serve: fix forced command lines containing BORG_ env vars

(cherry picked from commit 88dfb3e9c53bb335e339307232849e9ad0426bba)

borg/archiver.py

@@ -6,6 +6,7 @@ import argparse
 import faulthandler
 import functools
 import inspect
+import itertools
 import os
 import re
 import shlex
@@ -2060,6 +2061,9 @@ class Archiver:
         if cmd is not None and result.func == self.do_serve:
             forced_result = result
             argv = shlex.split(cmd)
+            # Drop environment variables (do *not* interpret them) before trying to parse
+            # the borg command line.
+            argv = list(itertools.dropwhile(lambda arg: '=' in arg, argv))
             result = self.parse_args(argv[1:])
             if result.func != forced_result.func:
                 # someone is trying to execute a different borg subcommand, don't do that!

borg/testsuite/archiver.py

@@ -1800,6 +1800,13 @@ def test_get_args():
                              'borg init /')
     assert args.func == archiver.do_serve
 
+    # Check that environment variables in the forced command don't cause issues. If the command
+    # were not forced, environment variables would be interpreted by the shell, but this does not
+    # happen for forced commands - we get the verbatim command line and need to deal with env vars.
+    args = archiver.get_args(['borg', 'serve', ],
+                             'BORG_HOSTNAME_IS_UNIQUE=yes borg serve --info')
+    assert args.func == archiver.do_serve
+
 
 class TestBuildFilter:
     def test_basic(self):