feat: set up SNMP with net-snmp

run_device_action("fix_snmp") and get_snmp_config for both drivers, as
decided in NetOrk/netork#800: net-snmp from packages rather than the base
daemons, because it answers UCD-SNMP-MIB, which netOrk's health metrics
read.

- fix_snmp installs net-snmp, writes the configuration netOrk uses on
  Linux (v2c, community "public", agentAddress udp:161) through stdin as
  root, enables and restarts the agent (FreeBSD `service snmpd`, OpenBSD
  `rcctl ... netsnmpd`, as `snmpd` is OpenBSD's own daemon) and asks it
  for sysDescr. It stops at the first step that fails and says why.
- get_snmp_config reports a running net-snmp's community and port.
- FreeBSD's install_package bootstraps pkg on a classic system that never
  had it instead of waiting for an answer that never comes.

Checked live: FreeBSD from a bare system, OpenBSD again over an existing
setup. Memory, swap and load answer on both; FreeBSD's ssCpuIdle about a
minute after start; OpenBSD's CPU figures from net-snmp are wrong (#800).

tests/test_drivers.py

@@ -274,7 +274,11 @@ class TestPackages:
     @pytest.mark.parametrize(
         ("cls", "install", "uninstall"),
         [
-            (FreeBSDDriver, "pkg install -y nginx", "pkg delete -y nginx"),
+            (
+                FreeBSDDriver,
+                "env ASSUME_ALWAYS_YES=yes pkg install -y nginx",
+                "pkg delete -y nginx",
+            ),
             (OpenBSDDriver, "pkg_add -I nginx", "pkg_delete nginx"),
         ],
     )
@@ -287,7 +291,7 @@ class TestPackages:
     def test_a_version_is_part_of_the_name(self):
         driver = _channel(FreeBSDDriver, {})
         driver.install_package("nginx", "1.28.0")
-        assert driver.calls == [("pkg install -y nginx-1.28.0", True)]
+        assert driver.calls == [("env ASSUME_ALWAYS_YES=yes pkg install -y nginx-1.28.0", True)]
 
     def test_a_failed_install_raises(self):
         driver = _channel(OpenBSDDriver, {("pkg_add -I nope", True): ("Can't find nope", 1)})
@@ -437,3 +441,94 @@ class TestServices:
         with pytest.raises(ValueError):
             driver.manage_service(name, action)
         assert driver.calls == []
+
+
+class TestSnmp:
+    """net-snmp from packages, as decided in NetOrk/netork#800: UCD-SNMP-MIB,
+    the same health metrics as on Linux."""
+
+    PROBE_OK = "STRING: FreeBSD host 15.1-RELEASE-p4 FreeBSD 15.1-RELEASE-p4 GENERIC amd64"
+
+    @pytest.mark.parametrize(
+        ("cls", "install", "conf", "start"),
+        [
+            (
+                FreeBSDDriver,
+                "env ASSUME_ALWAYS_YES=yes pkg install -y net-snmp",
+                "/usr/local/etc/snmp/snmpd.conf",
+                "sysrc snmpd_enable=YES && service snmpd restart",
+            ),
+            (
+                OpenBSDDriver,
+                "pkg_add -I net-snmp",
+                "/etc/snmp/snmpd.conf",
+                "rcctl enable netsnmpd && rcctl restart netsnmpd",
+            ),
+        ],
+    )
+    def test_fix_snmp_installs_configures_and_starts_net_snmp(self, cls, install, conf, start):
+        driver = _channel(cls, {BsdDriver.SNMP_PROBE: self.PROBE_OK})
+        stdin_seen = {}
+        run = driver.run_command
+
+        def recording(command, *, privileged=False, timeout=60, stdin=None):
+            if stdin is not None:
+                stdin_seen[command] = stdin
+            return run(command, privileged=privileged, timeout=timeout, stdin=stdin)
+
+        driver.run_command = recording  # type: ignore[method-assign]
+        result = driver.run_device_action("fix_snmp")
+
+        assert result["success"] is True
+        commands = [c for c, _ in driver.calls]
+        assert commands[0] == install
+        assert any(conf in c and "cat >" in c for c in commands)
+        assert start in commands
+        assert commands[-1] == BsdDriver.SNMP_PROBE
+        assert all(p for c, p in driver.calls if c != BsdDriver.SNMP_PROBE)  # root for the setup
+        written = next(v for k, v in stdin_seen.items() if conf in k).decode()
+        assert "agentAddress udp:161" in written and "rocommunity public" in written
+
+    def test_a_failed_install_stops_and_says_so(self):
+        driver = _channel(
+            FreeBSDDriver,
+            {
+                ("env ASSUME_ALWAYS_YES=yes pkg install -y net-snmp", True): (
+                    "pkg: No packages available",
+                    1,
+                )
+            },
+        )
+        result = driver.run_device_action("fix_snmp")
+        assert result["success"] is False
+        assert "No packages available" in result["output"]
+        assert len(driver.calls) == 1
+
+    def test_no_answer_from_the_agent_is_a_failure(self):
+        driver = _channel(
+            OpenBSDDriver, {BsdDriver.SNMP_PROBE: "Timeout: No Response from 127.0.0.1"}
+        )
+        assert driver.run_device_action("fix_snmp")["success"] is False
+
+    def test_unknown_action(self):
+        with pytest.raises(NotImplementedError):
+            _channel(FreeBSDDriver, {}).run_device_action("fix_apt_proxy")
+
+    def test_snmp_config_of_a_running_agent(self):
+        driver = _channel(
+            FreeBSDDriver,
+            {
+                "pgrep -f /usr/local/sbin/snmpd": "1234",
+                "cat /usr/local/etc/snmp/snmpd.conf": "agentAddress udp:1161\nrocommunity s3cret\n",
+            },
+        )
+        assert driver.get_snmp_config() == {
+            "running": True,
+            "community": "s3cret",
+            "port": 1161,
+            "version": "2c",
+        }
+
+    def test_no_agent_running_means_no_config(self):
+        driver = _channel(OpenBSDDriver, {"pgrep -f /usr/local/sbin/snmpd": ("", 1)})
+        assert driver.get_snmp_config() is None