stdlib: Update the stdlib resource downloader for versions

The 'resources.json' file in gem5 resources is used to lookup resources.
The stdlib resource downloader was hardcoded to the 'resources.json'
stored in the gem5 resources stable branch. This change allows the gem5
downloader to obtain the specific resources.json.

Change-Id: Ie119ed9326b359055bf5bc347ca89336a5cc2e5d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/54466
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

src/python/gem5/resources/downloader.py

@@ -41,13 +41,16 @@ This Python module contains functions used to download, list, and obtain
 information about resources from resources.gem5.org.
 """
 
+def _resources_json_version_required() -> str:
+    """
+    Specifies the version of resources.json to obtain.
+    """
+    return "21.2"
 
 def _get_resources_json_uri() -> str:
-    # TODO: This is hardcoded to develop. This will need updated for each
-    # release to the stable branch.
     uri = (
         "https://gem5.googlesource.com/public/gem5-resources/"
-        + "+/refs/heads/develop/resources.json?format=TEXT"
+        + "+/refs/heads/stable/resources.json?format=TEXT"
     )
 
     return uri
@@ -64,8 +67,27 @@ def _get_resources_json() -> Dict:
     # text. Therefore when we open the URL we receive the JSON in base64
     # format. Conversion is needed before it can be loaded.
     with urllib.request.urlopen(_get_resources_json_uri()) as url:
-        return json.loads(base64.b64decode(url.read()).decode("utf-8"))
+        to_return = json.loads(base64.b64decode(url.read()).decode("utf-8"))
 
+    # If the current version pulled is not correct, look up the
+    # "previous-versions" field to find the correct one.
+    version = _resources_json_version_required()
+    if to_return["version"] != version:
+        if version in to_return["previous-versions"].keys():
+            with urllib.request.urlopen(
+                    to_return["previous-versions"][version]
+                ) as url:
+                to_return = json.loads(
+                    base64.b64decode(url.read()).decode("utf-8")
+                )
+        else:
+            # This should never happen, but we thrown an exception to explain
+            # that we can't find the version.
+            raise Exception(
+                f"Version '{version}' of resources.json cannot be found."
+                )
+
+    return to_return
 
 def _get_url_base() -> str:
     """