feat: report how full /, /boot and /var are in the host status

A Proxmox host's root filesystem stood at 94 % with about 400 MB free for a
week; a kernel upgrade failed on it with "No space left on device" and left
dpkg halfway, and nothing had said the disk was full (NetOrk/netork#862).

HOST_STATUS_COMMAND now also runs POSIX `df -Pk / /boot /var`, without root.
get_host_status() returns `filesystems`: each filesystem once (a directory
that is not a mount of its own is the filesystem it lives on), with its
mountpoint, total, used and available bytes, or None when df said nothing.
`available` is what df says an ordinary user can still write, which on ext4
is less than total minus used. How full is too full is the caller's decision.

Version 4.4.0.

tests/test_host_status.py

@@ -428,3 +428,81 @@ class TestBusyCheck:
         """Run over an exec channel, the command is the command line of the shell
         that runs it, and ``ps -o args`` lists it."""
         assert not re.search(r"(^|[ /])unattended-upgrade( |$)", HOST_STATUS_COMMAND)
+
+
+class TestFilesystems:
+    """How full the filesystems an upgrade writes to are: ``/``, ``/boot`` and
+    ``/var``, read with ``df -Pk`` and no root.
+
+    pve-dual's root filesystem stood at 94 % with about 400 MB free for a week;
+    a kernel upgrade failed on it with "No space left on device" and left dpkg
+    halfway, and nothing had said the disk was full (NetOrk/netork#862). The
+    figures below are shaped after it (6.5 GiB, 400 MiB available): ext4 keeps
+    5 % for root, so what is available is less than total minus used.
+    """
+
+    HEADER = "Filesystem 1024-blocks Used Available Capacity Mounted on"
+    PVE_ROOT = "/dev/mapper/pve-root 6815744 6065357 409600 94% /"
+    BOOT = "/dev/sda2 1046512 104860 941652 11% /boot"
+
+    @staticmethod
+    def _report(*df_lines: str) -> str:
+        return _wire().replace("[timers]", "\n".join(["[filesystems]", *df_lines, "[timers]"]))
+
+    def test_the_root_filesystem_in_bytes(self):
+        status = parse_host_status(self._report(self.HEADER, self.PVE_ROOT))
+
+        assert status["filesystems"] == [
+            {
+                "mountpoint": "/",
+                "total": 6815744 * 1024,
+                "used": 6065357 * 1024,
+                "available": 409600 * 1024,
+            }
+        ]
+
+    def test_a_separate_boot_partition(self):
+        status = parse_host_status(self._report(self.HEADER, self.PVE_ROOT, self.BOOT))
+
+        assert [fs["mountpoint"] for fs in status["filesystems"]] == ["/", "/boot"]
+
+    def test_a_directory_on_root_is_root_once(self):
+        """``df /boot /var`` names the filesystem they live on, here ``/`` three times."""
+        status = parse_host_status(
+            self._report(self.HEADER, self.PVE_ROOT, self.PVE_ROOT, self.PVE_ROOT)
+        )
+
+        assert [fs["mountpoint"] for fs in status["filesystems"]] == ["/"]
+
+    def test_a_long_device_name_and_a_blank_in_the_mountpoint(self):
+        line = "/dev/disk/by-id/a very long name 1000 600 400 60% /mnt/my disk"
+
+        (fs,) = parse_host_status(self._report(self.HEADER, line))["filesystems"]
+
+        assert (fs["mountpoint"], fs["available"]) == ("/mnt/my disk", 400 * 1024)
+
+    def test_a_filesystem_without_size_is_left_out(self):
+        """What a pseudo filesystem reports; it cannot run full."""
+        status = parse_host_status(self._report(self.HEADER, "none 0 0 0 - /"))
+
+        assert status["filesystems"] is None
+
+    def test_without_the_section_it_is_unknown(self):
+        """An older command, or no df: None, not "nothing is full"."""
+        assert parse_host_status(_wire())["filesystems"] is None
+
+    def test_df_that_printed_nothing_is_unknown(self):
+        assert parse_host_status(self._report())["filesystems"] is None
+
+    def test_the_command_asks_for_the_three_without_root(self):
+        assert "df -Pk / /boot /var" in HOST_STATUS_COMMAND
+        assert "sudo" not in HOST_STATUS_COMMAND
+
+    def test_the_command_reads_them_on_a_real_shell(self):
+        """POSIX df the way a host's sh runs it: ``/`` is always there, whatever it is."""
+        out = subprocess.run(
+            ["sh", "-c", HOST_STATUS_COMMAND], capture_output=True, text=True, timeout=30
+        )
+
+        mountpoints = [fs["mountpoint"] for fs in parse_host_status(out.stdout)["filesystems"]]
+        assert "/" in mountpoints