Fix NRPE check return error after snap upgrade

A recent update to snapd changed the response snapd returns when
requesting the version of the Vault snap. This is due to the snapd
API talking http over a socket interface and the code in question
not actually understanding http protocol. Rather than add additional
dependencies on the machine for this check to run when invoked by
nagios invocations, the code is rewritten to parse the version of
the installed vault snap from the output of the snap info command.

Closes-Bug: 2060173
Change-Id: I1d6b9d05b960e7723fab30594dcb73769066112c

src/files/nagios/check_vault_health.py

@@ -8,11 +8,11 @@
 #
 
 import json
-import socket
+import re
 import ssl
+import subprocess
 import sys
 
-from textwrap import dedent
 from urllib.request import urlopen
 
 VAULT_HEALTH_URL = 'http://127.0.0.1:8220/v1/sys/health?standbycode=200&'\
@@ -22,26 +22,46 @@ VAULT_HEALTH_URL = 'http://127.0.0.1:8220/v1/sys/health?standbycode=200&'\
                    'uninitcode=200'
 VAULT_VERIFY_SSL = False
 
-SNAPD_INFO_REQUEST = dedent("""\
-    GET /v2/snaps/{snap} HTTP/1.1\r
-    Host:\r
-    \r
-    """)
-
-SNAPD_SOCKET = '/run/snapd.socket'
-
 
 def get_vault_snap_version():
-    with socket.socket(socket.AF_UNIX, socket.SOCK_STREAM) as snapd:
-        snapd.connect(SNAPD_SOCKET)
-        snapd.sendall(SNAPD_INFO_REQUEST.format(snap='vault').encode('utf-8'))
-        # TODO(pjdc): This should be a loop.
-        info = json.loads(
-            snapd.recv(1024 * 1024).decode('utf-8').split('\n')[-1])
-        version = info['result']['version']
-        if version.startswith('v'):
-            version = version[1:]
-        return version
+    """Returns the vault snap version installed.
+
+    Returns the version of the vault snap installed. This is taken
+    from the output of the `snap info vault` command.
+
+    :returns: the version string for the snap installed
+    :raises: subprocess.CalledProcessError if the command to get the
+             snap version information fails
+    :raises: ValueError if the version string cannot be determined.
+    """
+    try:
+        output = subprocess.check_output(["snap", "info", "vault"],
+                                         encoding=("utf-8"))
+    except subprocess.CalledProcessError:
+        # This is captured in the calling code and results in a
+        # CRITICAL printed to the output
+        raise
+
+    # Search for the line that starts with "installed: ..."
+    # to parse the version string. If the snap is not installed,
+    # this line will not be present in the output.
+    match = re.search(r'^installed:\s*(?P<version>[version:\w.-]*).*$',
+                      output, flags=re.M)
+    version = None
+    if match:
+        version = match.group('version')
+
+    if version is None:
+        raise ValueError("Unable to determine version from output. "
+                         "Is vault snap installed?")
+
+    # The snap versions adopt the actual vault version. If for some reason
+    # this starts with a v, then strip it off. Note: this is carry over
+    # from the previous incarnation of this code and likely unnecessary.
+    if version.startswith("v"):
+        version = version[1:]
+
+    return version
 
 
 def get_vault_server_health(verify=True):