Don't consider 429 as terminal failure for Location poller (#22651)

If the polling request is being throttled, preserve the current state
and return the *http.Response.

sdk/azcore/CHANGELOG.md

@@ -13,6 +13,7 @@
 ### Bugs Fixed
 
 * `runtime.SetMultipartFormData` won't try to stringify `[]byte` values.
+* Pollers that use the `Location` header won't consider `http.StatusTooManyRequests` a terminal failure.
 
 ### Other Changes
 

sdk/azcore/internal/pollers/loc/loc.go

@@ -103,6 +103,10 @@ func (p *Poller[T]) Poll(ctx context.Context) (*http.Response, error) {
 		} else if resp.StatusCode > 199 && resp.StatusCode < 300 {
 			// any 2xx other than a 202 indicates success
 			p.CurState = poller.StatusSucceeded
+		} else if resp.StatusCode == http.StatusTooManyRequests {
+			// the request is being throttled. we DO NOT want to
+			// include this as terminal failure so preserve the
+			// existing state and return the response.
 		} else {
 			p.CurState = poller.StatusFailed
 		}

sdk/azcore/internal/pollers/loc/loc_test.go

@@ -16,6 +16,7 @@ import (
 
 	"github.com/Azure/azure-sdk-for-go/sdk/azcore/internal/exported"
 	"github.com/Azure/azure-sdk-for-go/sdk/azcore/internal/shared"
+	"github.com/Azure/azure-sdk-for-go/sdk/internal/mock"
 	"github.com/Azure/azure-sdk-for-go/sdk/internal/poller"
 	"github.com/stretchr/testify/require"
 )
@@ -173,3 +174,26 @@ func TestSynchronousCompletion(t *testing.T) {
 	require.Equal(t, poller.StatusSucceeded, lp.CurState)
 	require.True(t, lp.Done())
 }
+
+func TestWithThrottling(t *testing.T) {
+	srv, close := mock.NewServer()
+	defer close()
+	srv.AppendResponse(mock.WithStatusCode(http.StatusTooManyRequests))
+	srv.AppendResponse(mock.WithStatusCode(http.StatusAccepted))
+	srv.AppendResponse(mock.WithStatusCode(http.StatusTooManyRequests))
+	srv.AppendResponse(mock.WithStatusCode(http.StatusOK))
+	resp := initialResponse()
+	resp.Header.Set(shared.HeaderLocation, srv.URL())
+	lp, err := New[struct{}](exported.NewPipeline(shared.TransportFunc(func(req *http.Request) (*http.Response, error) {
+		return srv.Do(req)
+	})), resp)
+	require.NoError(t, err)
+	respCount := 0
+	for !lp.Done() {
+		_, err = lp.Poll(context.Background())
+		require.NoError(t, err)
+		respCount++
+	}
+	require.EqualValues(t, 4, respCount)
+	require.EqualValues(t, poller.StatusSucceeded, lp.CurState)
+}