Reuse http client used with ASM API (Azure#3858)

* Reuse the http client for ASM calls. Throttle retries when ServiceUnavailable responses are received

* code cleanup:-
Removed the only other occurence in sendAzureRequest() since the httpClient was already setup in makeClient().
Removed the nil check inside the function as the parameter is passed in as a value type.

* Reverted the change as it was inadvertent during testing

* Code improvements.  Use the configurable Poll interval and check for both service unavaialbe and throttling messages

services/classic/management/client.go

@@ -21,6 +21,7 @@ package management
 import (
 	"errors"
 	"fmt"
+	"net/http"
 	"runtime"
 	"time"
 
@@ -44,6 +45,7 @@ const (
 type client struct {
 	publishSettings publishSettings
 	config          ClientConfig
+	httpClient      *http.Client
 }
 
 // Client is the base Azure Service Management API client instance that
@@ -154,10 +156,16 @@ func makeClient(subscriptionID string, managementCert []byte, config ClientConfi
 		config.UserAgent = DefaultUserAgent
 	}
 
-	return client{
+	clientObj := client{
 		publishSettings: publishSettings,
 		config:          config,
-	}, nil
+	}
+	var err error
+	clientObj.httpClient, err = clientObj.createHTTPClient()
+	if err != nil {
+		return nil, err
+	}
+	return clientObj, nil
 }
 
 func userAgent() string {

services/classic/management/http.go

@@ -21,6 +21,7 @@ import (
 	"crypto/tls"
 	"fmt"
 	"net/http"
+	"time"
 )
 
 const (
@@ -86,12 +87,7 @@ func (client client) sendAzureRequest(method, url, contentType string, data []by
 		return nil, fmt.Errorf(errParamNotSpecified, "url")
 	}
 
-	httpClient, err := client.createHTTPClient()
-	if err != nil {
-		return nil, err
-	}
-
-	response, err := client.sendRequest(httpClient, url, method, contentType, data, 5)
+	response, err := client.sendRequest(client.httpClient, url, method, contentType, data, 5)
 	if err != nil {
 		return nil, err
 	}
@@ -161,6 +157,10 @@ func (client client) sendRequest(httpClient *http.Client, url, requestType, cont
 				if numberOfRetries == 0 {
 					return nil, azureErr
 				}
+				if response.StatusCode == http.StatusServiceUnavailable || response.StatusCode == http.StatusTooManyRequests {
+					// Wait before retrying the operation
+					time.Sleep(client.config.OperationPollInterval)
+				}
 
 				return client.sendRequest(httpClient, url, requestType, contentType, data, numberOfRetries-1)
 			}