Distinguish GitHub-hosted runners from ones that look hosted

`isHostedRunner` infers hostedness from the runner name and the toolcache path, so it also matches self-hosted runners that are configured to resemble hosted ones. Rename it to `looksLikeHostedRunner` so callers can see they are getting a heuristic, and add `isGitHubHostedRunner`, which reads the `RUNNER_ENVIRONMENT` value the Actions service reports. The existing callers keep the heuristic, so there is no behaviour change.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

src/actions-util.ts

@@ -283,6 +283,19 @@ export function isSelfHostedRunner(env: Env = getEnv()) {
   return env.getOptional(ActionsEnvVars.RUNNER_ENVIRONMENT) === "self-hosted";
 }
 
+/**
+ * Whether the job is running on a runner that GitHub hosts, and whose toolcache is therefore thrown
+ * away once the job has finished.
+ *
+ * Unlike `looksLikeHostedRunner`, this is based on what the service reports for the job rather than
+ * on how the runner's filesystem happens to be laid out, so it does not match self-hosted runners
+ * that are configured to resemble hosted ones, such as those that mount a persistent volume at
+ * `/opt/hostedtoolcache`.
+ */
+export function isGitHubHostedRunner(env: Env = getEnv()) {
+  return env.getOptional(ActionsEnvVars.RUNNER_ENVIRONMENT) === "github-hosted";
+}
+
 /** Determines whether the workflow trigger is `dynamic`. */
 export function isDynamicWorkflow(env: Env = getEnv()): boolean {
   return getWorkflowEventName(env) === "dynamic";

src/caching-utils.ts

@@ -5,7 +5,7 @@ import * as core from "@actions/core";
 import { getOptionalInput, isDefaultSetup } from "./actions-util";
 import { EnvVar } from "./environment";
 import { Logger } from "./logging";
-import { isHostedRunner, tryGetFolderBytes } from "./util";
+import { looksLikeHostedRunner, tryGetFolderBytes } from "./util";
 
 /**
  * Returns the total size of all the specified paths.
@@ -109,7 +109,7 @@ export function getDependencyCachingEnabled(): CachingKind {
   if (dependencyCaching !== undefined) return getCachingKind(dependencyCaching);
 
   // On self-hosted runners which may have dependencies installed centrally, disable caching by default
-  if (!isHostedRunner()) return CachingKind.None;
+  if (!looksLikeHostedRunner()) return CachingKind.None;
 
   // Disable in advanced workflows by default.
   if (!isDefaultSetup()) return CachingKind.None;

src/config-utils.ts

@@ -90,9 +90,8 @@ import {
   Result,
   Success,
   Failure,
-  isHostedRunner,
+  looksLikeHostedRunner,
 } from "./util";
-
 export { type Config } from "./config/action-config";
 
 /**
@@ -938,7 +937,7 @@ export async function isTrapCachingEnabled(
   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;
+  if (!looksLikeHostedRunner()) return false;
 
   // If overlay analysis is enabled, then disable TRAP caching since overlay analysis supersedes it.
   // This change is gated behind a feature flag.

src/util.ts

@@ -842,9 +842,13 @@ export async function checkForTimeout() {
  * directory with the name hostedtoolcache which is present on
  * GitHub-hosted runners.
  *
- * @returns true iff the runner is hosted by GitHub
+ * Since this is a heuristic over how the runner happens to be named and laid out, it also matches
+ * self-hosted runners that are configured to resemble hosted ones. Prefer
+ * `isGitHubHostedRunner` when you need the answer the Actions service reports.
+ *
+ * @returns true iff the runner looks like it is hosted by GitHub
  */
-export function isHostedRunner() {
+export function looksLikeHostedRunner() {
   return (
     // Name of the runner on hosted Windows runners
     process.env["RUNNER_NAME"]?.includes("Hosted Agent") ||