Servlet.py

"""Abstract servlets"""

import os

from MiscUtils import AbstractError
from MiscUtils.Funcs import asclocaltime


class Servlet:
    """A general servlet.

    A servlet is a key portion of a server-based application that implements
    the semantics of a particular request by providing a response.
    This abstract class defines servlets at a very high level.
    Most often, developers will subclass HTTPServlet or even Page.

    Servlets can be created once, then used and destroyed, or they may be
    reused several times over (it's up to the server). Therefore, servlet
    developers should take the proper actions in awake() and sleep()
    so that reuse can occur.

    Objects that participate in a transaction include:
      * Application
      * Request
      * Transaction
      * Session
      * Servlet
      * Response

    The awake(), respond() and sleep() methods form a message sandwich.
    Each is passed an instance of Transaction which gives further access
    to all the objects involved.
    """

    # region Init

    def __init__(self):
        """Subclasses must invoke super."""
        self._serverSidePath = None
        self._factory = None
        self._busy = False

    def __repr__(self):
        name = self.name()
        try:
            path = self.serverSidePath()
        except AttributeError:
            path = None
        if path:
            name += f' at {path}'
        return f'<Servlet {name}>'

    # endregion Init

    # region Access

    def name(self):
        """Return the name which is simple the name of the class.

        Subclasses should *not* override this method.
        It is used for logging and debugging.
        """
        return self.__class__.__name__

    # endregion Access

    # region Request-response cycles

    @staticmethod
    def runTransaction(transaction):
        try:
            transaction.awake()
            transaction.respond()
        except Exception as first:
            try:
                transaction.sleep()
            except Exception as second:
                # The first exception is more important than the *second* one
                # that comes from sleep(). In fact, without this little trick
                # the first exception gets hidden by the second which is often
                # just a result of the first. Then you're stuck scratching your
                # head wondering what the first might have been.
                raise second from first
            # no problems with sleep() so raise the one and only exception
            raise
        transaction.sleep()

    def runMethodForTransaction(self, transaction, method, *args, **kw):
        self.awake(transaction)
        result = getattr(self, method)(*args, **kw)
        self.sleep(transaction)
        return result

    def awake(self, transaction):
        """Send the awake message.

        This message is sent to all objects that participate in the
        request-response cycle in a top-down fashion, prior to respond().
        Subclasses must invoke super.
        """
        self._transaction = transaction

    def respond(self, transaction):
        """Respond to a request."""
        raise AbstractError(self.__class__)

    def sleep(self, transaction):
        """Send the sleep message."""
        # base method does nothing

    # endregion Request-response cycles

    # region Log

    def log(self, message):
        """Log a message.

        This can be invoked to print messages concerning the servlet.
        This is often used by self to relay important information back
        to developers.
        """
        print(f'[{asclocaltime()}] [msg] {message}')

    # endregion Log

    # region Abilities

    def canBeThreaded(self):
        """Return whether the servlet can be multithreaded.

        This value should not change during the lifetime of the object.
        The default implementation returns False.
        Note: This is not currently used.
        """
        return False

    def canBeReused(self):
        """Returns whether a single servlet instance can be reused.

        The default is True, but subclasses con override to return False.
        Keep in mind that performance may seriously be degraded if instances
        can't be reused. Also, there's no known good reasons not to reuse
        an instance. Remember the awake() and sleep() methods are invoked
        for every transaction. But just in case, your servlet can refuse
        to be reused.
        """
        return True

    # endregion Abilities

    # region Server side filesystem

    def serverSidePath(self, path=None):
        """Return the filesystem path of the page on the server."""
        if self._serverSidePath is None:
            self._serverSidePath = self._transaction.request().serverSidePath()
        if path:
            if path.startswith('/'):
                path = path[1:]
            return os.path.normpath(os.path.join(
                os.path.dirname(self._serverSidePath), path))
        return self._serverSidePath

    # endregion Server side filesystem

    # region Private

    def open(self):
        self._busy = True

    def close(self):
        if self._busy and self._factory:
            self._busy = False
            self._factory.returnServlet(self)

    def setFactory(self, factory):
        self._factory = factory

    # endregion Private