Add health package: REST and gRPC health checks

- Handler(db) returns http.HandlerFunc: 200 ok / 503 unhealthy
- RegisterGRPC registers grpc.health.v1.Health on a gRPC server
- 4 tests: healthy, unhealthy (closed db), content type, gRPC registration

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

health/health.go

@@ -0,0 +1,48 @@
+// Package health provides standard health check implementations for
+// Metacircular services, supporting both REST and gRPC.
+package health
+
+import (
+	"database/sql"
+	"encoding/json"
+	"net/http"
+
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/health"
+	healthpb "google.golang.org/grpc/health/grpc_health_v1"
+)
+
+// Handler returns an http.HandlerFunc that checks database connectivity.
+// It returns 200 {"status":"ok"} if the database is reachable, or
+// 503 {"status":"unhealthy","error":"..."} if the ping fails.
+//
+// Mount it on whatever path the service uses (typically /healthz or
+// /v1/health).
+func Handler(database *sql.DB) http.HandlerFunc {
+	return func(w http.ResponseWriter, _ *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+
+		if err := database.Ping(); err != nil {
+			w.WriteHeader(http.StatusServiceUnavailable)
+			_ = json.NewEncoder(w).Encode(map[string]string{
+				"status": "unhealthy",
+				"error":  err.Error(),
+			})
+			return
+		}
+
+		w.WriteHeader(http.StatusOK)
+		_ = json.NewEncoder(w).Encode(map[string]string{
+			"status": "ok",
+		})
+	}
+}
+
+// RegisterGRPC registers the standard gRPC health checking service
+// (grpc.health.v1.Health) on the given gRPC server. The health server
+// is set to SERVING status immediately.
+func RegisterGRPC(srv *grpc.Server) {
+	hs := health.NewServer()
+	hs.SetServingStatus("", healthpb.HealthCheckResponse_SERVING)
+	healthpb.RegisterHealthServer(srv, hs)
+}

health/health_test.go

@@ -0,0 +1,97 @@
+package health
+
+import (
+	"database/sql"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"path/filepath"
+	"testing"
+
+	"google.golang.org/grpc"
+
+	"git.wntrmute.dev/kyle/mcdsl/db"
+)
+
+func TestHandlerHealthy(t *testing.T) {
+	database := openTestDB(t)
+
+	handler := Handler(database)
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodGet, "/healthz", nil)
+	handler.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status = %d, want %d", rec.Code, http.StatusOK)
+	}
+
+	var body map[string]string
+	if err := json.Unmarshal(rec.Body.Bytes(), &body); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if body["status"] != "ok" {
+		t.Fatalf("status = %q, want %q", body["status"], "ok")
+	}
+}
+
+func TestHandlerUnhealthy(t *testing.T) {
+	database := openTestDB(t)
+	// Close the database to simulate unhealthy state.
+	_ = database.Close()
+
+	handler := Handler(database)
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodGet, "/healthz", nil)
+	handler.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusServiceUnavailable {
+		t.Fatalf("status = %d, want %d", rec.Code, http.StatusServiceUnavailable)
+	}
+
+	var body map[string]string
+	if err := json.Unmarshal(rec.Body.Bytes(), &body); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if body["status"] != "unhealthy" {
+		t.Fatalf("status = %q, want %q", body["status"], "unhealthy")
+	}
+	if body["error"] == "" {
+		t.Fatal("expected error message")
+	}
+}
+
+func TestHandlerContentType(t *testing.T) {
+	database := openTestDB(t)
+
+	handler := Handler(database)
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodGet, "/healthz", nil)
+	handler.ServeHTTP(rec, req)
+
+	if ct := rec.Header().Get("Content-Type"); ct != "application/json" {
+		t.Fatalf("Content-Type = %q, want %q", ct, "application/json")
+	}
+}
+
+func TestRegisterGRPC(t *testing.T) {
+	srv := grpc.NewServer()
+	// Should not panic.
+	RegisterGRPC(srv)
+
+	info := srv.GetServiceInfo()
+	if _, ok := info["grpc.health.v1.Health"]; !ok {
+		t.Fatal("health service not registered")
+	}
+}
+
+func openTestDB(t *testing.T) *sql.DB {
+	t.Helper()
+	dir := t.TempDir()
+	path := filepath.Join(dir, "test.db")
+	database, err := db.Open(path)
+	if err != nil {
+		t.Fatalf("db.Open: %v", err)
+	}
+	t.Cleanup(func() { _ = database.Close() })
+	return database
+}