Fix execute_isolated_fn() against immutable databases

execute_isolated_fn() always opened its temporary connection with
write=True, which is not allowed for immutable databases - so APIs
that rely on it, like SQL analysis when storing a query, failed.

An immutable database can never receive writes, so there is no write
queue to block: in that case the function now opens a read-only
connection and runs it on the executor, bypassing the write thread
entirely. Mutable databases keep the existing write-thread behavior.

Also fixed a latent bug in the write thread where a connect() failure
for an isolated task would crash the thread instead of delivering the
exception back to the caller.

Closes #2768

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_internals_database.py

@@ -863,6 +863,39 @@ async def test_execute_isolated(db, disable_threads):
     assert not await db.execute_isolated_fn(table_exists_checker("created_by_isolated"))
 
 
+@pytest.mark.asyncio
+async def test_execute_isolated_connect_failure_does_not_kill_write_thread():
+    # A connect() failure for an isolated task should be returned to the
+    # caller as an exception, not crash the write thread
+    class ConnectError(Exception):
+        pass
+
+    ds = Datasette(memory=True)
+    db = ds.add_memory_database("test_isolated_connect_failure")
+    # Start the write thread with a healthy dedicated write connection
+    await db.execute_write("create table dogs (id integer primary key)")
+
+    original_connect = db.connect
+
+    def broken_connect(write=False):
+        raise ConnectError("Could not connect")
+
+    db.connect = broken_connect
+    try:
+        with pytest.raises(ConnectError):
+            await asyncio.wait_for(db.execute_isolated_fn(lambda conn: None), timeout=2)
+    finally:
+        db.connect = original_connect
+
+    # Write thread should still be alive and processing tasks
+    assert db._write_thread.is_alive()
+    await db.execute_write("insert into dogs (id) values (1)")
+    count = await db.execute_isolated_fn(
+        lambda conn: conn.execute("select count(*) from dogs").fetchone()[0]
+    )
+    assert count == 1
+
+
 @pytest.mark.asyncio
 async def test_analyze_sql():
     ds = Datasette(memory=True)