lazy logging in event module (#4210)

* switches on debug level to guard against expensive messages

* adds memoization to msg construction

core/dbt/events/functions.py

@@ -2,8 +2,9 @@
 import dbt.logger as logger  # type: ignore # TODO eventually remove dependency on this logger
 from dbt.events.history import EVENT_HISTORY
 from dbt.events.types import CliEventABC, Event, ShowException
+import dbt.flags as flags
 import os
-from typing import List
+from typing import Generator, List
 
 
 def env_secrets() -> List[str]:
@@ -22,57 +23,73 @@ def scrub_secrets(msg: str, secrets: List[str]) -> str:
     return scrubbed
 
 
+# this exists because some log messages are actually expensive to build.
+# for example, many debug messages call `dump_graph()` and we don't want to
+# do that in the event that those messages are never going to be sent to
+# the user because we are only logging info-level events.
+def gen_msg(e: CliEventABC) -> Generator[str, None, None]:
+    msg = None
+    if not msg:
+        msg = scrub_secrets(e.cli_msg(), env_secrets())
+    while True:
+        yield msg
+
+
 # top-level method for accessing the new eventing system
 # this is where all the side effects happen branched by event type
 # (i.e. - mutating the event history, printing to stdout, logging
 # to files, etc.)
 def fire_event(e: Event) -> None:
     EVENT_HISTORY.append(e)
     if isinstance(e, CliEventABC):
-        msg = scrub_secrets(e.cli_msg(), env_secrets())
+        msg = gen_msg(e)
         if e.level_tag() == 'test' and not isinstance(e, ShowException):
             # TODO after implmenting #3977 send to new test level
-            logger.GLOBAL_LOGGER.debug(logger.timestamped_line(msg))
+            logger.GLOBAL_LOGGER.debug(next(msg))
         elif e.level_tag() == 'test' and isinstance(e, ShowException):
             # TODO after implmenting #3977 send to new test level
             logger.GLOBAL_LOGGER.debug(
-                logger.timestamped_line(msg),
+                next(msg),
                 exc_info=e.exc_info,
                 stack_info=e.stack_info,
                 extra=e.extra
             )
+        # explicitly checking the debug flag here so that potentially expensive-to-construct
+        # log messages are not constructed if debug messages are never shown.
+        elif e.level_tag() == 'debug' and not flags.DEBUG:
+            return  # eat the message in case it was one of the expensive ones
         elif e.level_tag() == 'debug' and not isinstance(e, ShowException):
-            logger.GLOBAL_LOGGER.debug(logger.timestamped_line(msg))
+            logger.GLOBAL_LOGGER.debug(next(msg))
         elif e.level_tag() == 'debug' and isinstance(e, ShowException):
             logger.GLOBAL_LOGGER.debug(
-                logger.timestamped_line(msg),
+                next(msg),
                 exc_info=e.exc_info,
                 stack_info=e.stack_info,
                 extra=e.extra
             )
         elif e.level_tag() == 'info' and not isinstance(e, ShowException):
-            logger.GLOBAL_LOGGER.info(logger.timestamped_line(msg))
+            logger.GLOBAL_LOGGER.info(next(msg))
         elif e.level_tag() == 'info' and isinstance(e, ShowException):
             logger.GLOBAL_LOGGER.info(
-                logger.timestamped_line(msg),
+                next(msg),
                 exc_info=e.exc_info,
                 stack_info=e.stack_info,
                 extra=e.extra
             )
         elif e.level_tag() == 'warn' and not isinstance(e, ShowException):
-            logger.GLOBAL_LOGGER.warning(logger.timestamped_line(msg))
+            logger.GLOBAL_LOGGER.warning(next(msg))
         elif e.level_tag() == 'warn' and isinstance(e, ShowException):
             logger.GLOBAL_LOGGER.warning(
-                logger.timestamped_line(msg),
+                next(msg),
                 exc_info=e.exc_info,
                 stack_info=e.stack_info,
                 extra=e.extra
             )
         elif e.level_tag() == 'error' and not isinstance(e, ShowException):
-            logger.GLOBAL_LOGGER.error(logger.timestamped_line(msg))
+            logger.GLOBAL_LOGGER.error(next(msg))
         elif e.level_tag() == 'error' and isinstance(e, ShowException):
             logger.GLOBAL_LOGGER.error(
-                logger.timestamped_line(msg),
+                next(msg),
                 exc_info=e.exc_info,
                 stack_info=e.stack_info,
                 extra=e.extra