proof of concept: move saveCache to the post-action phase

i mocked this up while investigating
https://github.com/docker/setup-buildx-action/issues/293

i guess this is SLIGHTLY better in that it makes the 2 minutes pause
that i'm seeing happen AFTER tests, but the pause is still there.

Signed-off-by: Nick Santos <nick.santos@docker.com>

src/buildx/install.ts

@@ -330,8 +330,8 @@ class InstallCache {
     core.debug(`InstallCache.save cached to hosted tool cache ${htcPath}`);
 
     if (cache.isFeatureAvailable()) {
-      core.debug(`InstallCache.save caching ${this.ghaCacheKey} to GitHub Actions cache`);
+      core.debug(`InstallCache.save sending ${this.ghaCacheKey} to cache in post-action`);
-      await cache.saveCache([this.cacheDir], this.ghaCacheKey);
+      core.saveState('post-save-cache', JSON.stringify({dir: this.cacheDir, key: this.ghaCacheKey}));
     }
 
     return cachePath;

src/index.ts

@@ -15,6 +15,7 @@
  */
 
 import * as core from '@actions/core';
+import * as cache from '@actions/cache';
 
 const isPost = !!process.env['STATE_isPost'];
 if (!isPost) {
@@ -36,7 +37,22 @@ export async function run(main: () => Promise<void>, post?: () => Promise<void>)
     } catch (e) {
       core.setFailed(e.message);
     }
-  } else if (post) {
+  } else {
+    if (post) {
       await post();
     }
+
+    // Post-step, cache any created files.
+    let cacheObj = {dir: '', key: ''};
+    try {
+      cacheObj = JSON.parse(core.getState('post-save-cache'));
+    } catch (e) {
+      // do nothing
+    }
+    if (cacheObj && cacheObj.dir && cacheObj.key) {
+      core.info(`Cache dir: ${cacheObj.dir}`);
+      core.info(`Cache with the key: ${cacheObj.key}`);
+      await cache.saveCache([cacheObj.dir], cacheObj.key);
+    }
+  }
 }