Get rid of intermediate ack; update docstring

Author: ilevkivskyi

mypy/build.py

@@ -1234,7 +1234,6 @@ def wait_for_done_workers(
             assert data.result is not None
             results.update(data.result)
             if data.is_interface:
-                send(self.workers[idx].conn, AckMessage())
                 done_sccs.append(self.scc_by_id[scc_id])
         self.submit_to_workers(graph)  # advance after some workers are free.
         return (

mypy/build_worker/worker.py

@@ -5,12 +5,13 @@
 * Read (pickled) build options from command line.
 * Populate status file with pid and socket address.
 * Receive build sources from coordinator.
-* Load graph using the sources, and send ack to coordinator.
+* Initialize build manager with the sources, and send ack to coordinator.
+* Receive build graph from coordinator, and ack it.
 * Receive SCC structure from coordinator, and ack it.
-* Receive an SCC id from coordinator, process it, and send back the results.
-* Each SCC is processed in two phases: interface then implementation, with an ack in
-  between. (It is not 100% clear why the ack is needed, but deadlocks happen without it)
-* When prompted by coordinator (with a scc_id=None message), cleanup and shutdown.
+* In a loop:
+  - Receive an SCC id from coordinator, and start processing it.
+  - SCC is processed in two phases: interface and implementation, send a response after each.
+  - When prompted by coordinator (with a scc_id=None message), cleanup and shutdown.
 """
 
 from __future__ import annotations
@@ -30,7 +31,6 @@
 
 from mypy import util
 from mypy.build import (
-    ACK_MESSAGE,
     GRAPH_MESSAGE,
     SCC,
     SCC_REQUEST_MESSAGE,
@@ -201,9 +201,6 @@ def serve(server: IPCServer, ctx: ServerContext) -> None:
                 mod_results[id] = mod_result
                 meta_files.append(meta_file)
             send(server, SccResponseMessage(scc_id=scc_id, is_interface=True, result=mod_results))
-            # Only proceed with the implementations if there are no blockers so far.
-            if should_shutdown(receive(server), ACK_MESSAGE):
-                break
             try:
                 result = process_stale_scc_implementation(graph, stale, manager, meta_files)
                 # Both phases write cache, so we should commit here as well.

mypy/ipc.py

@@ -366,6 +366,10 @@ def ready_to_read(conns: list[IPCClient], timeout: float | None = None) -> list[
 
     Return index of each readable connection in the original list.
     """
+    unread_messages = [i for i, conn in enumerate(conns) if conn.buffer]
+    if unread_messages:
+        # If we already have unread messages in the buffer, return those first.
+        return unread_messages
     if sys.platform == "win32":
         # Windows doesn't support select() on named pipes. Instead, start an overlapped
         # ReadFile on each pipe (which internally creates an event via CreateEventW),