Retry policy will always clone the *http.Request (Azure#20843)

* Retry policy will always clone the *http.Request

This ensures that the entirety of the request is restored on retries.

* simplify test

sdk/azcore/CHANGELOG.md

@@ -18,6 +18,7 @@
 ### Breaking Changes
 
 ### Bugs Fixed
+* Retry policy always clones the underlying `*http.Request` before invoking the next policy.
 
 ### Other Changes
 

sdk/azcore/runtime/policy_retry.go

@@ -125,7 +125,8 @@ func (p *retryPolicy) Do(req *policy.Request) (resp *http.Response, err error) {
 		}
 
 		if options.TryTimeout == 0 {
-			resp, err = req.Next()
+			clone := req.Clone(req.Raw().Context())
+			resp, err = clone.Next()
 		} else {
 			// Set the per-try time for this particular retry operation and then Do the operation.
 			tryCtx, tryCancel := context.WithTimeout(req.Raw().Context(), options.TryTimeout)

sdk/azcore/runtime/policy_retry_test.go

@@ -810,6 +810,39 @@ func TestRetryableRequestBodyWithCloser(t *testing.T) {
 	require.True(t, tr.closeCalled)
 }
 
+func TestRetryPolicySuccessWithRetryPreserveHeaders(t *testing.T) {
+	srv, close := mock.NewServer()
+	defer close()
+	srv.AppendResponse(mock.WithStatusCode(http.StatusRequestTimeout))
+	srv.AppendResponse()
+	pl := exported.NewPipeline(srv, NewRetryPolicy(testRetryOptions()), exported.PolicyFunc(challengeLikePolicy))
+	req, err := NewRequest(context.Background(), http.MethodGet, srv.URL())
+	require.NoError(t, err)
+	body := newRewindTrackingBody("stuff")
+	require.NoError(t, req.SetBody(body, "text/plain"))
+	resp, err := pl.Do(req)
+	require.NoError(t, err)
+	require.EqualValues(t, http.StatusOK, resp.StatusCode)
+	require.EqualValues(t, 2, srv.Requests())
+	require.EqualValues(t, 1, body.rcount)
+	require.True(t, body.closed)
+}
+
+func challengeLikePolicy(req *policy.Request) (*http.Response, error) {
+	if req.Body() == nil {
+		return nil, errors.New("request body wasn't restored")
+	}
+	if req.Raw().Header.Get("content-type") != "text/plain" {
+		return nil, errors.New("content-type header wasn't restored")
+	}
+
+	// remove the body and header. the retry policy should restore them
+	if err := req.SetBody(nil, ""); err != nil {
+		return nil, err
+	}
+	return req.Next()
+}
+
 func newRewindTrackingBody(s string) *rewindTrackingBody {
 	// there are two rewinds that happen before rewinding for a retry
 	// 1. to get the body's size in SetBody()