Initial commit: stalwart-migrator design and scaffolding

In-place upgrade tool for Stalwart Mail Server (0.15.5 -> latest) with
checkpointed rollback and post-migration validation. Design stage; see
ARCHITECTURE.md.

internal/recovery/main_test.go

@@ -0,0 +1,51 @@
+package recovery
+
+import (
+	"fmt"
+	"net"
+	"net/http"
+	"os"
+	"os/signal"
+	"syscall"
+	"testing"
+)
+
+// TestMain lets this test binary also act as a fake Stalwart binary for
+// subprocess tests - the standard os/exec "helper process" technique (see
+// Go's own os/exec_test.go). When STALWART_MIGRATOR_TEST_HELPER=1 is set,
+// the binary runs a minimal HTTP server on STALWART_MIGRATOR_TEST_PORT
+// until it receives SIGTERM (or, if STALWART_MIGRATOR_TEST_IGNORE_SIGTERM=1,
+// ignores SIGTERM to exercise the SIGKILL escalation path) instead of
+// running the actual test suite. This avoids needing a real Stalwart binary
+// - or a fabricated stand-in for its HTTP behavior - anywhere in these
+// tests: it's real net/http and real process signaling, just running under
+// this package's own compiled binary.
+func TestMain(m *testing.M) {
+	if os.Getenv("STALWART_MIGRATOR_TEST_HELPER") == "1" {
+		runFakeStalwartServer()
+		return
+	}
+	os.Exit(m.Run())
+}
+
+func runFakeStalwartServer() {
+	port := os.Getenv("STALWART_MIGRATOR_TEST_PORT")
+	ln, err := net.Listen("tcp", "127.0.0.1:"+port)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, "fake stalwart: listen:", err)
+		os.Exit(1)
+	}
+	srv := &http.Server{Handler: http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	})}
+	go srv.Serve(ln)
+
+	sigCh := make(chan os.Signal, 1)
+	if os.Getenv("STALWART_MIGRATOR_TEST_IGNORE_SIGTERM") == "1" {
+		signal.Ignore(syscall.SIGTERM)
+		select {} // block forever; the test must SIGKILL this process itself
+	}
+	signal.Notify(sigCh, syscall.SIGTERM)
+	<-sigCh
+	os.Exit(0)
+}