Keep each test database when the pool replaces its connection (closes #209)

SQLite deletes a shared in-memory database when its last connection
closes. A query cancelled while it runs makes the pool close its
connection, so a test could lose all its tables and rows at random.

NewTestDatabaseURL in internal/db returns the URL of a new in-memory
database with a random name. It holds one connection to it open until
the test ends. The db, handlers, ircserver, server and service tests
now take their database URL from it.

A new test cancels a query on such a database, checks with a TEMP
table that the connection was replaced, and checks that a row is
still there.

Model: opus-5-5

internal/db/testing.go

@@ -2,10 +2,40 @@ package db
 
 import (
 	"context"
+	"crypto/rand"
 	"database/sql"
 	"log/slog"
+	"testing"
 )
 
+// NewTestDatabaseURL returns the URL of a new, empty in-memory
+// database for one test. SQLite deletes an in-memory database when
+// its last connection closes. A pool closes a connection when a query
+// on it is cancelled while it runs. So this function holds a
+// connection of its own open until the test ends.
+func NewTestDatabaseURL(tb testing.TB) string {
+	tb.Helper()
+
+	// A random name, so that no other test can share the database.
+	dbURL := "file:" + rand.Text() + "?mode=memory&cache=shared"
+
+	pool, err := sql.Open("sqlite", dbURL)
+	if err != nil {
+		tb.Fatalf("open test database: %v", err)
+	}
+
+	tb.Cleanup(func() { _ = pool.Close() })
+
+	conn, err := pool.Conn(tb.Context())
+	if err != nil {
+		tb.Fatalf("connect to test database: %v", err)
+	}
+
+	tb.Cleanup(func() { _ = conn.Close() })
+
+	return dbURL
+}
+
 // NewTestDatabaseFromConn creates a Database wrapping an
 // existing *sql.DB connection.  Intended for integration
 // tests in other packages.