TRAP Caching: Disable on self-hosted runners

src/init-action.ts

@@ -39,6 +39,7 @@ import {
   getRequiredEnvParam,
   getThreadsFlagValue,
   initializeEnvironment,
+  isHostedRunner,
   Mode,
 } from "./util";
 
@@ -320,10 +321,14 @@ async function run() {
 async function getTrapCachingEnabled(
   featureEnablement: FeatureEnablement
 ): Promise<boolean> {
+  // If the workflow specified something always respect that
   const trapCaching = getOptionalInput("trap-caching");
-  if (trapCaching !== undefined) {
-    return trapCaching === "true";
-  }
+  if (trapCaching !== undefined) return trapCaching === "true";
+
+  // On self-hosted runners which may have slow network access, disable TRAP caching by default
+  if (!isHostedRunner()) return false;
+
+  // On hosted runners, respect the feature flag
   return await featureEnablement.getValue(Feature.TrapCachingEnabled);
 }
 

src/util.ts

@@ -892,3 +892,24 @@ export async function withTimeout<T>(
 
   return await Promise.race([mainTask(), timeout]);
 }
+
+/**
+ * This function implements a heuristic to determine whether the
+ * runner we are on is hosted by GitHub. It does this by checking
+ * the name of the runner against the list of known GitHub-hosted
+ * runner names. It also checks for the presence of a toolcache
+ * directory with the name hostedtoolcache which is present on
+ * GitHub-hosted runners.
+ *
+ * @returns true iff the runner is hosted by GitHub
+ */
+export function isHostedRunner() {
+  return (
+    // Name of the runner on hosted Windows runners
+    process.env["RUNNER_NAME"]?.includes("Hosted Agent") ||
+    // Name of the runner on hosted POSIX runners
+    process.env["RUNNER_NAME"]?.includes("GitHub Actions") ||
+    // Segment of the path to the tool cache on all hosted runners
+    process.env["RUNNER_TOOL_CACHE"]?.includes("hostedtoolcache")
+  );
+}