Use atomic write for state file to prevent corruption on crash

internal/server/router.go

@@ -8,6 +8,7 @@ import (
 	"log/slog"
 	"net/http"
 	"os"
+	"path/filepath"
 	"strings"
 	"sync"
 	"time"
@@ -350,17 +351,34 @@ func (r *Router) saveStateSnapshot() error {
 		return nil
 	})
 
-	f, err := os.Create(r.statePath)
+	tmp, err := os.CreateTemp(filepath.Dir(r.statePath), ".kamal-proxy.state.*")
 	if err != nil {
+		slog.Error("Unable to create temp state file", "error", err)
 		return err
 	}
+	defer os.Remove(tmp.Name()) // clean up on any failure path
 
-	err = json.NewEncoder(f).Encode(services)
+	err = json.NewEncoder(tmp).Encode(services)
 	if err != nil {
+		tmp.Close()
 		slog.Error("Unable to save state", "error", err, "path", r.statePath)
 		return err
 	}
 
+	err = tmp.Sync()
+	if err != nil {
+		tmp.Close()
+		slog.Error("Unable to sync state file", "error", err)
+		return err
+	}
+	tmp.Close()
+
+	err = os.Rename(tmp.Name(), r.statePath)
+	if err != nil {
+		slog.Error("Unable to rename state file", "error", err)
+		return err
+	}
+
 	slog.Debug("Saved state", "path", r.statePath)
 	return nil
 }

internal/server/router_test.go

@@ -778,6 +778,32 @@ func testRouter(t *testing.T) *Router {
 	return NewRouter(statePath)
 }
 
+func TestRouter_StateFileSurvivesRestart(t *testing.T) {
+	statePath := filepath.Join(t.TempDir(), "state.json")
+	_, target := testBackend(t, "first", http.StatusOK)
+
+	router := NewRouter(statePath)
+	require.NoError(t, router.DeployService("service1", []string{target},
+		defaultEmptyReaders, defaultServiceOptions, defaultTargetOptions, defaultDeploymentOptions))
+
+	// Verify state file exists and is valid JSON
+	f, err := os.Open(statePath)
+	require.NoError(t, err)
+	defer f.Close()
+
+	var services []*Service
+	require.NoError(t, json.NewDecoder(f).Decode(&services))
+	assert.Len(t, services, 1)
+
+	// Verify no temp files left behind
+	entries, err := os.ReadDir(filepath.Dir(statePath))
+	require.NoError(t, err)
+	for _, entry := range entries {
+		assert.False(t, strings.HasPrefix(entry.Name(), ".kamal-proxy.state."),
+			"temp file should not remain: %s", entry.Name())
+	}
+}
+
 func sendGETRequest(router *Router, url string) (int, string) {
 	req := httptest.NewRequest(http.MethodGet, url, nil)
 	return sendRequest(router, req)