Fix route-controller by handling multi-ID inputs in the batcher

The describeInstanceBatcher currently rejects DescribeInstances inputs
with anything other than a single instance ID, which breaks every caller
of getInstancesByIDs that legitimately passes multiple IDs (notably the
route controller's UpdateRoutes path). See #1351.

Move the fan-out logic into the batcher itself: multi-ID inputs are
split into one batcher submission per ID so the batcher can coalesce
them with other in-flight requests, and the aggregated results (and
joined errors, if any) are returned to the caller. The caller in
getInstancesByIDs is unchanged.

Adds a regression test exercising a multi-ID input through the batcher.

Author: Hannah Lyons

pkg/providers/v1/describe_instance_batch.go

@@ -18,6 +18,7 @@ package aws
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"sync"
 	"time"
@@ -51,16 +52,50 @@ func newdescribeInstanceBatcher(ctx context.Context, ec2api iface.EC2) *describe
 	return &describeInstanceBatcher{batcher: batcher.NewBatcher(ctx, options)}
 }
 
-// DescribeInstances adds describe instances input to batcher
+// DescribeInstances adds describe instances input to batcher. Inputs with
+// multiple instance IDs are split into one batcher submission per ID so the
+// batcher can coalesce them with other in-flight requests; a single
+// aggregated slice (and joined error, if any) is returned to the caller.
 func (b *describeInstanceBatcher) DescribeInstances(ctx context.Context, input *ec2.DescribeInstancesInput) ([]*ec2types.Instance, error) {
-	if len(input.InstanceIds) != 1 {
-		return nil, fmt.Errorf("expected to receive a single instance only, found %d", len(input.InstanceIds))
+	if len(input.InstanceIds) == 0 {
+		return nil, fmt.Errorf("expected at least one instance ID, found 0")
 	}
-	result := b.batcher.Add(ctx, input)
-	if result.Output == nil {
-		return nil, result.Err
+	if len(input.InstanceIds) == 1 {
+		result := b.batcher.Add(ctx, input)
+		if result.Output == nil {
+			return nil, result.Err
+		}
+		return []*ec2types.Instance{result.Output}, result.Err
+	}
+
+	var (
+		mu        sync.Mutex
+		wg        sync.WaitGroup
+		instances []*ec2types.Instance
+		errs      []error
+	)
+	for _, id := range input.InstanceIds {
+		wg.Add(1)
+		go func(id string) {
+			defer wg.Done()
+			subInput := *input
+			subInput.InstanceIds = []string{id}
+			result := b.batcher.Add(ctx, &subInput)
+			mu.Lock()
+			defer mu.Unlock()
+			if result.Err != nil {
+				errs = append(errs, result.Err)
+			}
+			if result.Output != nil {
+				instances = append(instances, result.Output)
+			}
+		}(id)
+	}
+	wg.Wait()
+	if len(errs) > 0 {
+		return instances, errors.Join(errs...)
 	}
-	return []*ec2types.Instance{result.Output}, result.Err
+	return instances, nil
 }
 
 // DescribeInstanceHasher generates hash for different describe instances inputs

pkg/providers/v1/instances_v2_test.go

@@ -506,6 +506,42 @@ func TestDescribeInstanceBatchingWithBatchedRequestFail(t *testing.T) {
 	mockedEC2API.AssertNumberOfCalls(t, "DescribeInstances", 3)
 }
 
+// TestDescribeInstanceBatchingWithMultipleInstanceIDs verifies that an input
+// containing multiple instance IDs is fanned out through the batcher and
+// returns aggregated results. Regression test for #1351.
+func TestDescribeInstanceBatchingWithMultipleInstanceIDs(t *testing.T) {
+	mockedEC2API := newMockedEC2API()
+	batcher := newdescribeInstanceBatcher(context.Background(), &awsSdkEC2{ec2: mockedEC2API})
+
+	mockedEC2API.On("DescribeInstances", mock.Anything).Return(&ec2.DescribeInstancesOutput{
+		Reservations: []ec2types.Reservation{
+			{
+				Instances: []ec2types.Instance{
+					{InstanceId: aws.String("Test-1")},
+					{InstanceId: aws.String("Test-2")},
+					{InstanceId: aws.String("Test-3")},
+				},
+			},
+		},
+	}, nil)
+
+	res, err := batcher.DescribeInstances(context.Background(), &ec2.DescribeInstancesInput{
+		InstanceIds: []string{"Test-1", "Test-2", "Test-3"},
+	})
+	assert.NoError(t, err)
+	assert.Len(t, res, 3)
+
+	gotIDs := map[string]bool{}
+	for _, instance := range res {
+		gotIDs[aws.ToString(instance.InstanceId)] = true
+	}
+	assert.True(t, gotIDs["Test-1"])
+	assert.True(t, gotIDs["Test-2"])
+	assert.True(t, gotIDs["Test-3"])
+
+	mockedEC2API.AssertNumberOfCalls(t, "DescribeInstances", 1)
+}
+
 func getCloudWithMockedDescribeInstances(instanceExists bool, instanceState ec2types.InstanceStateName, instanceID string) *Cloud {
 	mockedEC2API := newMockedEC2API()
 	c := &Cloud{ec2: &awsSdkEC2{ec2: mockedEC2API}, describeInstanceBatcher: newdescribeInstanceBatcher(context.Background(), &awsSdkEC2{ec2: mockedEC2API})}