Capture reason if zstd fails unexpectedly

lib/setup-codeql.js

@@ -479,17 +479,27 @@ function getCanonicalToolcacheVersion(cliVersion, bundleVersion, logger) {
  */
 async function setupCodeQLBundle(toolsInput, apiDetails, tempDir, variant, defaultCliVersion, features, logger) {
     const zstdAvailability = await tar.isZstdAvailable(logger);
+    let zstdFailureReason;
     // If we think the installed version of tar supports zstd, try to use zstd,
     // but be prepared to fall back to gzip in case we were wrong.
     if (zstdAvailability.available) {
         try {
+            // To facilitate testing the fallback, fail here if a testing environment variable is set.
+            if (process.env.CODEQL_ACTION_FORCE_ZSTD_FAILURE === "true") {
+                throw new Error("Failing since CODEQL_ACTION_FORCE_ZSTD_FAILURE is true.");
+            }
             return await setupCodeQLBundleWithCompressionMethod(toolsInput, apiDetails, tempDir, variant, defaultCliVersion, features, logger, zstdAvailability, true);
         }
         catch (e) {
+            zstdFailureReason = util.getErrorMessage(e) || "unknown error";
             logger.warning(`Failed to set up CodeQL tools with zstd. Falling back to gzipped version. Error: ${util.getErrorMessage(e)}`);
         }
     }
-    return await setupCodeQLBundleWithCompressionMethod(toolsInput, apiDetails, tempDir, variant, defaultCliVersion, features, logger, zstdAvailability, false);
+    const result = await setupCodeQLBundleWithCompressionMethod(toolsInput, apiDetails, tempDir, variant, defaultCliVersion, features, logger, zstdAvailability, false);
+    if (result.toolsDownloadStatusReport && zstdFailureReason) {
+        result.toolsDownloadStatusReport.zstdFailureReason = zstdFailureReason;
+    }
+    return result;
 }
 async function setupCodeQLBundleWithCompressionMethod(toolsInput, apiDetails, tempDir, variant, defaultCliVersion, features, logger, zstdAvailability, useTarIfAvailable) {
     const source = await getCodeQLSource(toolsInput, defaultCliVersion, apiDetails, variant, useTarIfAvailable, features, logger);