vpp: T9484: keep net.core.rmem_max at VPP's netlink buffer size

VPP requests a 256 MiB netlink receive buffer (nl-rx-buffer-size), but
the kernel caps it at net.core.rmem_max, which VyOS sets to 2 MiB, so
VPP gets 4 MiB. A large routing table over VPP interfaces can overflow
it, and linux-cp then falls back to a full resync.

Setting the sysctl from the CLI does not help at boot: system sysctl
runs after vpp, so VPP has already opened its socket.

- vpp.py raises net.core.rmem_max to the requested size before VPP is
  (re)started; a higher value is kept.
- VPP re-opens its netlink socket on every resync and sizes it again, so
  system sysctl no longer applies a lower value while VPP is configured;
  it uses the required size and prints a warning.
- When VPP is removed, vpp.py sets net.core.rmem_max back to the value
  from system sysctl, or to the default from sysctl.d, read with the new
  vyos.utils.system.sysctl_read_default().
- startup.conf.j2 takes nl-rx-buffer-size from the same value
  (default_resource_map) instead of its own constant.

1 files changed, 26 insertions, 0 deletions

diff --git a/python/vyos/utils/system.py b/python/vyos/utils/system.py
index 90ab74ea6..fc6507328 100644
--- a/python/vyos/utils/system.py
+++ b/python/vyos/utils/system.py
@@ -33,6 +33,32 @@ def sysctl_read(name: list[str]) -> str:
     tmp = run(['sysctl', '-nb', key], capture_output=True)
     return tmp.stdout.decode().strip()
 
+
+def sysctl_read_default(name: list[str]) -> str | None:
+    """Read the value the sysctl.d files set for a sysctl() option
+
+    systemd-analyze lists the files in the order systemd-sysctl applies them,
+    so the last assignment wins.
+
+    Args:
+        name (list[str]): sysctl key name components
+
+    Returns:
+        str | None: sysctl key value, None if no file sets it
+    """
+    key = _sysctl_key(name)
+    tmp = run(['systemd-analyze', 'cat-config', 'sysctl.d'], capture_output=True)
+    value = None
+    for line in tmp.stdout.decode().splitlines():
+        line = line.strip()
+        if not line or line.startswith(('#', ';')):
+            continue
+        tmp_key, sep, tmp_value = line.partition('=')
+        # a leading '-' only tells systemd-sysctl to ignore errors
+        if sep and tmp_key.strip().lstrip('-') == key:
+            value = tmp_value.strip()
+    return value
+
 def sysctl_write(name: list[str], value: str | int) -> bool:
     """Change value via sysctl()