index.json.hash, no fatal error if key cannot be fetched (#34643)

lib/spack/spack/binary_distribution.py

@@ -2389,33 +2389,35 @@ def __init__(self, url, local_hash, urlopen=web_util.urlopen):
         self.url = url
         self.local_hash = local_hash
         self.urlopen = urlopen
+        self.headers = {"User-Agent": web_util.SPACK_USER_AGENT}
 
-    def conditional_fetch(self):
-        # Do an intermediate fetch for the hash
-        # and a conditional fetch for the contents
-        if self.local_hash:
+    def get_remote_hash(self):
+        # Failure to fetch index.json.hash is not fatal
         url_index_hash = url_util.join(self.url, _build_cache_relative_path, "index.json.hash")
-
         try:
-                response = self.urlopen(urllib.request.Request(url_index_hash))
-            except urllib.error.URLError as e:
-                raise FetchIndexError("Could not fetch {}".format(url_index_hash), e) from e
+            response = self.urlopen(urllib.request.Request(url_index_hash, headers=self.headers))
+        except urllib.error.URLError:
+            return None
 
         # Validate the hash
         remote_hash = response.read(64)
         if not re.match(rb"[a-f\d]{64}$", remote_hash):
-                raise FetchIndexError("Invalid hash format in {}".format(url_index_hash))
-            remote_hash = remote_hash.decode("utf-8")
+            return None
+        return remote_hash.decode("utf-8")
 
-            # No need to update further
-            if remote_hash == self.local_hash:
+    def conditional_fetch(self):
+        # Do an intermediate fetch for the hash
+        # and a conditional fetch for the contents
+
+        # Early exit if our cache is up to date.
+        if self.local_hash and self.local_hash == self.get_remote_hash():
             return FetchIndexResult(etag=None, hash=None, data=None, fresh=True)
 
         # Otherwise, download index.json
         url_index = url_util.join(self.url, _build_cache_relative_path, "index.json")
 
         try:
-            response = self.urlopen(urllib.request.Request(url_index))
+            response = self.urlopen(urllib.request.Request(url_index, headers=self.headers))
         except urllib.error.URLError as e:
             raise FetchIndexError("Could not fetch index from {}".format(url_index), e)
 

lib/spack/spack/test/bindist.py

@@ -854,8 +854,7 @@ def urlopen(request: urllib.request.Request):
         url="https://www.example.com", local_hash=index_json_hash, urlopen=urlopen
     )
 
-    with pytest.raises(bindist.FetchIndexError, match="Invalid hash format"):
-        fetcher.conditional_fetch()
+    assert fetcher.get_remote_hash() is None
 
 
 def test_default_index_json_404():