Bug 1305695 - ./mach test --debugger=<debugger> doesn't fail if <debu…

…gger> isn't available r=gbrown

- added checkers in python/mach/mach/main.py prior to calling registrar.py.
- added internal function to check if specified debugger is installed.
- support both ./mach test <test_name> and ./mach <test_category> styles.

Differential Revision: https://phabricator.services.mozilla.com/D7234

--HG--
extra : moz-landing-system : lando

python/mach/mach/main.py

@@ -452,6 +452,42 @@ def _run(self, argv):
             # to command line handling (e.g alias, defaults) will be ignored.
             self.load_settings(args.settings_file)
 
+        def _check_debugger(program):
+            """Checks if debugger specified in command line is installed.
+
+            This internal function calls an in-tree library 'which'.
+
+            If the call does not raise any exceptions, mach is permitted
+            to continue execution.
+
+            Otherwise, mach execution is halted.
+
+            Args:
+                program (str): debugger program name.
+            """
+            from which import which, WhichError
+            try:
+                which(program)
+            except WhichError:
+                print("Specified debugger '{}' is not found.\n".format(program) +
+                      "Is it installed? Is it in your PATH?")
+                sys.exit(1)
+
+        # For the codepath where ./mach <test_type> --debugger=<program>,
+        # checks if the debugger specified is installed on system.
+        if (hasattr(args.command_args, "debugger") and
+                getattr(args.command_args, "debugger") is not None):
+            _check_debugger(getattr(args.command_args, "debugger"))
+        # For the codepath where ./mach test --debugger=<program> <test_type>,
+        # checks if the debugger specified is installed on system.
+        elif (hasattr(args.command_args, "extra_args") and
+                getattr(args.command_args, "extra_args")):
+            extra_args = getattr(args.command_args, "extra_args")
+            # This supports common use case where one debugger is specified.
+            debugger = [ea.split("=")[1] for ea in extra_args if "debugger" in ea]
+            if debugger:
+                _check_debugger(''.join(debugger))
+
         if not hasattr(args, 'mach_handler'):
             raise MachError('ArgumentParser result missing mach handler info.')