feat(implementation-sweep): run one campaign against every implementation of a requirement

installs, builds and serves each implementation on its own port, then hands the campaign tool the same seed slice, step budget and generator for all of them, so a difference between implementations is not a difference in exploration. the generator and platform are fixed rather than exposed.

cmd/internal-tools/implementation-sweep/serve_test.go

@@ -0,0 +1,119 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+	"os"
+	"path/filepath"
+	"syscall"
+	"testing"
+	"time"
+)
+
+// bunSpawningAServer serves from a child process and then waits, which is the
+// shape of `bun run preview`: the port belongs to something below the process
+// the sweep started.
+const bunSpawningAServer = `#!/bin/sh
+port=""
+previous=""
+for argument in "$@"; do
+  if [ "$previous" = "--port" ]; then port="$argument"; fi
+  previous="$argument"
+done
+SWEEP_TEST_SERVE_PORT="$port" "%[1]s" &
+wait
+`
+
+func TestServerStop_TakesTheProcessBelowItWithTheServer(t *testing.T) {
+	directory := t.TempDir()
+	testBinary, err := filepath.Abs(os.Args[0])
+	if err != nil {
+		t.Fatal(err)
+	}
+	bunPath := writeScript(
+		t,
+		filepath.Join(directory, "stub-bun"),
+		fmt.Sprintf(bunSpawningAServer, testBinary),
+	)
+	port := freePortRange(t, 1)
+	target := implementation{Name: "impl-01", Directory: directory, Port: port}
+
+	// Background rather than the test context: only stop() may end this
+	// server, or a leak would be hidden by the context being cancelled.
+	running, err := startServer(
+		context.Background(),
+		config{bunPath: bunPath},
+		target,
+		filepath.Join(directory, "serve.log"),
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(
+		func() { syscall.Kill(-running.command.Process.Pid, syscall.SIGKILL) },
+	)
+	if err := running.waitReady(context.Background(), readinessURL(port)); err != nil {
+		t.Fatal(err)
+	}
+
+	running.stop()
+
+	client := &http.Client{Timeout: time.Second}
+	deadline := time.Now().Add(5 * time.Second)
+	for time.Now().Before(deadline) {
+		response, err := client.Get(readinessURL(port))
+		if err != nil {
+			return
+		}
+		response.Body.Close()
+		time.Sleep(100 * time.Millisecond)
+	}
+	t.Fatalf(
+		"port %d is still served after stop(): the server below bun outlived the sweep and holds the port",
+		port,
+	)
+}
+
+func TestServerWaitReady_ReportsAServerThatExited(t *testing.T) {
+	directory := t.TempDir()
+	bunPath := writeScript(
+		t,
+		filepath.Join(directory, "stub-bun"),
+		"#!/bin/sh\necho 'port is already in use' >&2\nexit 1\n",
+	)
+	port := freePortRange(t, 1)
+
+	running, err := startServer(
+		context.Background(),
+		config{bunPath: bunPath},
+		implementation{
+			Name:      "impl-01",
+			Directory: directory,
+			Port:      port,
+		},
+		filepath.Join(directory, "serve.log"),
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer running.stop()
+
+	started := time.Now()
+	err = running.waitReady(context.Background(), readinessURL(port))
+	if err == nil {
+		t.Fatal(
+			"a server that exited should not be waited on until the start timeout",
+		)
+	}
+	if elapsed := time.Since(started); elapsed > 30*time.Second {
+		t.Errorf("waited %s for a server that had already exited", elapsed)
+	}
+	log, err := os.ReadFile(filepath.Join(directory, "serve.log"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(log) == "" {
+		t.Error("serve.log did not capture why the server exited")
+	}
+}