[nepet]

Checklist

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Fixes #9351.

With an explicit --conf in a directory lightningd can't write, the first setconfig fails with "Cannot write to config file", as it should. But config_not_writable() hands the file name to the command with tal_steal(), and with --conf that name is ld->config_filename itself, so freeing the failed command frees it. The next setconfig reads the freed name, passes the writability check against the wrong directory, and lightningd aborts in append_to_file().

This returns a copy instead, like the other branch of config_not_writable() already does. Introduced in 5e79cd4.

The test starts a node with --conf in a read-only directory and calls setconfig twice: both calls must fail with "Cannot write to config file", and the node must still be running.

This doesn't conflict with #9356, which makes the persistence path return errors instead of calling fatal(). That PR doesn't fix this bug on its own: with it, the second call still reads freed memory.