refactor: decouple conflicting ModuleContext requirements

The ModuleContext was designed to be an abstract data model in the
Controller for the resources required by a module, but along the way it
started to be used for storing DB connections for use by the go-runtime.
This change cleanly separates those requirements so that the go-runtime
is entirely responsible for creating new connections from the DSN
provided by the ModuleContext.

go-runtime/ftl/database.go

@@ -5,6 +5,7 @@ import (
 	"database/sql"
 	"fmt"
 
+	"github.com/alecthomas/types/once"
 	_ "github.com/jackc/pgx/v5/stdlib" // Register Postgres driver
 
 	"github.com/TBD54566975/ftl/internal/modulecontext"
@@ -13,24 +14,37 @@ import (
 type Database struct {
 	Name   string
 	DBType modulecontext.DBType
+
+	db *once.Handle[*sql.DB]
 }
 
 // PostgresDatabase returns a handler for the named database.
 func PostgresDatabase(name string) Database {
 	return Database{
 		Name:   name,
 		DBType: modulecontext.DBTypePostgres,
+		db: once.Once(func(ctx context.Context) (*sql.DB, error) {
+			provider := modulecontext.FromContext(ctx).CurrentContext()
+			dsn, err := provider.GetDatabase(name, modulecontext.DBTypePostgres)
+			if err != nil {
+				return nil, fmt.Errorf("failed to get database %q: %w", name, err)
+			}
+			db, err := sql.Open("pgx", dsn)
+			if err != nil {
+				return nil, fmt.Errorf("failed to open database %q: %w", name, err)
+			}
+			return db, nil
+		}),
 	}
 }
 
 func (d Database) String() string { return fmt.Sprintf("database %q", d.Name) }
 
-// Get returns the sql db connection for the database.
+// Get returns the SQL DB connection for the database.
 func (d Database) Get(ctx context.Context) *sql.DB {
-	provider := modulecontext.FromContext(ctx).CurrentContext()
-	db, err := provider.GetDatabase(d.Name, d.DBType)
+	db, err := d.db.Get(ctx)
 	if err != nil {
-		panic(err.Error())
+		panic(err)
 	}
 	return db
 }

internal/modulecontext/database.go

@@ -1,7 +1,6 @@
 package modulecontext
 
 import (
-	"database/sql"
 	"fmt"
 	"strconv"
 
@@ -14,19 +13,13 @@ type Database struct {
 	DSN      string
 	DBType   DBType
 	isTestDB bool
-	db       *sql.DB
 }
 
 // NewDatabase creates a Database that can be added to ModuleContext
 func NewDatabase(dbType DBType, dsn string) (Database, error) {
-	db, err := sql.Open("pgx", dsn)
-	if err != nil {
-		return Database{}, fmt.Errorf("failed to bring up DB connection: %w", err)
-	}
 	return Database{
 		DSN:    dsn,
 		DBType: dbType,
-		db:     db,
 	}, nil
 }
 

internal/modulecontext/module_context.go

@@ -2,7 +2,6 @@ package modulecontext
 
 import (
 	"context"
-	"database/sql"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -130,22 +129,23 @@ func (m ModuleContext) GetSecret(name string, value any) error {
 	return json.Unmarshal(data, value)
 }
 
-// GetDatabase gets a database connection
+// GetDatabase gets a database DSN by name and type.
 //
-// Returns an error if no database with that name is found or it is not the expected type
-// When in a testing context (via ftltest), an error is returned if the database is not a test database
-func (m ModuleContext) GetDatabase(name string, dbType DBType) (*sql.DB, error) {
+// Returns an error if no database with that name is found or it is not the
+// expected type. When in a testing context (via ftltest), an error is returned
+// if the database is not a test database.
+func (m ModuleContext) GetDatabase(name string, dbType DBType) (string, error) {
 	db, ok := m.databases[name]
 	if !ok {
-		return nil, fmt.Errorf("missing DSN for database %s", name)
+		return "", fmt.Errorf("missing DSN for database %s", name)
 	}
 	if db.DBType != dbType {
-		return nil, fmt.Errorf("database %s does not match expected type of %s", name, dbType)
+		return "", fmt.Errorf("database %s does not match expected type of %s", name, dbType)
 	}
 	if m.isTesting && !db.isTestDB {
-		return nil, fmt.Errorf("accessing non-test database %q while testing: try adding ftltest.WithDatabase(db) as an option with ftltest.Context(...)", name)
+		return "", fmt.Errorf("accessing non-test database %q while testing: try adding ftltest.WithDatabase(db) as an option with ftltest.Context(...)", name)
 	}
-	return db.db, nil
+	return db.DSN, nil
 }
 
 // LeaseClient is the interface for acquiring, heartbeating and releasing leases