Fix viewcode extension importing modules more than once (#13380)

Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>

AUTHORS.rst

@@ -45,6 +45,7 @@ Contributors
 * Daniel Eades -- improved static typing
 * Daniel Hahler -- testing and CI improvements
 * Daniel Pizetta -- inheritance diagram improvements
+* Dave Hoese -- ``sphinx.ext.viewcode`` bug fix
 * Dave Kuhlman -- original LaTeX writer
 * Dimitri Papadopoulos Orfanos -- linting and spelling
 * Dmitry Shachnev -- modernisation and reproducibility

CHANGES.rst

@@ -19,6 +19,8 @@ Bugs fixed
 * #13377: Restore support for using ``sphinx.testing.path`` paths with
   ``sphinx.testing.fixtures``.
   Patch by Kazuya Takei.
+* #13380: viewcode: Fix importing modules more than once.
+  Patch by Dave Hoese.
 
 Testing
 -------

sphinx/ext/viewcode.py

@@ -2,7 +2,7 @@
 
 from __future__ import annotations
 
-import importlib.util
+import importlib
 import operator
 import posixpath
 import traceback
@@ -62,16 +62,20 @@ def _get_full_modname(modname: str, attribute: str) -> str | None:
         num_parts = len(module_path)
         for i in range(num_parts, 0, -1):
             mod_root = '.'.join(module_path[:i])
-            module_spec = importlib.util.find_spec(mod_root)
-            if module_spec is not None:
+            try:
+                # import_module() caches the module in sys.modules
+                module = importlib.import_module(mod_root)
                 break
+            except ModuleNotFoundError:
+                continue
+            except BaseException as exc:
+                # Importing modules may cause any side effects, including
+                # SystemExit, so we need to catch all errors.
+                msg = f"viewcode failed to import '{mod_root}'."
+                raise ImportError(msg) from exc
         else:
             return None
-        # Load and execute the module
-        module = importlib.util.module_from_spec(module_spec)
-        if module_spec.loader is None:
-            return None
-        module_spec.loader.exec_module(module)
+
         if i != num_parts:
             for mod in module_path[i:]:
                 module = getattr(module, mod)