ci: compile the unit tests before running them

.github/workflows/ci.yml

@@ -184,6 +184,17 @@ jobs:
       - name: Fetch dependencies (with retries)
         shell: bash
         run: bash scripts/ci-retry.sh zig build --fetch
+      # Every test binary is compiled before any of them runs, so no compiler is alive beside a
+      # running test. On Windows, Zig 0.16 spawns each child inheriting every inheritable handle
+      # the build runner holds, the pipes it is making for its other children included. A
+      # compiler spawned while a test's pipes exist keeps a copy of that test's stdout: the test
+      # passes and exits, but its stdout reaches EOF only when the compiler exits, and the runner
+      # gives up after a minute with "test runner failed to respond", on whichever tiny test was
+      # unlucky, about one run in thirty. `check` is exactly the compiles `test` needs, so the
+      # step after it spawns only test binaries, which live for milliseconds.
+      - name: Compile tests
+        shell: bash
+        run: bash scripts/ci-retry.sh zig build check --summary all
       - name: Run tests
         shell: bash
         run: bash scripts/ci-retry.sh zig build test --summary all

core/transport/NativeTransport.zig

@@ -441,6 +441,15 @@ test "cancel returns at once while the server is still holding the connection" {
     const Hold = struct {
         fn run(s: *std.Io.net.Server, held_io: std.Io, release: *std.atomic.Value(bool)) void {
             const stream = s.accept(held_io) catch return;
+            // Read the request before holding: a socket closed with bytes still unread is reset,
+            // not closed, and Zig 0.16 reads a reset on Windows as `error.Unexpected`, dumping a
+            // stack trace into the test's stderr that reads as a failure.
+            var in_buf: [1024]u8 = undefined;
+            var out_buf: [64]u8 = undefined;
+            var reader = stream.reader(held_io, &in_buf);
+            var writer = stream.writer(held_io, &out_buf);
+            var http_server = std.http.Server.init(&reader.interface, &writer.interface);
+            _ = http_server.receiveHead() catch {};
             while (!release.load(.acquire)) std.Io.sleep(held_io, .fromMicroseconds(1000), .awake) catch {};
             stream.close(held_io);
         }

docs/PITFALLS.md

@@ -15,6 +15,13 @@ goes beside the code if it has an area, here if not.
   for function-local structs with `var`s inside generic functions.
 - **Tests run only from files their root module analyzes** (`tests/AGENTS.md`).
   `zig build check-tests-reached` names any that don't.
+- **On Windows, "test runner failed to respond" with every test passed means a leaked pipe.**
+  Zig 0.16's Windows spawn makes each child inherit every inheritable handle the build runner
+  holds, its other children's pipes among them. A long-lived child (a compiler, a sleeper)
+  spawned beside a test keeps that test's stdout open, so after the test exits the runner waits a
+  minute for EOF and fails it. CI compiles the tests (`zig build check`) before running them for
+  this; don't fold the two steps back together, or start anything long-lived inside `zig build
+  test`.
 
 ## Running a second fizzy