intersphinx: Handle the case where intersphinx_cache_limit is negative

The documentation said:

  Set this (intersphinx_cache_limit) to a negative value to cache inventories
  for unlimited time.

In the current implementation, a negative intersphinx_cache_limit causes
inventories always expire, this patch ensures that it behaves as documented.

CHANGES.rst

@@ -109,6 +109,9 @@ Bugs fixed
 * #12425: Use Docutils' SVG processing in the HTML builder
   and remove Sphinx's custom logic.
   Patch by Tunç Başar Köse.
+* #12514: intersphinx: fix the meaning of a negative value for
+  :confval:`intersphinx_cache_limit`.
+  Patch by Shengyu Zhang.
 
 Testing
 -------

sphinx/ext/intersphinx/_load.py

@@ -107,7 +107,10 @@ def fetch_inventory_group(
     app: Sphinx,
     now: int,
 ) -> bool:
-    cache_time = now - app.config.intersphinx_cache_limit * 86400
+    if app.config.intersphinx_cache_limit < 0:
+        cache_time = now - app.config.intersphinx_cache_limit * 86400
+    else:
+        cache_time = 0
     failures = []
     try:
         for inv in invs:

tests/test_extensions/test_ext_intersphinx.py

@@ -1,6 +1,8 @@
 """Test the intersphinx extension."""
 
 import http.server
+import time
+from typing import TYPE_CHECKING
 from unittest import mock
 
 import pytest
@@ -10,6 +12,7 @@
 from sphinx.builders.html import INVENTORY_FILENAME
 from sphinx.ext.intersphinx import (
     fetch_inventory,
+    fetch_inventory_group,
     inspect_main,
     load_mappings,
     missing_reference,
@@ -26,6 +29,9 @@
 )
 from tests.utils import http_server
 
+if TYPE_CHECKING:
+    from sphinx.ext.intersphinx._shared import InventoryCacheEntry
+
 
 def fake_node(domain, type, target, content, **attrs):
     contnode = nodes.emphasis(content, content)
@@ -603,3 +609,21 @@ def test_intersphinx_role(app, warning):
 
     # explicit title
     assert html.format('index.html#foons') in content
+
+
+def test_intersphinx_cache_limit(app):
+    url = 'https://example.org/'
+    app.config.intersphinx_mapping = {
+        'inv': (url, None),
+    }
+    # load the inventory and check if it's done correctly
+    normalize_intersphinx_mapping(app, app.config)
+    intersphinx_cache: dict[str, InventoryCacheEntry] = {
+        url: (None, 0, {}),  # 0 is a timestamp, make sure the entry is expired
+    }
+    now = int(time.time())
+
+    app.config.intersphinx_cache_limit = -1
+    for name, (uri, invs) in app.config.intersphinx_mapping.values():
+        # no need to read from remote
+        assert not fetch_inventory_group(name, uri, invs, intersphinx_cache, app, now)