intersphinx: Allow strict prefix matching

Strictly using prefix for intersphinx links gives a better control on
external links - only links explicitely declared will point to the
external documentation.

Fixes sphinx-doc#2068

doc/usage/extensions/intersphinx.rst

@@ -148,6 +148,20 @@ linking:
       exception is raised if the server has not issued a response for timeout
       seconds.
 
+.. confval:: intersphinx_strict_prefix
+
+   .. versionadded:: 4.0.0
+
+   Turn on strict matching of references in Sphinx - all intersphinx links
+   needs to have a prefix. This disables looking up the non-prefixed targets in
+   intersphinx and will fail the build if such target is not present.
+
+   .. hint::
+
+      Strictly using prefix for intersphinx links gives a better control on
+      external links - only links explicitely declared will point to the
+      external documentation.
+
 
 Showing all links of an Intersphinx mapping file
 ------------------------------------------------

sphinx/ext/intersphinx.py

@@ -314,6 +314,8 @@ def missing_reference(app: Sphinx, env: BuildEnvironment, node: pending_xref,
                 full_qualified_name = env.get_domain(domain).get_full_qualified_name(node)
                 if full_qualified_name:
                     to_try.append((inventories.named_inventory[setname], full_qualified_name))
+    elif app.config.intersphinx_strict_prefix:
+        return None
     for inventory, target in to_try:
         for objtype in objtypes:
             if objtype not in inventory or target not in inventory[objtype]:
@@ -379,6 +381,7 @@ def setup(app: Sphinx) -> Dict[str, Any]:
     app.add_config_value('intersphinx_mapping', {}, True)
     app.add_config_value('intersphinx_cache_limit', 5, False)
     app.add_config_value('intersphinx_timeout', None, False)
+    app.add_config_value('intersphinx_strict_prefix', False, True)
     app.connect('config-inited', normalize_intersphinx_mapping, priority=800)
     app.connect('builder-inited', load_mappings)
     app.connect('missing-reference', missing_reference)
@@ -399,6 +402,7 @@ def inspect_main(argv: List[str]) -> None:
 
     class MockConfig:
         intersphinx_timeout = None  # type: int
+        intersphinx_strict_prefix = False
         tls_verify = False
         user_agent = None
 

tests/test_ext_intersphinx.py

@@ -97,6 +97,7 @@ def test_missing_reference(tempdir, app, status, warning):
         'py3krelparent': ('../../py3k', inv_file),  # relative path, parent dir
     }
     app.config.intersphinx_cache_limit = 0
+    app.config.intersphinx_strict_prefix = False
 
     # load the inventory and check if it's done correctly
     normalize_intersphinx_mapping(app, app.config)
@@ -165,6 +166,13 @@ def test_missing_reference(tempdir, app, status, warning):
     rn = reference_check(app, 'std', 'doc', 'docname', 'docname')
     assert rn['refuri'] == 'https://docs.python.org/docname.html'
 
+    # check resolution when strict prefix is enabled
+    app.config.intersphinx_strict_prefix = True
+    rn = reference_check(app, 'py', 'func', 'module1.func', 'foo')
+    assert rn is None
+    rn = reference_check(app, 'py', 'func', 'py3k:module1.func', 'foo')
+    assert rn is not None
+
 
 def test_missing_reference_pydomain(tempdir, app, status, warning):
     inv_file = tempdir / 'inventory'
@@ -173,6 +181,7 @@ def test_missing_reference_pydomain(tempdir, app, status, warning):
         'https://docs.python.org/': inv_file,
     }
     app.config.intersphinx_cache_limit = 0
+    app.config.intersphinx_strict_prefix = False
 
     # load the inventory and check if it's done correctly
     normalize_intersphinx_mapping(app, app.config)
@@ -212,6 +221,7 @@ def test_missing_reference_stddomain(tempdir, app, status, warning):
         'cmd': ('https://docs.python.org/', inv_file),
     }
     app.config.intersphinx_cache_limit = 0
+    app.config.intersphinx_strict_prefix = False
 
     # load the inventory and check if it's done correctly
     normalize_intersphinx_mapping(app, app.config)
@@ -244,6 +254,7 @@ def test_missing_reference_cppdomain(tempdir, app, status, warning):
         'https://docs.python.org/': inv_file,
     }
     app.config.intersphinx_cache_limit = 0
+    app.config.intersphinx_strict_prefix = False
 
     # load the inventory and check if it's done correctly
     normalize_intersphinx_mapping(app, app.config)
@@ -271,6 +282,7 @@ def test_missing_reference_jsdomain(tempdir, app, status, warning):
         'https://docs.python.org/': inv_file,
     }
     app.config.intersphinx_cache_limit = 0
+    app.config.intersphinx_strict_prefix = False
 
     # load the inventory and check if it's done correctly
     normalize_intersphinx_mapping(app, app.config)
@@ -297,6 +309,7 @@ def test_inventory_not_having_version(tempdir, app, status, warning):
         'https://docs.python.org/': inv_file,
     }
     app.config.intersphinx_cache_limit = 0
+    app.config.intersphinx_strict_prefix = False
 
     # load the inventory and check if it's done correctly
     normalize_intersphinx_mapping(app, app.config)
@@ -326,6 +339,7 @@ def test_load_mappings_warnings(tempdir, app, status, warning):
     }
 
     app.config.intersphinx_cache_limit = 0
+    app.config.intersphinx_strict_prefix = False
     # load the inventory and check if it's done correctly
     normalize_intersphinx_mapping(app, app.config)
     load_mappings(app)
@@ -336,6 +350,7 @@ def test_load_mappings_fallback(tempdir, app, status, warning):
     inv_file = tempdir / 'inventory'
     inv_file.write_bytes(inventory_v2)
     app.config.intersphinx_cache_limit = 0
+    app.config.intersphinx_strict_prefix = False
 
     # connect to invalid path
     app.config.intersphinx_mapping = {