Fix HA volume placement in scale command

When scaling apps with volumes, flyctl was hardcoding RequireUniqueZone=false,
causing all volumes (and their attached machines) to be placed on the same host.
This defeated the purpose of HA when scaling with volumes.

The fix detects HA scenarios by checking if total machines in a region > 1:
- Single machine (existingCount + delta == 1): RequireUniqueZone=false
- HA scenario (existingCount + delta > 1): RequireUniqueZone=true

This ensures volumes are distributed across different hosts, and since
machines follow their volumes via RequireVolumeID, the machines will
also be distributed for proper HA.

Examples:
- Scale 1→3: Existing=1, Delta=2, Total=3 → RequireUniqueZone=true ✅
- Deploy with 1: Existing=0, Delta=1, Total=1 → RequireUniqueZone=false ✅
- Scale 0→3: Existing=0, Delta=3, Total=3 → RequireUniqueZone=true ✅

Fixes: TestFlyScaleTo3/With_Volume where all 3 machines ended up on one host

Author: mattp-fly

internal/command/scale/count_machines.go

@@ -317,14 +317,15 @@ func computeActions(appName string, machines []*fly.Machine, expectedGroupCounts
 		delete(mConfig.Env, "FLY_STANDBY_FOR")
 
 		for region, delta := range regionDiffs {
+			existingMachinesInRegion := perRegionMachines[region]
 			actions = append(actions, &planItem{
 				GroupName:           groupName,
 				Region:              region,
 				Delta:               delta,
-				Machines:            perRegionMachines[region],
+				Machines:            existingMachinesInRegion,
 				LaunchMachineInput:  &fly.LaunchMachineInput{Region: region, Config: mConfig, MinSecretsVersion: minvers},
 				Volumes:             defaults.PopAvailableVolumes(mConfig, region, delta),
-				CreateVolumeRequest: defaults.CreateVolumeRequest(mConfig, region, delta),
+				CreateVolumeRequest: defaults.CreateVolumeRequest(mConfig, region, delta, len(existingMachinesInRegion)),
 			})
 		}
 	}
@@ -352,7 +353,7 @@ func computeActions(appName string, machines []*fly.Machine, expectedGroupCounts
 				Delta:               delta,
 				LaunchMachineInput:  &fly.LaunchMachineInput{Region: region, Config: mConfig, MinSecretsVersion: minvers},
 				Volumes:             defaults.PopAvailableVolumes(mConfig, region, delta),
-				CreateVolumeRequest: defaults.CreateVolumeRequest(mConfig, region, delta),
+				CreateVolumeRequest: defaults.CreateVolumeRequest(mConfig, region, delta, 0), // No existing machines for new groups
 			})
 		}
 	}

internal/command/scale/machine_defaults.go

@@ -118,17 +118,23 @@ func (d *defaultValues) PopAvailableVolumes(mConfig *fly.MachineConfig, region s
 	return availableVolumes
 }
 
-func (d *defaultValues) CreateVolumeRequest(mConfig *fly.MachineConfig, region string, delta int) *fly.CreateVolumeRequest {
+func (d *defaultValues) CreateVolumeRequest(mConfig *fly.MachineConfig, region string, delta int, existingMachineCount int) *fly.CreateVolumeRequest {
 	if len(mConfig.Mounts) == 0 || delta <= 0 {
 		return nil
 	}
 	mount := mConfig.Mounts[0]
+
+	// Enable RequireUniqueZone for HA scenarios (when total machines in region > 1)
+	// This ensures volumes (and their attached machines) are distributed across different hosts
+	totalMachinesInRegion := existingMachineCount + delta
+	requireUniqueZone := totalMachinesInRegion > 1
+
 	return &fly.CreateVolumeRequest{
 		Name:                mount.Name,
 		Region:              region,
 		SizeGb:              &mount.SizeGb,
 		Encrypted:           fly.Pointer(mount.Encrypted),
-		RequireUniqueZone:   fly.Pointer(false),
+		RequireUniqueZone:   fly.Pointer(requireUniqueZone),
 		SnapshotID:          d.snapshotID,
 		ComputeRequirements: mConfig.Guest,
 		ComputeImage:        mConfig.Image,