OCPBUGS-17227: gcp: fix validation of custom instance types

Users can create their own instance types in GCloud. Those types are not
returned when listing the machine types for a given project/region. This
would cause those installs with custom types to fail with the error:

```
Internal error: failed to fetch instance type, this error usually occurs if the region or the instance type is not found
```

after the validations introduced by
#7317 and
#7096.

To fix that, let's fallback to the previous way of fetching the specific
machine type whenever the aggregated list returns 0 elements.

pkg/asset/installconfig/gcp/client.go

@@ -104,9 +104,6 @@ func GetMachineTypeList(ctx context.Context, svc *compute.Service, project, regi
 		}
 		return nil
 	})
-	if len(machines) == 0 {
-		return nil, errors.New("failed to fetch instance type, this error usually occurs if the region or the instance type is not found")
-	}
 
 	return machines, err
 }
@@ -118,25 +115,38 @@ func (c *Client) GetMachineTypeWithZones(ctx context.Context, project, region, m
 		return nil, nil, err
 	}
 
+	pz, err := GetZones(ctx, svc, project, fmt.Sprintf("region eq .*%s", region))
+	if err != nil {
+		return nil, nil, err
+	}
+	projZones := sets.New[string]()
+	for _, zone := range pz {
+		projZones.Insert(zone.Name)
+	}
+
 	machines, err := GetMachineTypeList(ctx, svc, project, region, machineType, "")
 	if err != nil {
 		return nil, nil, err
 	}
 
+	// Custom machine types are not included in aggregated lists, so let's try
+	// to get the machine type directly before returning an error. Also
+	// fallback to all the zones in the project
+	if len(machines) == 0 {
+		cctx, cancel := context.WithTimeout(ctx, defaultTimeout)
+		defer cancel()
+		machine, err := svc.MachineTypes.Get(project, pz[0].Name, machineType).Context(cctx).Do()
+		if err != nil {
+			return nil, nil, fmt.Errorf("failed to fetch instance type: %w", err)
+		}
+		return machine, projZones, nil
+	}
+
 	zones := sets.New[string]()
 	for _, machine := range machines {
 		zones.Insert(machine.Zone)
 	}
-
 	// Restrict to zones avaialable in the project
-	pz, err := GetZones(ctx, svc, project, fmt.Sprintf("region eq .*%s", region))
-	if err != nil {
-		return nil, nil, err
-	}
-	projZones := sets.New[string]()
-	for _, zone := range pz {
-		projZones.Insert(zone.Name)
-	}
 	zones = zones.Intersection(projZones)
 
 	return machines[0], zones, nil

pkg/asset/machines/gcp/zones.go

@@ -64,25 +64,29 @@ func ZonesForInstanceType(project, region, instanceType string) ([]string, error
 	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Minute)
 	defer cancel()
 
+	pZones, err := gcpconfig.GetZones(ctx, svc, project, fmt.Sprintf("(region eq .*%s) (status eq UP)", region))
+	if err != nil {
+		return nil, fmt.Errorf("failed to get zones for project: %w", err)
+	}
+	pZoneNames := sets.New[string]()
+	for _, z := range pZones {
+		pZoneNames.Insert(z.Name)
+	}
+
 	machines, err := gcpconfig.GetMachineTypeList(ctx, svc, project, region, instanceType, "items/*/machineTypes(zone),nextPageToken")
 	if err != nil {
 		return nil, fmt.Errorf("failed to get zones for instance type: %w", err)
 	}
+	// Custom machine types do not show up in the list. Let's fallback to the project zones
+	if len(machines) == 0 {
+		return sets.List(pZoneNames), nil
+	}
 
 	zones := sets.New[string]()
 	for _, machine := range machines {
 		zones.Insert(machine.Zone)
 	}
 
 	// Not all instance zones might be available in the project
-	pZones, err := gcpconfig.GetZones(ctx, svc, project, fmt.Sprintf("(region eq .*%s) (status eq UP)", region))
-	if err != nil {
-		return nil, fmt.Errorf("failed to get zones for project: %w", err)
-	}
-	pZoneNames := sets.New[string]()
-	for _, z := range pZones {
-		pZoneNames.Insert(z.Name)
-	}
-
 	return sets.List(zones.Intersection(pZoneNames)), nil
 }