Existing reference and idempotency

[TiTi]

Bicep version 0.26.54 (5e20b29b58)

Describe the bug When using an existing reference to compute properties for the resources to create in bicep, module is not idempotent. Indeed, successive re-execution of the whatif will show modifications even though it would resolve to the same values. For instance, i see this in the whatif:

~ name:                        "toAzure10.55" => "[format('toAzure{0}', join(take(split(parseCidr(reference(extensionResourceId(format('/subscriptions/{0}/resourceGroups/{1}', parameters('hubVnetSubscriptionID'), parameters('hubVnetResourceGroupName')), 'Microsoft.Network/virtualNetworks', parameters('hubVnetName')), '2023-04-01').addressSpace.addressPrefixes[0]).network, '.'), 2), '.'))]"

if i apply the deployment, no change appear as the computed name is (as expected) : "toAzure10.55"

To Reproduce

resource vnetHub 'Microsoft.Network/virtualNetworks@2023-04-01' existing = {
  scope: resourceGroup(hubVnetSubscriptionID, hubVnetResourceGroupName)
  name: hubVnetName
}

var networkBase = join(take(split(parseCidr(vnetHub.properties.addressSpace.addressPrefixes[0]).network, '.'), 2), '.') // ex: 10.128
var dnsServers = vnetHub.properties.dhcpOptions.dnsServers

resource routeTable 'Microsoft.Network/routeTables@2023-04-01' = {
  name: routeTableName
  location: location
  tags: tags
  properties: {
    disableBgpRoutePropagation: false
    routes: [
      {
        name: 'toAzure${networkBase}'
        properties: {
          addressPrefix: '${networkBase}.0.0/16'
          hasBgpOverride: false
          nextHopIpAddress: '${networkBase}.0.4'
          nextHopType: 'VirtualAppliance'
        }
      }
      # other routes...
    ]
  }
}

resource vnetSpoke 'Microsoft.Network/virtualNetworks@2023-04-01' = {
  name: vnetSpokeName
  location: location
  tags: tags
  properties: {
    addressSpace: {
      addressPrefixes: [
        addressSpace
      ]
    }
    dhcpOptions: empty(dnsServers) ? null : {
      dnsServers: dnsServers         # value from hub vnet
    }
    subnets: [
      # ....
    ]
  }
  dependsOn: [nsg, routeTable]
}

dnsServers also tried to replace computed value with

=> "[if(empty(reference(extensionResourceId(format('/subscriptions/{0}/resourceGroups/{1}', parameters('hubVnetSubscriptionID'), parameters('hubVnetResourceGroupName')), 'Microsoft.Network/virtualNetworks', parameters('hubVnetName')), '2023-04-01').dhcpOptions.dnsServers), null(), createObject('dnsServers', reference(extensionResourceId(format('/subscriptions/{0}/resourceGroups/{1}', parameters('hubVnetSubscriptionID'), parameters('hubVnetResourceGroupName')), 'Microsoft.Network/virtualNetworks', parameters('hubVnetName')), '2023-04-01').dhcpOptions.dnsServers))]"

despite resolving to the same values....

Creation is working fine. My issue is the non-idempotence when re-executing the same bicep a second time with whatif, to check infra is in proper/expected state.

[ahelland]

Yeah, there are known issues (ACKed bugs) with what-if - it's predictions are sort of wonky.

[alex-frankel]

Closing as a dup of #83