refactor/fix subprocess env preparation

refactor: make a generally usable function

fix: remove support code for ancient pyinstaller

the "else" branch was needed for pyinstaller < 20160820 because it did
not have the LD_LIBRARY_PATH_ORIG env var, so we just killed LDLP
because we had no better way.

but with borg tests running under fakeroot, this is troublesome as
fakeroot uses this also and can't find its library without it.

so, just remove it, we do not need to support old pyinstaller.

(cherry picked from commit ba941b0801)

src/borg/helpers.py

@@ -2303,6 +2303,32 @@ def popen_with_error_handling(cmd_line: str, log_prefix='', **kwargs):
         return
 
 
+def prepare_subprocess_env(system, env=None):
+    """
+    Prepare the environment for a subprocess we are going to create.
+
+    :param system: True for preparing to invoke system-installed binaries,
+                   False for stuff inside the pyinstaller environment (like borg, python).
+    :param env: optionally give a environment dict here. if not given, default to os.environ.
+    :return: a modified copy of the environment
+    """
+    env = dict(env if env is not None else os.environ)
+    if system:
+        # a pyinstaller binary's bootloader modifies LD_LIBRARY_PATH=/tmp/_ME...,
+        # but we do not want that system binaries (like ssh or other) pick up
+        # (non-matching) libraries from there.
+        # thus we install the original LDLP, before pyinstaller has modified it:
+        lp_key = 'LD_LIBRARY_PATH'
+        lp_orig = env.get(lp_key + '_ORIG')  # pyinstaller >= 20160820 has this
+        if lp_orig is not None:
+            env[lp_key] = lp_orig
+    # security: do not give secrets to subprocess
+    env.pop('BORG_PASSPHRASE', None)
+    # for information, give borg version to the subprocess
+    env['BORG_VERSION'] = borg_version
+    return env
+
+
 def dash_open(path, mode):
     assert '+' not in mode  # the streams are either r or w, but never both
     if path == '-':

src/borg/remote.py

@@ -30,6 +30,7 @@ from .helpers import replace_placeholders
 from .helpers import sysinfo
 from .helpers import format_file_size
 from .helpers import truncate_and_unlink
+from .helpers import prepare_subprocess_env
 from .logger import create_logger, setup_logging
 from .repository import Repository
 from .version import parse_version, format_version
@@ -537,21 +538,12 @@ class RemoteRepository:
         self.server_version = parse_version('1.0.8')  # fallback version if server is too old to send version information
         self.p = None
         testing = location.host == '__testsuite__'
+        # when testing, we invoke and talk to a borg process directly (no ssh).
+        # when not testing, we invoke the system-installed ssh binary to talk to a remote borg.
+        env = prepare_subprocess_env(system=not testing)
         borg_cmd = self.borg_cmd(args, testing)
-        env = dict(os.environ)
         if not testing:
             borg_cmd = self.ssh_cmd(location) + borg_cmd
-            # pyinstaller binary modifies LD_LIBRARY_PATH=/tmp/_ME... but we do not want
-            # that the system's ssh binary picks up (non-matching) libraries from there.
-            # thus we install the original LDLP, before pyinstaller has modified it:
-            lp_key = 'LD_LIBRARY_PATH'
-            lp_orig = env.get(lp_key + '_ORIG')  # pyinstaller >= 20160820 has this
-            if lp_orig is not None:
-                env[lp_key] = lp_orig
-            else:
-                env.pop(lp_key, None)
-        env.pop('BORG_PASSPHRASE', None)  # security: do not give secrets to subprocess
-        env['BORG_VERSION'] = __version__
         logger.debug('SSH command line: %s', borg_cmd)
         self.p = Popen(borg_cmd, bufsize=0, stdin=PIPE, stdout=PIPE, stderr=PIPE, env=env)
         self.stdin_fd = self.p.stdin.fileno()