su_to_zulip: Fix detection of zulip user ID.

Apparently, while upgrade-zulip-from-git always ensures that zulip
deployment directories are owned by the Zulip user, unpack-zulip (aka
the tarball code path) has them owned by root.

The user ID detection logic in su_to_zulip's helper get_zulip_uid was
intended to support both development environments (where the user ID
might vary) and production environments.  For development
environments, the existing code is fine, but given this unpack-zulip
permissions issue, we need to have code to fallback to 'zulip' if the
detection logic detects the "zulip" user has having UID 0.

scripts/lib/zulip_tools.py

@@ -110,8 +110,15 @@ def subprocess_text_output(args):
     # type: (Sequence[str]) -> str
     return subprocess.check_output(args, universal_newlines=True).strip()
 
-def get_zulip_uid() -> int:
-    return os.stat(get_deploy_root()).st_uid
+def get_zulip_pwent() -> pwd.struct_passwd:
+    deploy_root_uid = os.stat(get_deploy_root()).st_uid
+    if deploy_root_uid != 0:
+        return pwd.getpwuid(deploy_root_uid)
+
+    # In the case that permissions got messed up and the deployment
+    # directory is unexpectedly owned by root, we fallback to the
+    # `zulip` user as that's the correct value in production.
+    return pwd.getpwnam("zulip")
 
 def su_to_zulip(save_suid=False):
     # type: (bool) -> None
@@ -120,7 +127,7 @@ def su_to_zulip(save_suid=False):
     installation).  It should never be run from the installer or other
     production contexts before /home/zulip/deployments/current is
     created."""
-    pwent = pwd.getpwuid(get_zulip_uid())
+    pwent = get_zulip_pwent()
     os.setgid(pwent.pw_gid)
     if save_suid:
         os.setresuid(pwent.pw_uid, pwent.pw_uid, os.getuid())
@@ -429,7 +436,7 @@ def run_as_root(args, **kwargs):
 def assert_not_running_as_root() -> None:
     script_name = os.path.abspath(sys.argv[0])
     if is_root():
-        pwent = pwd.getpwuid(get_zulip_uid())
+        pwent = get_zulip_pwent()
         msg = ("{shortname} should not be run as root. Use `su {user}` to switch to the 'zulip'\n"
                "user before rerunning this, or use \n  su {user} -c '{name} ...'\n"
                "to switch users and run this as a single command.").format(