-negative integer of milliseconds)
  • Decoder: UTF-8 with replacement, tolerant of CR/LF/CRLF line endings
• EventSource interface
  • new EventSource(url, { withCredentials = false }?)
  • Properties: url, readyState (CONNECTING = 0, OPEN = 1, CLOSED = 2), withCredentials
  • close() permanently closes the connection
  • onopen, onmessage, onerror plus addEventListener equivalents
  • Custom event types via the event: field are dispatched as their own type
• HTTP behaviour
  • Request: Accept: text/event-stream, Cache-Control: no-store, send Last-Event-ID header on reconnect when one is known
  • Reject non-text/event-stream responses with a CLOSED failure (no reconnect)
  • On network error or successful close, schedule a reconnect with the current retry timeout (default 3 s, capped to a reasonable maximum)
  • 204 / 401 / 403 / 404 / 410 / 5xx and many other non-2xx statuses follow the spec's reconnect-or-fail rules — implement the failure case (no reconnect) per spec for the obvious terminal codes
  • CORS: respect withCredentials, send Origin, accept Access-Control-Allow-Origin / Access-Control-Allow-Credentials
• Lifecycle
  • GC root active EventSource instances similar to WebSocket

Acceptance Criteria#

• Connects to a text/event-stream endpoint and dispatches message events as the server pushes data
• Multi-line data: blocks are joined with \\n
• event: foo\\ndata: x\\n\\n dispatches a custom foo event with data === 'x'
• After a server disconnect, the client reconnects automatically using the configured retry timeout and sends Last-Event-ID
• close() ends the connection and prevents further reconnects
• Cross-origin requests with withCredentials: false succeed only when the server returns the required CORS headers
• Unit tests for parser edge cases (CR/LF/CRLF mixing, multi-line data, retry parsing, NUL in id, comments) + an integration test against an in-process SSE server
• Conventions
  • cargo fmt --all --check
  • cargo clippy --workspace -- -D warnings
  • cargo test --workspace