vmtest.githubapi: add environment variable for trusting cache

Specifying a vmtest kernel version that isn't an exact match (e.g., with
a wildcard) currently always makes a GitHub API request to get the
vmtest release list. This is annoying if you have no or slow network
connectivity and don't want to check what the exact version is
(especially if you just downloaded it). Add a hack to bypass this
request by setting VMTEST_TRUST_CACHE=1 in the environment.

Signed-off-by: Omar Sandoval <[email protected]>

vmtest/githubapi.py

@@ -2,6 +2,7 @@
 # SPDX-License-Identifier: LGPL-2.1-or-later
 
 import json
+import os
 from pathlib import Path
 import typing
 from typing import Any, Dict, Mapping, Optional, Union
@@ -68,6 +69,15 @@ def _cached_get_headers(
                 return {**self._headers, "If-Modified-Since": cached["last_modified"]}
         return self._headers
 
+    @staticmethod
+    def _trust_cache(cached: Any) -> bool:
+        # If the request was cached and the VMTEST_TRUST_CACHE environment
+        # variable is non-zero, assume the cache is still valid.
+        try:
+            return cached is not None and int(os.getenv("VMTEST_TRUST_CACHE", "0")) != 0
+        except ValueError:
+            return False
+
     def _write_cache(
         self, cache: _CACHE, body: Any, headers: Mapping[str, str]
     ) -> None:
@@ -122,6 +132,8 @@ def _request(
 
     def _cached_get_json(self, endpoint: str, cache: _CACHE) -> Any:
         cached = self._read_cache(cache)
+        if self._trust_cache(cached):
+            return cached["body"]
         try:
             with urllib.request.urlopen(
                 urllib.request.Request(
@@ -164,6 +176,8 @@ def _request(
 
     async def _cached_get_json(self, endpoint: str, cache: _CACHE) -> Any:
         cached = self._read_cache(cache)
+        if self._trust_cache(cached):
+            return cached["body"]
         async with self._session.get(
             self._HOST + "/" + endpoint,
             headers=self._cached_get_headers(cached),