add: synchronize calls to DiscoverPollEndpoint

Agents that share ECSClients can call DiscoverPollEndpoint (DPE) multiple
times per task. Each routine that calls DPE will first check the cache
before performing the actual API call over the network. The intention
here is that only one actual API call is performed (by the first
routine to call DPE).

However, it is possible for multiple routines to race and effectively
make many actual API calls. This is because the `pollEndpointCache` is
only updated when the first API call _returns_.

This change enforces the intended behavior by making subsequent
routines wait for the cache to be updated (or not) by the first
thread, eliminating simultaneous calls to DPE.

ecs-agent/api/ecs/client/ecs_client.go

@@ -19,6 +19,7 @@ import (
 	"fmt"
 	"net/http"
 	"strings"
+	"sync"
 	"time"
 
 	"github.com/aws/aws-sdk-go/aws"
@@ -74,6 +75,7 @@ type ecsClient struct {
 	ec2metadata                      ec2.EC2MetadataClient
 	httpClient                       *http.Client
 	pollEndpointCache                async.TTLCache
+	pollEndpointLock                 sync.Mutex
 	isFIPSDetected                   bool
 	shouldExcludeIPv6PortBinding     bool
 	sascCustomRetryBackoff           func(func() error) error
@@ -731,6 +733,8 @@ func (client *ecsClient) DiscoverSystemLogsEndpoint(containerInstanceArn string,
 
 func (client *ecsClient) discoverPollEndpoint(containerInstanceArn string,
 	availabilityZone string) (*ecsmodel.DiscoverPollEndpointOutput, error) {
+	client.pollEndpointLock.Lock()
+	defer client.pollEndpointLock.Unlock()
 	// Try getting an entry from the cache.
 	cachedEndpoint, expired, found := client.pollEndpointCache.Get(containerInstanceArn)
 	if !expired && found {