Bug 1750153 - Prevent unwanted errors from showing up in ./mach show-log after closing less before the entire log file has been piped to it r=firefox-build-system-reviewers,mhentges

The main thread is blocking (for an unknown reason) without piping the entirety of the log
file to `less` (unless the user manually scrolls to almost the bottom). Since the piping
cannot always complete without a user terminating, we must hide the errors that arise
from using the `stdin` stream for the `less` process after the process is terminated.

Differential Revision: https://phabricator.services.mozilla.com/D135979

python/mozbuild/mozbuild/mach_commands.py

@@ -17,6 +17,7 @@ import subprocess
 import sys
 import tempfile
 import time
+import errno
 
 import mozbuild.settings  # noqa need @SettingsProvider hook to execute
 import mozpack.path as mozpath
@@ -361,31 +362,60 @@ def show_log(command_context, log_file=None):
             ["less"], stdin=subprocess.PIPE, env=env, encoding="UTF-8"
         )
 
-        # Create a new logger handler with the stream being the stdin of our 'less'
-        # process so that we can pipe the logger output into 'less'
-        less_handler = logging.StreamHandler(stream=less.stdin)
-        less_handler.setFormatter(
-            command_context.log_manager.terminal_handler.formatter
-        )
-        less_handler.setLevel(command_context.log_manager.terminal_handler.level)
+        try:
+            # Create a new logger handler with the stream being the stdin of our 'less'
+            # process so that we can pipe the logger output into 'less'
+            less_handler = logging.StreamHandler(stream=less.stdin)
+            less_handler.setFormatter(
+                command_context.log_manager.terminal_handler.formatter
+            )
+            less_handler.setLevel(command_context.log_manager.terminal_handler.level)
 
-        # replace the existing terminal handler with the new one for 'less' while
-        # still keeping the original one to set back later
-        original_handler = command_context.log_manager.replace_terminal_handler(
-            less_handler
-        )
+            # replace the existing terminal handler with the new one for 'less' while
+            # still keeping the original one to set back later
+            original_handler = command_context.log_manager.replace_terminal_handler(
+                less_handler
+            )
 
-        # Parses the log file line by line and streams (to less.stdin) the relevant records we want
-        handle_log_file(command_context, log_file)
+            # Save this value so we can set it back to the original value later
+            original_logging_raise_exceptions = logging.raiseExceptions
 
-        # Set the handler (and contained stream) back to the original one
-        command_context.log_manager.replace_terminal_handler(original_handler)
+            # We need to explicitly disable raising exceptions inside logging so
+            # that we can catch them here ourselves to ignore the ones we want
+            logging.raiseExceptions = False
 
-        # At this point we've piped the entire log file to 'less', so we can close the input stream
-        less.stdin.close()
-        # Wait for the user to manually terminate `less`
-        less.wait()
+            # Parses the log file line by line and streams
+            # (to less.stdin) the relevant records we want
+            handle_log_file(command_context, log_file)
+
+            # At this point we've piped the entire log file to
+            # 'less', so we can close the input stream
+            less.stdin.close()
+
+            # Wait for the user to manually terminate `less`
+            less.wait()
+        except OSError as os_error:
+            # (POSIX)   errno.EPIPE: BrokenPipeError: [Errno 32] Broken pipe
+            # (Windows) errno.EINVAL: OSError:        [Errno 22] Invalid argument
+            if os_error.errno == errno.EPIPE or os_error.errno == errno.EINVAL:
+                # If the user manually terminates 'less' before the entire log file
+                # is piped (without scrolling close enough to the bottom) we will get
+                # one of these errors (depends on the OS) because the logger will still
+                # attempt to stream to the now invalid less.stdin. To prevent a bunch
+                # of errors being shown after a user terminates 'less', we just catch
+                # the first of those exceptions here, and stop parsing the log file.
+                pass
+            else:
+                raise
+        except Exception:
+            raise
+        finally:
+            # Ensure these values are changed back to the originals, regardless of outcome
+            command_context.log_manager.replace_terminal_handler(original_handler)
+            logging.raiseExceptions = original_logging_raise_exceptions
     else:
+        # Not in a terminal context, so just handle the log file with the
+        # default stream without piping it to a pager (less)
         handle_log_file(command_context, log_file)