fix: Only call getResumptionRequestFn once (#1593)

* Only call getResumptionRequestFn once

The current code calls getResumptionRequestFn twice when the result from the first call can be reused. For clients implementing retries, we only want one call to getResumptionRequestFn to be done per request because we don’t want the state that getResumptionRequestFn relies on to be modified twice.

* fix: Only call getResumptionRequestFn once

* Add a sinon spy to make sure the fn is called once

* Remove only

* ran linter

---------

Co-authored-by: Leah E. Cole <6719667+leahecole@users.noreply.github.com>

gax/src/streamingCalls/streaming.ts

@@ -133,7 +133,7 @@ export class StreamProxy extends duplexify implements GRPCCallResult {
       const resumptionRetryArgument =
         retry.getResumptionRequestFn(retryArgument);
       if (resumptionRetryArgument !== undefined) {
-        retryArgument = retry.getResumptionRequestFn(retryArgument);
+        retryArgument = resumptionRetryArgument;
       }
     }
     this.resetStreams(stream);

gax/test/unit/streaming.ts

@@ -1157,10 +1157,10 @@ describe('streaming', () => {
       true // new retry behavior enabled
     );
     // resumption strategy is to pass a different arg to the function
-    const getResumptionRequestFn = (originalRequest: RequestType) => {
+    const getResumptionRequestFn = sinon.spy((originalRequest: RequestType) => {
       assert.strictEqual(originalRequest.arg, 0);
       return {arg: 2};
-    };
+    });
     const s = apiCall(
       {arg: 0},
       {
@@ -1194,6 +1194,7 @@ describe('streaming', () => {
         receivedData.join(' '),
         'Hello World testing retries'
       );
+      assert.strictEqual(getResumptionRequestFn.callCount, 1);
       done();
     });
   });