fix(middleware-signing): attempt secondary authscheme selection during request signing (#5895)

* fix(middleware-signing): attempt secondary authscheme selection during request signing

* chore: typo

Co-authored-by: Steven Yuan 

* fix(middware-signing): do not set input signing region when incoming authscheme is sigv4a

---------

Co-authored-by: Steven Yuan

Author: kuhe

packages/middleware-signing/src/awsAuthConfiguration.ts

@@ -194,19 +194,30 @@ export const resolveAwsAuthConfig = <T>(
         authScheme
       );
 
+      const isSigv4a = authScheme?.name === "sigv4a";
+
       const signingRegion = authScheme.signingRegion;
       const signingService = authScheme.signingName;
-      // update client's singing region and signing service config if they are resolved.
-      // signing region resolving order: user supplied signingRegion -> endpoints.json inferred region -> client region
-      input.signingRegion = input.signingRegion || signingRegion;
+
+      let regionForSigner: string | undefined;
+
+      if (isSigv4a) {
+        regionForSigner = input.signingRegion || signingRegion;
+      } else {
+        // update client's signing region and signing service config if they are resolved.
+        // signing region resolving order: user supplied signingRegion -> endpoints.json inferred region -> client region
+        input.signingRegion = input.signingRegion || signingRegion;
+        regionForSigner = input.signingRegion;
+      }
+
       // signing name resolving order:
       // user supplied signingName -> endpoints.json inferred (credential scope -> model arnNamespace) -> model service id
       input.signingName = input.signingName || signingService || input.serviceId;
 
       const params: SignatureV4Init & SignatureV4CryptoInit = {
         ...input,
         credentials: normalizedCreds,
-        region: input.signingRegion,
+        region: regionForSigner,
         service: input.signingName,
         sha256,
         uriEscapePath: signingEscapePath,

packages/middleware-signing/src/awsAuthMiddleware.ts

@@ -10,6 +10,7 @@ import {
   HttpRequest as IHttpRequest,
   Pluggable,
   RelativeMiddlewareOptions,
+  RequestSigner,
 } from "@smithy/types";
 
 import { AwsAuthResolvedConfig } from "./awsAuthConfiguration";
@@ -25,14 +26,40 @@ export const awsAuthMiddleware =
       if (!HttpRequest.isInstance(args.request)) return next(args);
 
       // TODO(identityandauth): call authScheme resolver
-      const authScheme: AuthScheme | undefined = context.endpointV2?.properties?.authSchemes?.[0];
+      let authScheme: AuthScheme | undefined;
+      let signer: RequestSigner | undefined;
+
+      const firstAuthScheme = context.endpointV2?.properties?.authSchemes?.[0];
+      const secondAuthScheme = context.endpointV2?.properties?.authSchemes?.[1];
+      const firstAuthSchemeIsSigv4a = firstAuthScheme?.name === "sigv4a";
+
+      if (firstAuthSchemeIsSigv4a && secondAuthScheme) {
+        signer = await options.signer((authScheme = firstAuthScheme));
+        const uncheckedSigner = signer as any;
+        const sigv4aAvailable = (() => {
+          if (typeof uncheckedSigner?.getSigv4aSigner === "function") {
+            if (uncheckedSigner?.signerOptions?.runtime !== "node") {
+              return false;
+            }
+            try {
+              uncheckedSigner.getSigv4aSigner();
+              return true;
+            } catch (e: unknown) {}
+          }
+          return false;
+        })();
+        if (!sigv4aAvailable) {
+          signer = await options.signer((authScheme = secondAuthScheme));
+        }
+      } else {
+        signer = await options.signer((authScheme = firstAuthScheme));
+      }
+
+      let signedRequest: IHttpRequest;
 
       const multiRegionOverride: string | undefined =
         authScheme?.name === "sigv4a" ? authScheme?.signingRegionSet?.join(",") : undefined;
 
-      const signer = await options.signer(authScheme);
-
-      let signedRequest: IHttpRequest;
       const signingOptions = {
         signingDate: getSkewCorrectedDate(options.systemClockOffset),
         signingRegion: multiRegionOverride || context["signing_region"],