Windows: write SSH keys with LF endings — CRLF keys are rejected (#347)

Remote SSH repos failed from Windows agents with 'Connection closed by
remote host. Is borg working on the server?' while local repos worked
and Linux agents used the same remote host fine. The per-job remote SSH
key is normalized to LF and then written with text-mode open(), which
on Windows converts every newline back to CRLF — and OpenSSH rejects
CRLF private keys ('invalid format'), so pubkey auth never happens and
the remote host closes the connection. Local backups were unaffected
only because the installer downloads the agent's own key byte-for-byte.

All five key writes (agent key download fallback, backup task, and the
three database-restore handlers) now open with newline="\n", which
disables newline translation on every platform.

Author: marcpope

agent/bbs-agent.py

@@ -598,7 +598,7 @@ def download_ssh_key(config):
     try:
         key_dir = os.path.dirname(SSH_KEY_PATH)
         os.makedirs(key_dir, exist_ok=True)
-        with open(SSH_KEY_PATH, "w") as f:
+        with open(SSH_KEY_PATH, "w", newline="\n") as f:  # LF endings — CRLF keys are rejected by ssh (#347)
             f.write(private_key)
         if IS_WINDOWS:
             _lockdown_key_windows(SSH_KEY_PATH)
@@ -2573,7 +2573,7 @@ def execute_restore_pg(config, task):
             normalized_key = remote_ssh_key.replace("\r\n", "\n").replace("\r", "\n").rstrip() + "\n"
             if IS_WINDOWS:
                 _clear_stale_key_windows(remote_key_path)
-            with open(remote_key_path, "w") as kf:
+            with open(remote_key_path, "w", newline="\n") as kf:  # LF endings — CRLF keys are rejected by ssh (#347)
                 kf.write(normalized_key)
             if IS_WINDOWS:
                 _lockdown_key_windows(remote_key_path)
@@ -2790,7 +2790,7 @@ def execute_restore_mysql(config, task):
             normalized_key = remote_ssh_key.replace("\r\n", "\n").replace("\r", "\n").rstrip() + "\n"
             if IS_WINDOWS:
                 _clear_stale_key_windows(remote_key_path)
-            with open(remote_key_path, "w") as kf:
+            with open(remote_key_path, "w", newline="\n") as kf:  # LF endings — CRLF keys are rejected by ssh (#347)
                 kf.write(normalized_key)
             if IS_WINDOWS:
                 _lockdown_key_windows(remote_key_path)
@@ -3020,7 +3020,7 @@ def execute_restore_mongo(config, task):
             normalized_key = remote_ssh_key.replace("\r\n", "\n").replace("\r", "\n").rstrip() + "\n"
             if IS_WINDOWS:
                 _clear_stale_key_windows(remote_key_path)
-            with open(remote_key_path, "w") as kf:
+            with open(remote_key_path, "w", newline="\n") as kf:  # LF endings — CRLF keys are rejected by ssh (#347)
                 kf.write(normalized_key)
             if IS_WINDOWS:
                 _lockdown_key_windows(remote_key_path)
@@ -3593,7 +3593,7 @@ def _execute_task_inner(config, task, job_id, task_type, command, env_vars,
             normalized_key = remote_ssh_key.replace("\r\n", "\n").replace("\r", "\n").rstrip() + "\n"
             if IS_WINDOWS:
                 _clear_stale_key_windows(remote_key_path)
-            with open(remote_key_path, "w") as kf:
+            with open(remote_key_path, "w", newline="\n") as kf:  # LF endings — CRLF keys are rejected by ssh (#347)
                 kf.write(normalized_key)
             if IS_WINDOWS:
                 _lockdown_key_windows(remote_key_path)