Explain why it was detected as self-hosted

* Since checking for self-hosted is based on guessing as there is
  no proper predicate for it in GitHub Actions (AFAIK).

action.yml

@@ -30,6 +30,11 @@ inputs:
     description: |
       Arbitrary string that will be added to the cache key of the bundler cache. Set or change it if you need
       to invalidate the cache.
+  self-hosted:
+    description: |
+      Consider the runner as a self-hosted runner, which means not using prebuilt Ruby binaries which only work
+      on GitHub-hosted runners or self-hosted runners with a very similar image to the ones used by GitHub runners.
+      The default is to detect this automatically based on the OS, OS version and $RUNNER_TOOL_CACHE.
 outputs:
   ruby-prefix:
     description: 'The prefix of the installed ruby'

common.js

@@ -13,6 +13,10 @@ export const windows = (os.platform() === 'win32')
 // Extract to SSD on Windows, see https://github.com/ruby/setup-ruby/pull/14
 export const drive = (windows ? (process.env['GITHUB_WORKSPACE'] || 'C')[0] : undefined)
 
+export const inputs = {
+  selfHosted: undefined
+}
+
 export function partition(string, separator) {
   const i = string.indexOf(separator)
   if (i === -1) {
@@ -162,9 +166,29 @@ const GitHubHostedPlatforms = [
   'windows-2022-x64',
 ]
 
-// Actually a self-hosted runner which does not correspond to a GitHub-hosted runner image
+// Actually a self-hosted runner for which either
+// * the OS and OS version does not correspond to a GitHub-hosted runner image,
+// * or the hosted tool cache is different from the default tool cache path
 export function isSelfHostedRunner() {
-  return !GitHubHostedPlatforms.includes(getOSNameVersionArch())
+  if (inputs.selfHosted === undefined) {
+    throw new Error('inputs.selfHosted should have been already set')
+  }
+
+  return inputs.selfHosted === 'true' ||
+    !GitHubHostedPlatforms.includes(getOSNameVersionArch()) ||
+    getRunnerToolCache() !== getDefaultToolCachePath()
+}
+
+export function selfHostedRunnerReason() {
+  if (inputs.selfHosted === 'true') {
+    return 'the self-hosted input was set'
+  } else if (!GitHubHostedPlatforms.includes(getOSNameVersionArch())) {
+    return 'the platform does not match a GitHub-hosted runner image (or that image is deprecated and no longer supported)'
+  } else if (getRunnerToolCache() !== getDefaultToolCachePath()) {
+    return 'the $RUNNER_TOOL_CACHE is different than the default tool cache path (they must be the same to reuse prebuilt Ruby binaries)'
+  } else {
+    return 'unknown reason'
+  }
 }
 
 let virtualEnvironmentName = undefined
@@ -213,28 +237,31 @@ export function shouldUseToolCache(engine, version) {
   return (engine === 'ruby' && !isHeadVersion(version)) || isSelfHostedRunner()
 }
 
-function getPlatformToolCache(platform) {
-  if (isSelfHostedRunner()) {
-    const runnerToolCache = process.env['RUNNER_TOOL_CACHE']
-    if (!runnerToolCache) {
-      throw new Error('$RUNNER_TOOL_CACHE must be set on self-hosted runners')
-    }
-    return runnerToolCache
+export function getRunnerToolCache() {
+  const runnerToolCache = process.env['RUNNER_TOOL_CACHE']
+  if (!runnerToolCache) {
+    throw new Error('$RUNNER_TOOL_CACHE must be set')
   }
-  // Hardcode paths rather than using $RUNNER_TOOL_CACHE because the prebuilt Rubies cannot be moved anyway
+  return runnerToolCache
+}
+
+// Rubies prebuilt by this action embed this path rather than using $RUNNER_TOOL_CACHE,
+// so they can only be used if the two paths are the same
+function getDefaultToolCachePath() {
+  const platform = getVirtualEnvironmentName()
   if (platform.startsWith('ubuntu-')) {
     return '/opt/hostedtoolcache'
   } else if (platform.startsWith('macos-')) {
     return '/Users/runner/hostedtoolcache'
   } else if (platform.startsWith('windows-')) {
-    return 'C:/hostedtoolcache/windows'
+    return 'C:\\hostedtoolcache\\windows'
   } else {
     throw new Error('Unknown platform')
   }
 }
 
 export function getToolCacheRubyPrefix(platform, engine, version) {
-  const toolCache = getPlatformToolCache(platform)
+  const toolCache = getRunnerToolCache()
   const name = {
     ruby: 'Ruby',
     jruby: 'JRuby',

dist/index.js

@@ -291,10 +291,12 @@ __nccwpck_require__.d(__webpack_exports__, {
   "drive": () => (/* binding */ drive),
   "floatVersion": () => (/* binding */ floatVersion),
   "getOSNameVersionArch": () => (/* binding */ getOSNameVersionArch),
+  "getRunnerToolCache": () => (/* binding */ getRunnerToolCache),
   "getToolCacheRubyPrefix": () => (/* binding */ getToolCacheRubyPrefix),
   "getVirtualEnvironmentName": () => (/* binding */ getVirtualEnvironmentName),
   "hasBundlerDefaultGem": () => (/* binding */ hasBundlerDefaultGem),
   "hashFile": () => (/* binding */ hashFile),
+  "inputs": () => (/* binding */ inputs),
   "isBundler1Default": () => (/* binding */ isBundler1Default),
   "isBundler2Default": () => (/* binding */ isBundler2Default),
   "isBundler2dot2Default": () => (/* binding */ isBundler2dot2Default),
@@ -303,6 +305,7 @@ __nccwpck_require__.d(__webpack_exports__, {
   "isStableVersion": () => (/* binding */ isStableVersion),
   "measure": () => (/* binding */ measure),
   "partition": () => (/* binding */ partition),
+  "selfHostedRunnerReason": () => (/* binding */ selfHostedRunnerReason),
   "setupPath": () => (/* binding */ setupPath),
   "shouldUseToolCache": () => (/* binding */ shouldUseToolCache),
   "targetRubyVersion": () => (/* binding */ targetRubyVersion),
@@ -365,6 +368,10 @@ const windows = (os.platform() === 'win32')
 // Extract to SSD on Windows, see https://github.com/ruby/setup-ruby/pull/14
 const drive = (windows ? (process.env['GITHUB_WORKSPACE'] || 'C')[0] : undefined)
 
+const inputs = {
+  selfHosted: undefined
+}
+
 function partition(string, separator) {
   const i = string.indexOf(separator)
   if (i === -1) {
@@ -514,9 +521,29 @@ const GitHubHostedPlatforms = [
   'windows-2022-x64',
 ]
 
-// Actually a self-hosted runner which does not correspond to a GitHub-hosted runner image
+// Actually a self-hosted runner for which either
+// * the OS and OS version does not correspond to a GitHub-hosted runner image,
+// * or the hosted tool cache is different from the default tool cache path
 function isSelfHostedRunner() {
-  return !GitHubHostedPlatforms.includes(getOSNameVersionArch())
+  if (inputs.selfHosted === undefined) {
+    throw new Error('inputs.selfHosted should have been already set')
+  }
+
+  return inputs.selfHosted === 'true' ||
+    !GitHubHostedPlatforms.includes(getOSNameVersionArch()) ||
+    getRunnerToolCache() !== getDefaultToolCachePath()
+}
+
+function selfHostedRunnerReason() {
+  if (inputs.selfHosted === 'true') {
+    return 'the self-hosted input was set'
+  } else if (!GitHubHostedPlatforms.includes(getOSNameVersionArch())) {
+    return 'the platform does not match a GitHub-hosted runner image (or that image is deprecated and no longer supported)'
+  } else if (getRunnerToolCache() !== getDefaultToolCachePath()) {
+    return 'the $RUNNER_TOOL_CACHE is different than the default tool cache path (they must be the same to reuse prebuilt Ruby binaries)'
+  } else {
+    return 'unknown reason'
+  }
 }
 
 let virtualEnvironmentName = undefined
@@ -565,28 +592,31 @@ function shouldUseToolCache(engine, version) {
   return (engine === 'ruby' && !isHeadVersion(version)) || isSelfHostedRunner()
 }
 
-function getPlatformToolCache(platform) {
-  if (isSelfHostedRunner()) {
-    const runnerToolCache = process.env['RUNNER_TOOL_CACHE']
-    if (!runnerToolCache) {
-      throw new Error('$RUNNER_TOOL_CACHE must be set on self-hosted runners')
-    }
-    return runnerToolCache
+function getRunnerToolCache() {
+  const runnerToolCache = process.env['RUNNER_TOOL_CACHE']
+  if (!runnerToolCache) {
+    throw new Error('$RUNNER_TOOL_CACHE must be set')
   }
-  // Hardcode paths rather than using $RUNNER_TOOL_CACHE because the prebuilt Rubies cannot be moved anyway
+  return runnerToolCache
+}
+
+// Rubies prebuilt by this action embed this path rather than using $RUNNER_TOOL_CACHE,
+// so they can only be used if the two paths are the same
+function getDefaultToolCachePath() {
+  const platform = getVirtualEnvironmentName()
   if (platform.startsWith('ubuntu-')) {
     return '/opt/hostedtoolcache'
   } else if (platform.startsWith('macos-')) {
     return '/Users/runner/hostedtoolcache'
   } else if (platform.startsWith('windows-')) {
-    return 'C:/hostedtoolcache/windows'
+    return 'C:\\hostedtoolcache\\windows'
   } else {
     throw new Error('Unknown platform')
   }
 }
 
 function getToolCacheRubyPrefix(platform, engine, version) {
-  const toolCache = getPlatformToolCache(platform)
+  const toolCache = getRunnerToolCache()
   const name = {
     ruby: 'Ruby',
     jruby: 'JRuby',
@@ -68253,8 +68283,8 @@ async function install(platform, engine, version) {
       if (common.isSelfHostedRunner()) {
         const rubyBuildDefinition = engine === 'ruby' ? version : `${engine}-${version}`
         core.error(
-          `The current runner (${common.getOSNameVersionArch()}) was detected as self-hosted and not matching a GitHub-hosted runner image.\n` +
-          `In such a case, you should install Ruby in the $RUNNER_TOOL_CACHE yourself, for example using https://github.com/rbenv/ruby-build:\n` +
+          `The current runner (${common.getOSNameVersionArch()}, RUNNER_TOOL_CACHE=${common.getRunnerToolCache()}) was detected as self-hosted because ${common.selfHostedRunnerReason()}.\n` +
+          `In such a case, you should install Ruby in the $RUNNER_TOOL_CACHE yourself, for example using https://github.com/rbenv/ruby-build\n` +
           `You can take inspiration from this workflow for more details: https://github.com/ruby/ruby-builder/blob/master/.github/workflows/build.yml\n` +
           `$ ruby-build ${rubyBuildDefinition} ${toolCacheRubyPrefix}\n` +
           `Once that completes successfully, mark it as complete with:\n` +
@@ -69015,6 +69045,7 @@ const inputDefaults = {
   'bundler-cache': 'false',
   'working-directory': '.',
   'cache-version': bundler.DEFAULT_CACHE_VERSION,
+  'self-hosted': 'false',
 }
 
 // entry point when this action is run on its own
@@ -69038,6 +69069,7 @@ async function setupRuby(options = {}) {
       inputs[key] = core.getInput(key) || inputDefaults[key]
     }
   }
+  common.inputs.selfHosted = inputs['self-hosted']
 
   process.chdir(inputs['working-directory'])
 

index.js

@@ -16,6 +16,7 @@ const inputDefaults = {
   'bundler-cache': 'false',
   'working-directory': '.',
   'cache-version': bundler.DEFAULT_CACHE_VERSION,
+  'self-hosted': 'false',
 }
 
 // entry point when this action is run on its own
@@ -39,6 +40,7 @@ export async function setupRuby(options = {}) {
       inputs[key] = core.getInput(key) || inputDefaults[key]
     }
   }
+  common.inputs.selfHosted = inputs['self-hosted']
 
   process.chdir(inputs['working-directory'])
 

ruby-builder.js

@@ -28,8 +28,8 @@ export async function install(platform, engine, version) {
       if (common.isSelfHostedRunner()) {
         const rubyBuildDefinition = engine === 'ruby' ? version : `${engine}-${version}`
         core.error(
-          `The current runner (${common.getOSNameVersionArch()}) was detected as self-hosted and not matching a GitHub-hosted runner image.\n` +
-          `In such a case, you should install Ruby in the $RUNNER_TOOL_CACHE yourself, for example using https://github.com/rbenv/ruby-build:\n` +
+          `The current runner (${common.getOSNameVersionArch()}, RUNNER_TOOL_CACHE=${common.getRunnerToolCache()}) was detected as self-hosted because ${common.selfHostedRunnerReason()}.\n` +
+          `In such a case, you should install Ruby in the $RUNNER_TOOL_CACHE yourself, for example using https://github.com/rbenv/ruby-build\n` +
           `You can take inspiration from this workflow for more details: https://github.com/ruby/ruby-builder/blob/master/.github/workflows/build.yml\n` +
           `$ ruby-build ${rubyBuildDefinition} ${toolCacheRubyPrefix}\n` +
           `Once that completes successfully, mark it as complete with:\n` +