Add WheelCache method to inform which cache was used

Return whether the link was found in the persistent or ephemeral cache.

src/pip/_internal/cache.py

@@ -270,6 +270,16 @@ class EphemWheelCache(SimpleWheelCache):
         )
 
 
+class CacheEntry(object):
+    def __init__(
+        self,
+        link,  # type: Link
+        persistent,  # type: bool
+    ):
+        self.link = link
+        self.persistent = persistent
+
+
 class WheelCache(Cache):
     """Wraps EphemWheelCache and SimpleWheelCache into a single Cache
 
@@ -304,16 +314,36 @@ class WheelCache(Cache):
         supported_tags,  # type: List[Tag]
     ):
         # type: (...) -> Link
+        cache_entry = self.get_cache_entry(link, package_name, supported_tags)
+        if cache_entry is None:
+            return link
+        return cache_entry.link
+
+    def get_cache_entry(
+        self,
+        link,            # type: Link
+        package_name,    # type: Optional[str]
+        supported_tags,  # type: List[Tag]
+    ):
+        # type: (...) -> Optional[CacheEntry]
+        """Returns a CacheEntry with a link to a cached item if it exists or
+        None. The cache entry indicates if the item was found in the persistent
+        or ephemeral cache.
+        """
         retval = self._wheel_cache.get(
             link=link,
             package_name=package_name,
             supported_tags=supported_tags,
         )
         if retval is not link:
-            return retval
+            return CacheEntry(retval, persistent=True)
 
-        return self._ephem_cache.get(
+        retval = self._ephem_cache.get(
             link=link,
             package_name=package_name,
             supported_tags=supported_tags,
         )
+        if retval is not link:
+            return CacheEntry(retval, persistent=False)
+
+        return None

tests/unit/test_cache.py

@@ -90,3 +90,26 @@ def test_get_with_legacy_entry_only(tmpdir):
         os.path.normcase(os.path.dirname(cached_link.file_path)) ==
         os.path.normcase(legacy_path)
     )
+
+
+def test_get_cache_entry(tmpdir):
+    wc = WheelCache(tmpdir, FormatControl())
+    persi_link = Link("https://g.c/o/r/persi")
+    persi_path = wc.get_path_for_link(persi_link)
+    ensure_dir(persi_path)
+    with open(os.path.join(persi_path, "persi-1.0.0-py3-none-any.whl"), "w"):
+        pass
+    ephem_link = Link("https://g.c/o/r/ephem")
+    ephem_path = wc.get_ephem_path_for_link(ephem_link)
+    ensure_dir(ephem_path)
+    with open(os.path.join(ephem_path, "ephem-1.0.0-py3-none-any.whl"), "w"):
+        pass
+    other_link = Link("https://g.c/o/r/other")
+    supported_tags = [Tag("py3", "none", "any")]
+    assert (
+        wc.get_cache_entry(persi_link, "persi", supported_tags).persistent
+    )
+    assert (
+        not wc.get_cache_entry(ephem_link, "ephem", supported_tags).persistent
+    )
+    assert wc.get_cache_entry(other_link, "other", supported_tags) is None