Capture reason if zstd fails unexpectedly

src/setup-codeql.ts

@@ -486,6 +486,7 @@ export interface ToolsDownloadStatusReport {
   downloadDurationMs: number;
   extractionDurationMs: number;
   toolsUrl: string;
+  zstdFailureReason?: string;
 }
 
 // Exported using `export const` for testing purposes. Specifically, we want to
@@ -655,6 +656,7 @@ export interface SetupCodeQLResult {
   toolsSource: ToolsSource;
   toolsVersion: string;
   zstdAvailability: tar.ZstdAvailability;
+  zstdFailureReason?: string;
 }
 
 /**
@@ -672,11 +674,18 @@ export async function setupCodeQLBundle(
   logger: Logger,
 ): Promise<SetupCodeQLResult> {
   const zstdAvailability = await tar.isZstdAvailable(logger);
+  let zstdFailureReason: string | undefined;
 
   // 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,
@@ -689,6 +698,7 @@ export async function setupCodeQLBundle(
         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,
@@ -697,7 +707,7 @@ export async function setupCodeQLBundle(
     }
   }
 
-  return await setupCodeQLBundleWithCompressionMethod(
+  const result = await setupCodeQLBundleWithCompressionMethod(
     toolsInput,
     apiDetails,
     tempDir,
@@ -708,6 +718,10 @@ export async function setupCodeQLBundle(
     zstdAvailability,
     false,
   );
+  if (result.toolsDownloadStatusReport && zstdFailureReason) {
+    result.toolsDownloadStatusReport.zstdFailureReason = zstdFailureReason;
+  }
+  return result;
 }
 
 async function setupCodeQLBundleWithCompressionMethod(