From b78fb210e54252079362cb0ae132314d81e12fec Mon Sep 17 00:00:00 2001
From: John Downey <jdowney@gmail.com>
Date: Mon, 1 Jun 2026 11:39:24 -0500
Subject: [PATCH] Fix arm64 Linux runner OS detection

---
 dist/index.js           | 10 ++++++++--
 src/setup-beam.js       | 10 ++++++++--
 test/setup-beam.test.js | 38 ++++++++++++++++++++++++++++++++++++++
 3 files changed, 54 insertions(+), 4 deletions(-)

diff --git a/dist/index.js b/dist/index.js
index cd1fe4a5..f1b10b11 100644
--- a/dist/index.js
+++ b/dist/index.js
@@ -55370,10 +55370,15 @@ function getRunnerOSVersion() {
     ubuntu18: 'ubuntu-18.04',
     ubuntu20: 'ubuntu-20.04',
   }
-  const containerFromEnvImageOS = ImageOSToContainer[process.env.ImageOS]
+  // actions/runner-images PR #14113 gave arm64 images distinct ImageOS values
+  // (e.g. ImageOS=ubuntu24-arm64 instead of ubuntu24). The CPU architecture is
+  // resolved separately via RUNNER_ARCH, so the arm64 variants map to the same
+  // OS container as their x86 counterparts.
+  const imageOS = (process.env.ImageOS || '').replace(/-arm64$/, '')
+  const containerFromEnvImageOS = ImageOSToContainer[imageOS]
   if (!containerFromEnvImageOS) {
     const deprecatedContainerFromEnvImageOS =
-      deprecatedImageOSToContainer[process.env.ImageOS]
+      deprecatedImageOSToContainer[imageOS]
     if (deprecatedContainerFromEnvImageOS) {
       warning(
         `You are using deprecated ImageOS ${deprecatedContainerFromEnvImageOS}. ` +
@@ -55993,6 +55998,7 @@ function debugLoggingEnabled() {
   getGleamVersion,
   getOTPVersion,
   getRebar3Version,
+  getRunnerOSVersion,
   getVersionFromSpec,
   githubAMDRunnerArchs,
   githubARMRunnerArchs,
diff --git a/src/setup-beam.js b/src/setup-beam.js
index 966a4b46..c7eaf0f7 100644
--- a/src/setup-beam.js
+++ b/src/setup-beam.js
@@ -676,10 +676,15 @@ function getRunnerOSVersion() {
     ubuntu18: 'ubuntu-18.04',
     ubuntu20: 'ubuntu-20.04',
   }
-  const containerFromEnvImageOS = ImageOSToContainer[process.env.ImageOS]
+  // actions/runner-images PR #14113 gave arm64 images distinct ImageOS values
+  // (e.g. ImageOS=ubuntu24-arm64 instead of ubuntu24). The CPU architecture is
+  // resolved separately via RUNNER_ARCH, so the arm64 variants map to the same
+  // OS container as their x86 counterparts.
+  const imageOS = (process.env.ImageOS || '').replace(/-arm64$/, '')
+  const containerFromEnvImageOS = ImageOSToContainer[imageOS]
   if (!containerFromEnvImageOS) {
     const deprecatedContainerFromEnvImageOS =
-      deprecatedImageOSToContainer[process.env.ImageOS]
+      deprecatedImageOSToContainer[imageOS]
     if (deprecatedContainerFromEnvImageOS) {
       core.warning(
         `You are using deprecated ImageOS ${deprecatedContainerFromEnvImageOS}. ` +
@@ -1299,6 +1304,7 @@ export default {
   getGleamVersion,
   getOTPVersion,
   getRebar3Version,
+  getRunnerOSVersion,
   getVersionFromSpec,
   githubAMDRunnerArchs,
   githubARMRunnerArchs,
diff --git a/test/setup-beam.test.js b/test/setup-beam.test.js
index 35d8a690..161a6ff2 100644
--- a/test/setup-beam.test.js
+++ b/test/setup-beam.test.js
@@ -587,6 +587,44 @@ describe('OTP arch-specific install', () => {
   })
 })
 
+describe('.getRunnerOSVersion(_)', () => {
+  it('maps x86 ImageOS values to their container', async () => {
+    const previousImageOS = process.env.ImageOS
+
+    process.env.ImageOS = 'ubuntu22'
+    assert.strictEqual(setupBeam.getRunnerOSVersion(), 'ubuntu-22.04')
+
+    process.env.ImageOS = 'ubuntu24'
+    assert.strictEqual(setupBeam.getRunnerOSVersion(), 'ubuntu-24.04')
+
+    process.env.ImageOS = previousImageOS
+  })
+
+  it('maps arm64 ImageOS variants to the same container as x86', async () => {
+    const previousImageOS = process.env.ImageOS
+
+    process.env.ImageOS = 'ubuntu22-arm64'
+    assert.strictEqual(setupBeam.getRunnerOSVersion(), 'ubuntu-22.04')
+
+    process.env.ImageOS = 'ubuntu24-arm64'
+    assert.strictEqual(setupBeam.getRunnerOSVersion(), 'ubuntu-24.04')
+
+    process.env.ImageOS = previousImageOS
+  })
+
+  it('throws for genuinely unknown ImageOS values', async () => {
+    const previousImageOS = process.env.ImageOS
+
+    process.env.ImageOS = 'plan9'
+    assert.throws(
+      () => setupBeam.getRunnerOSVersion(),
+      /Tried to map a target OS/,
+    )
+
+    process.env.ImageOS = previousImageOS
+  })
+})
+
 describe('.getOTPVersion(_) - Elixir', () => {
   let got
   let expected