yanger: Start the modularization process with ietf-system

The yanger source grown significantly, code is disorganized, the
namespace is all over the place, etc.

Take a stab at remedying the situation by splitting out the individual
YANG models in separate Python modules, starting with ietf-system.

src/statd/python/yanger/host.py

@@ -0,0 +1,133 @@
+import abc
+import datetime
+import json
+import os
+import subprocess
+
+from . import common
+
+HOST = None
+
+class Host(abc.ABC):
+    """Host system API"""
+
+    @abc.abstractmethod
+    def now(self):
+        """Get the current time as a `datetime`"""
+        pass
+
+    @abc.abstractmethod
+    def run(self, cmd, default=None, log=True):
+        """Get stdout of cmd
+
+        Run cmd, provided as an argv, and return the output it
+        produces. On error, return the default value if provided,
+        otherwise raise an exception.
+
+        """
+        pass
+
+    def run_multiline(self, cmd, default=None):
+        """Get lines of stdout of cmd"""
+        try:
+            txt = self.run(cmd, log=(default is None))
+            return txt.splitlines()
+        except:
+            if default is not None:
+                return default
+            raise
+
+    def run_json(self, cmd, default=None):
+        """Get JSON object from stdout of cmd"""
+        try:
+            txt = self.run(cmd, log=(default is None))
+            return json.loads(txt)
+        except:
+            if default is not None:
+                return default
+            raise
+
+    @abc.abstractmethod
+    def read(self, path):
+        """Get the contents of path
+
+        Returns `None` if the file is not readable for any reason.
+
+        """
+        pass
+
+    def read_json(self, path, default=None):
+        """Get JSON object from path """
+        try:
+            txt = self.read(path)
+            return json.loads(txt)
+        except:
+            if default is not None:
+                return default
+            raise
+
+class Localhost(Host):
+    def now(self):
+        return datetime.datetime.now(tz=datetime.timezone.utc)
+
+    def run(self, cmd, default=None, log=True):
+        try:
+            result = subprocess.run(cmd, check=True, text=True,
+                                    stdin=subprocess.DEVNULL,
+                                    stdout=subprocess.PIPE,
+                                    stderr=subprocess.DEVNULL)
+            return result.stdout
+        except subprocess.CalledProcessError as err:
+            if default is not None:
+                return default
+
+            if log:
+                common.LOG.error(f"Failed to run {err}")
+            raise
+
+    def read(self, path):
+        try:
+            with open(path, 'r') as f:
+                data = f.read().strip()
+                return data
+        except FileNotFoundError:
+            # This is considered OK
+            pass
+        except IOError:
+            common.LOG.error(f"Failed to read \"{path}\"")
+
+        return None
+
+class Testhost(Host):
+    def __init__(self, basedir):
+        self.basedir = basedir
+
+    def now(self):
+        return datetime.datetime(2023, 1, 1, 12, 0, 0, tzinfo=datetime.timezone.utc)
+
+    def run(self, cmd, default=None, log=True):
+        slug = "_".join(cmd).replace("/", "+").replace(" ", "-")
+        path = os.path.join(self.basedir, "run", slug)
+
+        try:
+            with open(path, 'r') as f:
+                return f.read()
+        except:
+            if default is not None:
+                return default
+
+            if log:
+                common.LOG.error(f"No recording found for run \"{path}\"")
+            raise
+
+    def read(self, path):
+        path = os.path.join(self.basedir, "rootfs", path[1:])
+        try:
+            with open(path, 'r') as f:
+                return f.read()
+        except FileNotFoundError:
+            # This is considered OK
+            pass
+        except:
+            common.LOG.error(f"No recording found for file \"{path}\"")
+            raise