Merge pull request #55 from PrivateStorageio/docs-system-tests-debugging

Docs: Add a few pointers to help new devs getting started.

README.rst

@@ -33,9 +33,24 @@ Unit tests are also run on CI.
 
 The system tests are run using this command::
 
-  $ nix-build nixos/system-tests.nix
+  $ sudo --preserve-env nix-build nixos/system-tests.nix
 
 The system tests boot QEMU VMs which prevents them from running on CI at this time.
+Starting VMs require elevated privileges,
+which is the reason for the suggested ``sudo`` command,
+together with the ``--preserve-env`` option to avoid escaping the ``nix-shell`` set up at the top.
+The build requires > 10 GB of disk space,
+and the VMs might be timing out on slow or busy machines.
+If you run into timeouts,
+try `raising the number of retries <https://github.com/PrivateStorageio/PrivateStorageio/blob/e8233d2/nixos/modules/tests/run-introducer.py#L55-L62>`_.
+
+It is also possible go through the testing script interactively - useful for debugging::
+
+  $ sudo --preserve-env nix-build -A private-storage.driver nixos/system-tests.nix
+
+This will give you a result symlink in the current directory.
+Inside that is bin/nixos-test-driver which gives you a kind of REPL for interacting with the VMs.
+The kind of `Perl in this testScript <https://github.com/PrivateStorageio/PrivateStorageio/blob/78881a3/nixos/modules/tests/private-storage.nix#L180>`_ is what you can enter into this REPL.
 
 Deployment
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