login: T8086: replace getpwall() user enumeration to avoid NSS/TACACS timeouts

The previous implementation of "system login" relied on Python's pwd.getpwall()
to enumerate user accounts. This forces a full walk through the NSS stack,
which is acceptable in general but problematic for our use-case. VyOS only
needs information about locally created accounts and not remote accounts
provided via AAA backends such as TACACS or RADIUS.

When TACACS servers are unreachable, NSS lookups become extremely slow due to
repeated timeouts. As a result, any operation triggering pwd.getpwall()
(including configuration commits) can stall for several minutes.

This change introduces a dedicated helper, get_local_passwd_entries(), which
reads /etc/passwd directly and avoids NSS entirely. Since only local UIDs are
relevant, this provides all required data with no external dependencies.

Performance improvement on VyOS 1.4.3 with two unreachable TACACS servers:

  # set system login tacacs server 192.168.1.50 key test123
  # set system login tacacs server 192.168.1.51 key test123

  # time commit

  Before:
    real    3m29.825s
    user    0m0.329s
    sys     0m0.246s

  After:
    real    0m1.464s
    user    0m0.337s
    sys     0m0.195s

This significantly improves commit performance and removes sensitivity to AAA
server outages.

1 files changed, 56 insertions, 3 deletions

diff --git a/python/vyos/utils/auth.py b/python/vyos/utils/auth.py
index 462332332..4aa446455 100644
--- a/python/vyos/utils/auth.py
+++ b/python/vyos/utils/auth.py
@@ -18,10 +18,13 @@ import math
 import re
 import string
 
-from enum import StrEnum
+from dataclasses import dataclass
 from decimal import Decimal
-from pwd import getpwall
 from pwd import getpwnam
+from enum import StrEnum
+from typing import List
+from typing import Optional
+
 from vyos.utils.process import cmd
 
 # Minimum UID used when adding system users
@@ -146,12 +149,62 @@ def get_current_user() -> str:
         current_user = os.environ['USER']
     return current_user
 
+@dataclass
+class PasswdEntry:
+    pw_name: str
+    pw_passwd: str
+    pw_uid: int
+    pw_gid: int
+    pw_gecos: str
+    pw_dir: str
+    pw_shell: str
+
+def get_local_passwd_entries(uid: Optional[int] = None) -> PasswdEntry | List[PasswdEntry] | None:
+    """
+    If uid is None: return a list of all passwd entries.
+    If uid is given: return the matching entry or None.
+    """
+    entries = []
+    with open('/etc/passwd', 'r') as f:
+        for line in f:
+            line = line.strip()
+            if not line or line.startswith("#"):
+                continue
+            parts = line.split(":")
+            if len(parts) != 7:
+                continue
+
+            try:
+                entry = PasswdEntry(
+                    pw_name=parts[0],
+                    pw_passwd=parts[1],
+                    pw_uid=int(parts[2]),
+                    pw_gid=int(parts[3]),
+                    pw_gecos=parts[4],
+                    pw_dir=parts[5],
+                    pw_shell=parts[6],
+                )
+            except ValueError:
+                # Skip entries with non-numeric UID or GID
+                continue
+
+            # If searching for a specific UID, return immediately if found
+            if uid is not None and entry.pw_uid == uid:
+                return entry
+
+            entries.append(entry)
+
+    # uid given but not found
+    if uid is not None:
+        return None
+
+    return entries
 
 def get_local_users(min_uid=MIN_USER_UID, max_uid=MAX_USER_UID) -> list:
     """Return list of dynamically allocated users (see Debian Policy Manual)"""
     local_users = []
 
-    for s_user in getpwall():
+    for s_user in get_local_passwd_entries():
         if s_user.pw_uid < min_uid:
             continue
         if s_user.pw_uid > max_uid: