nixos/tests: extend shell_interact to accept alternative socat addresses

`shell_interact()` is currently not nice to use.  If you try to cancel
the socat process, it will also break the nixos test. Furthermore
ptpython creates it's own terminal that subprocesses are running in,
which breaks some of the terminal features of socat.
Hence this commit extends `shell_interact` to allow also to connect to
arbitrary servers i.e. tcp servers started by socat.

nixos/doc/manual/development/running-nixos-tests-interactively.section.md

@@ -24,6 +24,39 @@ back into the test driver command line upon its completion. This allows
 you to inspect the state of the VMs after the test (e.g. to debug the
 test script).
 
+## Shell access in interactive mode {#sec-nixos-test-shell-access}
+
+The function `<yourmachine>.shell_interact()` grants access to a shell running
+inside a virtual machine. To use it, replace `<yourmachine>` with the name of a
+virtual machine defined in the test, for example: `machine.shell_interact()`.
+Keep in mind that this shell may not display everything correctly as it is
+running within an interactive Python REPL, and logging output from the virtual
+machine may overwrite input and output from the guest shell:
+
+```py
+>>> machine.shell_interact()
+machine: Terminal is ready (there is no initial prompt):
+$ hostname
+machine
+```
+
+As an alternative, you can proxy the guest shell to a local TCP server by first
+starting a TCP server in a terminal using the command:
+
+```ShellSession
+$ socat 'READLINE,PROMPT=$ ' tcp-listen:4444,reuseaddr`
+```
+
+In the terminal where the test driver is running, connect to this server by
+using:
+
+```py
+>>> machine.shell_interact("tcp:127.0.0.1:4444")
+```
+
+Once the connection is established, you can enter commands in the socat terminal
+where socat is running.
+
 ## Reuse VM state {#sec-nixos-test-reuse-vm-state}
 
 You can re-use the VM states coming from a previous run by setting the

nixos/doc/manual/from_md/development/running-nixos-tests-interactively.section.xml

@@ -25,6 +25,46 @@ $ ./result/bin/nixos-test-driver
     completion. This allows you to inspect the state of the VMs after
     the test (e.g. to debug the test script).
   </para>
+  <section xml:id="sec-nixos-test-shell-access">
+    <title>Shell access in interactive mode</title>
+    <para>
+      The function
+      <literal>&lt;yourmachine&gt;.shell_interact()</literal> grants
+      access to a shell running inside a virtual machine. To use it,
+      replace <literal>&lt;yourmachine&gt;</literal> with the name of a
+      virtual machine defined in the test, for example:
+      <literal>machine.shell_interact()</literal>. Keep in mind that
+      this shell may not display everything correctly as it is running
+      within an interactive Python REPL, and logging output from the
+      virtual machine may overwrite input and output from the guest
+      shell:
+    </para>
+    <programlisting language="python">
+&gt;&gt;&gt; machine.shell_interact()
+machine: Terminal is ready (there is no initial prompt):
+$ hostname
+machine
+</programlisting>
+    <para>
+      As an alternative, you can proxy the guest shell to a local TCP
+      server by first starting a TCP server in a terminal using the
+      command:
+    </para>
+    <programlisting>
+$ socat 'READLINE,PROMPT=$ ' tcp-listen:4444,reuseaddr`
+</programlisting>
+    <para>
+      In the terminal where the test driver is running, connect to this
+      server by using:
+    </para>
+    <programlisting language="python">
+&gt;&gt;&gt; machine.shell_interact(&quot;tcp:127.0.0.1:4444&quot;)
+</programlisting>
+    <para>
+      Once the connection is established, you can enter commands in the
+      socat terminal where socat is running.
+    </para>
+  </section>
   <section xml:id="sec-nixos-test-reuse-vm-state">
     <title>Reuse VM state</title>
     <para>

nixos/lib/test-driver/test_driver/machine.py

@@ -549,18 +549,27 @@ class Machine:
 
         return (rc, output.decode())
 
-    def shell_interact(self) -> None:
-        """Allows you to interact with the guest shell
+    def shell_interact(self, address: Optional[str] = None) -> None:
+        """Allows you to interact with the guest shell for debugging purposes.
 
-        Should only be used during test development, not in the production test."""
+        @address string passed to socat that will be connected to the guest shell.
+        Check the `Running Tests interactivly` chapter of NixOS manual for an example.
+        """
         self.connect()
-        self.log("Terminal is ready (there is no initial prompt):")
+
+        if address is None:
+            address = "READLINE,prompt=$ "
+            self.log("Terminal is ready (there is no initial prompt):")
 
         assert self.shell
-        subprocess.run(
-            ["socat", "READLINE,prompt=$ ", f"FD:{self.shell.fileno()}"],
-            pass_fds=[self.shell.fileno()],
-        )
+        try:
+            subprocess.run(
+                ["socat", address, f"FD:{self.shell.fileno()}"],
+                pass_fds=[self.shell.fileno()],
+            )
+            # allow users to cancel this command without breaking the test
+        except KeyboardInterrupt:
+            pass
 
     def console_interact(self) -> None:
         """Allows you to interact with QEMU's stdin