[sonic-py-common] get_platform(): Refactor method of retrieving platform identifier (#5094)

Applications running in the host OS can read the platform identifier from /host/machine.conf. When loading configuration, sonic-config-engine *needs* to read the platform identifier from machine.conf, as it it responsible for populating the value in Config DB.

When an application is running inside a Docker container, the machine.conf file is not accessible, as the /host directory is not mounted. So we need to retrieve the platform identifier from Config DB if get_platform() is called from inside a Docker 
container. However, we can't simply check that we're running in a Docker container because the host OS of the SONiC virtual switch is running inside a Docker container. So I refactored `get_platform()` to:
    1. Read from the `PLATFORM` environment variable if it exists (which is defined in a virtual switch Docker container)
    2. Read from machine.conf if possible (works in the host OS of a standard SONiC image, critical for sonic-config-engine at boot)
    3. Read the value from Config DB (needed for Docker containers running in SONiC, as machine.conf is not accessible to them)

- Also fix typo in daemon_base.py
- Also changes to align `get_hwsku()` with `get_platform()`

platform/vs/docker-sonic-vs/start.sh

@@ -2,8 +2,8 @@
 
 # generate configuration
 
-PLATFORM=x86_64-kvm_x86_64-r0
-HWSKU=Force10-S6000
+export PLATFORM=x86_64-kvm_x86_64-r0
+export HWSKU=Force10-S6000
 
 ln -sf /usr/share/sonic/device/$PLATFORM/$HWSKU /usr/share/sonic/hwsku
 

src/sonic-py-common/sonic_py_common/daemon_base.py

@@ -56,7 +56,7 @@ class DaemonBase(Logger):
         platform_util = None
 
         # Get path to platform and hwsku
-        (platform_path, hwsku_path) = device_info.get_paths_to_platform_and_hwsku()
+        (platform_path, hwsku_path) = device_info.get_paths_to_platform_and_hwsku_dirs()
 
         try:
             module_file = "/".join([platform_path, "plugins", module_name + ".py"])

src/sonic-py-common/sonic_py_common/device_info.py

@@ -58,6 +58,17 @@ def get_platform():
     Returns:
         A string containing the device's platform identifier
     """
+
+    # If we are running in a virtual switch Docker container, the environment
+    # variable 'PLATFORM' will be defined and will contain the platform
+    # identifier.
+    platform_env = os.getenv("PLATFORM")
+    if platform_env:
+        return platform_env
+
+    # If 'PLATFORM' env variable is not defined, we try to read the platform
+    # identifier from machine.conf. This is critical for sonic-config-engine,
+    # because it is responsible for populating this value in Config DB.
     machine_info = get_machine_info()
     if machine_info:
         if 'onie_platform' in machine_info:
@@ -65,6 +76,21 @@ def get_platform():
         elif 'aboot_platform' in machine_info:
             return machine_info['aboot_platform']
 
+    # If we fail to read from machine.conf, we may be running inside a Docker
+    # container in SONiC, where the /host directory is not mounted. In this
+    # case the value should already be populated in Config DB so we finally
+    # try reading it from there.
+    try:
+        config_db = ConfigDBConnector()
+        config_db.connect()
+
+        metadata = config_db.get_table('DEVICE_METADATA')
+
+        if 'localhost' in metadata and 'platform' in metadata['localhost']:
+            return metadata['localhost']['platform']
+    except Exception:
+        pass
+
     return None
 
 
@@ -75,15 +101,18 @@ def get_hwsku():
     Returns:
         A string containing the device's hardware SKU identifier
     """
-    config_db = ConfigDBConnector()
-    config_db.connect()
+    try:
+        config_db = ConfigDBConnector()
+        config_db.connect()
 
-    metadata = config_db.get_table('DEVICE_METADATA')
+        metadata = config_db.get_table('DEVICE_METADATA')
 
-    if 'localhost' in metadata and 'hwsku' in metadata['localhost']:
-        return metadata['localhost']['hwsku']
+        if 'localhost' in metadata and 'hwsku' in metadata['localhost']:
+            return metadata['localhost']['hwsku']
+    except Exception:
+        pass
 
-    return ""
+    return None
 
 
 def get_platform_and_hwsku():