bpo-44461: Check early that a pdb target is valid for execution. (#27227)

* bpo-44461: Fix bug with pdb's handling of import error due to a package which does not have a __main__ module

* 📜🤖 Added by blurb_it.

* remove "else"

Co-authored-by: Jason R. Coombs <jaraco@jaraco.com>

* If running as a module, first check that it can run as a module. Alternate fix for bpo-44461.

Co-authored-by: Irit Katriel <iritkatriel@yahoo.com>
Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>
Co-authored-by: Irit Katriel <1055913+iritkatriel@users.noreply.github.com>

Lib/pdb.py

@@ -169,7 +169,11 @@ class ScriptTarget(str):
 
 class ModuleTarget(str):
     def check(self):
-        pass
+        try:
+            self._details
+        except Exception:
+            traceback.print_exc()
+            sys.exit(1)
 
     @functools.cached_property
     def _details(self):

Lib/test/test_pdb.py

@@ -1729,6 +1729,20 @@ def bœr():
         self.assertIn("ImportError: No module named t_main.__main__",
                       stdout.splitlines())
 
+    def test_package_without_a_main(self):
+        pkg_name = 't_pkg'
+        module_name = 't_main'
+        os_helper.rmtree(pkg_name)
+        modpath = pkg_name + '/' + module_name
+        os.makedirs(modpath)
+        with open(modpath + '/__init__.py', 'w') as f:
+            pass
+        self.addCleanup(os_helper.rmtree, pkg_name)
+        stdout, stderr = self._run_pdb(['-m', modpath.replace('/', '.')], "")
+        self.assertIn(
+            "'t_pkg.t_main' is a package and cannot be directly executed",
+            stdout)
+
     def test_blocks_at_first_code_line(self):
         script = """
                 #This is a comment, on line 2

Misc/NEWS.d/next/Library/2021-06-29-21-17-17.bpo-44461.acqRnV.rst

@@ -0,0 +1 @@
+Fix bug with :mod:`pdb`'s handling of import error due to a package which does not have a ``__main__`` module