Turned NewDBConnection into a singleton, added database tests.

internal/db/db.go

@@ -2,34 +2,48 @@ package db
 
 import (
 	"fmt"
-	"gorm.io/gorm/logger"
 	"log"
+	"sync"
 
 	"github.com/glebarez/sqlite"
 	"gorm.io/driver/postgres"
 	"gorm.io/gorm"
+	"gorm.io/gorm/logger"
 )
 
-// TODO: Turn this into a singleton class.
-// Only one database connection should be created and used throughout the application.
+const DB_SQLITE_FALLBACK_WARNING = "[db] DATABASE_URL not set – falling back to embedded SQLite ./mcp.db"
+const DB_SQLITE_CONN_STRING = "mcp.db?_busy_timeout=5000&_journal_mode=WAL"
+
+var db *gorm.DB
+var err error
+var once sync.Once
 
 // NewDBConnection creates a new database connection based on the provided DSN.
 // If the DSN is empty, it falls back to an embedded SQLite database at "./mcp.db".
-func NewDBConnection(dsn string) (*gorm.DB, error) {
-	var dialector gorm.Dialector
-	if dsn == "" {
-		log.Println("[db] DATABASE_URL not set – falling back to embedded SQLite ./mcp.db")
-		dialector = sqlite.Open("mcp.db?_busy_timeout=5000&_journal_mode=WAL")
-	} else {
-		dialector = postgres.Open(dsn)
-	}
+func NewDBConnection(dsnString string) (*gorm.DB, error) {
+	var dbErr error
+	once.Do(func() {
+		var dialector gorm.Dialector
 
-	c := &gorm.Config{
-		Logger: logger.Default.LogMode(logger.Silent),
-	}
-	db, err := gorm.Open(dialector, c)
-	if err != nil {
-		return nil, fmt.Errorf("failed to connect to database: %w", err)
+		if dsnString == "" {
+			log.Println(DB_SQLITE_FALLBACK_WARNING)
+			dialector = sqlite.Open(DB_SQLITE_CONN_STRING)
+		} else {
+			dialector = postgres.Open(dsnString)
+		}
+
+		c := &gorm.Config{
+			Logger: logger.Default.LogMode(logger.Silent),
+		}
+
+		db, err = gorm.Open(dialector, c)
+
+		if err != nil {
+			dbErr = fmt.Errorf("failed to connect to database: %w", err)
+		}
+	})
+	if dbErr != nil {
+		return nil, dbErr
 	}
-	return db, nil
+	return db, err
 }

internal/db/db_test.go

@@ -0,0 +1,29 @@
+package db
+
+import (
+	"os"
+	"testing"
+)
+
+func TestNewDBConnection_SQLiteFallback(t *testing.T) {
+	// Unset DATABASE_URL to force fallback
+	os.Unsetenv("DATABASE_URL")
+	db, err := NewDBConnection("")
+	if err != nil {
+		t.Fatalf("expected no error, got: %v", err)
+	}
+	if db == nil {
+		t.Fatal("expected db instance, got nil")
+	}
+}
+
+func TestNewDBConnection_PostgresDSN(t *testing.T) {
+	// Use a fake DSN, expect connection error
+	db, err := NewDBConnection("postgres://invalid:invalid@localhost:5432/invalid?sslmode=disable")
+	if err == nil {
+		t.Error("expected error for invalid DSN, got nil")
+	}
+	if db != nil {
+		t.Error("expected nil db for invalid DSN")
+	}
+}