resources, stdlib: Add support for local files in obtain_resource (#204)

This patch allows a local JSON file to specify a local path in the JSON
object of a Resource, through the "url" field.

Local paths can be entered with the prefix "file:" in the "url" field.

If the local path exists, then the Resource from there is copied into
the resource directory defined in the
function earlier.

This behavior is the same as using specific Resource classes (ex.
BinaryResource) and passing a local_path into the function.

But, the above class does not allow simultaneous creation of local
Resources and Workloads of those local Resources.

With this patch, someone can use a local JSON, specify the location of
local Resources and create a Workload from those Resources and test both
together.

src/python/gem5/resources/downloader.py

@@ -34,6 +34,7 @@ import random
 from pathlib import Path
 import tarfile
 from urllib.error import HTTPError
+from urllib.parse import urlparse
 from typing import List, Optional, Dict
 
 from _m5 import core
@@ -310,19 +311,34 @@ def get_resource(
         if run_unzip:
             download_dest += zip_extension
 
-        # TODO: Might be nice to have some kind of download status bar here.
-        # TODO: There might be a case where this should be silenced.
-        print(
-            "Resource '{}' was not found locally. Downloading to '{}'...".format(
-                resource_name, download_dest
+        file_uri_path = _file_uri_to_path(resource_json["url"])
+        if file_uri_path:
+            if not file_uri_path.exists():
+                raise Exception(
+                    f"Could not find file at path '{file_uri_path}'"
+                )
+            print(
+                "Resource '{}' is being copied from '{}' to '{}'...".format(
+                    resource_name,
+                    urlparse(resource_json["url"]).path,
+                    download_dest,
+                )
+            )
+            shutil.copy(file_uri_path, download_dest)
+        else:
+            # TODO: Might be nice to have some kind of download status bar here.
+            # TODO: There might be a case where this should be silenced.
+            print(
+                "Resource '{}' was not found locally. Downloading to '{}'...".format(
+                    resource_name, download_dest
+                )
             )
-        )
 
-        # Get the URL.
-        url = resource_json["url"]
+            # Get the URL.
+            url = resource_json["url"]
 
-        _download(url=url, download_to=download_dest)
-        print(f"Finished downloading resource '{resource_name}'.")
+            _download(url=url, download_to=download_dest)
+            print(f"Finished downloading resource '{resource_name}'.")
 
         if run_unzip:
             print(
@@ -368,3 +384,27 @@ def get_resource(
 
                 safe_extract(f, unpack_to)
             os.remove(download_dest)
+
+
+def _file_uri_to_path(uri: str) -> Optional[Path]:
+    """
+    If the URI uses the File scheme (e.g, `file://host/path`) then
+    a Path object for the local path is returned, otherwise None.
+
+    **Note:** Only files from localhost are permitted. An exception
+    is thrown otherwise.
+
+    :param uri: The file URI to convert.
+
+    :returns: The path to the file.
+    """
+
+    if urlparse(uri).scheme == "file":
+        if urlparse(uri).netloc == "" or urlparse(uri).netloc == "localhost":
+            local_path = urlparse(uri).path
+            return Path(local_path)
+        raise Exception(
+            f"File URI '{uri}' specifies host '{urlparse(uri).netloc}'. "
+            "Only localhost is permitted."
+        )
+    return None