Remove Tails 3.x admin virtualenv when upgrading to Tails 4.x

Fixes #4910

admin/bootstrap.py

@@ -20,6 +20,7 @@
 import argparse
 import logging
 import os
+import shutil
 import subprocess
 import sys
 
@@ -75,6 +76,32 @@ def is_tails():
     return id == 'Tails'
 
 
+def clean_up_tails3_venv():
+    """
+    Tails 3.x, based on debian stretch uses libpython3.5, whereas Tails 4.x is
+    based on Debian Buster and uses libpython3.7. This means that the Tails 3.x
+    virtualenv will not work under Tails 4.x, and will need to be destroyed and
+    rebuilt. We can detect if the version of libpython is 3.5 in the
+    admin/.venv3/ folder, and delete it if that's the case. This will ensure a
+    smooth upgrade from Tails 3.x to Tails 4.x.
+    """
+    if is_tails():
+        try:
+            dist = subprocess.check_output('lsb_release --codename --short',
+                                           shell=True).strip()
+        except subprocess.CalledProcessError:
+            dist = None
+
+        # tails4 is based on buster
+        if dist == 'buster':
+            python_lib_path = os.path.join(VENV_DIR, "lib/python3.5")
+            if os.path.exists(os.path.join(python_lib_path)):
+                sdlog.info("Tails 3 Python 3 virtualenv detected.")
+                shutil.rmtree(VENV_DIR)
+                sdlog.info("Tails 3 Python 3 virtualenv deleted. It will be "
+                           "rebuilt to complete migration to Tails 4.x")
+
+
 def maybe_torify():
     if is_tails():
         return ['torify']
@@ -130,6 +157,9 @@ def envsetup(args):
     Ansible is available to the Admin on subsequent boots without requiring
     installation of packages again.
     """
+    # clean up tails 3.x venv when migrating to tails 4.x
+    clean_up_tails3_venv()
+
     # virtualenv doesnt exist? Install dependencies and create
     if not os.path.exists(VENV_DIR):