SSD Health: Retrieve SSD health and temperature values from generic SSD info (#229)

The problem:
    The current SSD health API uses a SSD vendor specific utility to retrieve
    values for overall SSD health and internal SSD temperature. These utilities
    are typically made available only in binary form and are not always
    available for all platform architectures (amd64, arm64, armhf, etc.)
    Vendor licensing agreements can prevent the inclusion of utility source
    code in the Sonic build.

The solution presented here is to retrieve SSD health and temperature values
from the generic SSD info retrieved via the open-source smartmontools utility
(smartctl). These values can be overwritten with values retrieved using a
vendor specific utility when available. smartmontools is already included in
the Sonic source tree and is being built for all platform architectures.

Author: dflynn-Nokia

sonic_platform_base/sonic_ssd/ssd_generic.py

@@ -63,6 +63,9 @@ def __init__(self, diskdev):
     def _execute_shell(self, cmd):
         process = subprocess.Popen(cmd.split(), universal_newlines=True, stdout=subprocess.PIPE)
         output, error = process.communicate()
+        exit_code = process.returncode
+        if exit_code:
+            return None
         return output
 
     def _parse_re(self, pattern, buffer):
@@ -72,10 +75,13 @@ def _parse_re(self, pattern, buffer):
     def fetch_generic_ssd_info(self, diskdev):
         self.ssd_info = self._execute_shell(self.vendor_ssd_utility["Generic"]["utility"].format(diskdev))
 
+    # Health and temperature values may be overwritten with vendor specific data
     def parse_generic_ssd_info(self):
         self.model = self._parse_re('Device Model:\s*(.+?)\n', self.ssd_info)
         self.serial = self._parse_re('Serial Number:\s*(.+?)\n', self.ssd_info)
         self.firmware = self._parse_re('Firmware Version:\s*(.+?)\n', self.ssd_info)
+        self.health = self._parse_re('Remaining_Lifetime_Perc\s*(.+?)\n', self.ssd_info).split()[-1]
+        self.temperature = self._parse_re('Temperature_Celsius\s*(.+?)\n', self.ssd_info).split()[-6]
 
     def parse_innodisk_info(self):
         self.health = self._parse_re('Health:\s*(.+?)%', self.vendor_ssd_info)
@@ -94,7 +100,8 @@ def fetch_vendor_ssd_info(self, diskdev, model):
         self.vendor_ssd_info = self._execute_shell(self.vendor_ssd_utility[model]["utility"].format(diskdev))
 
     def parse_vendor_ssd_info(self, model):
-        self.vendor_ssd_utility[model]["parser"]()
+        if self.vendor_ssd_info:
+            self.vendor_ssd_utility[model]["parser"]()
 
     def get_health(self):
         """