Looking through the operations in node_resource_apply and
node_resource_destroy, there are multiple nil checks for diffApply, so
one we need to continue to assume that the diff can be nil through there
until we ensure that it is non-nill in all cases.
So regardless of how we came to get a nil diff in the UiHook PreApply
method, we need to check it.
We've supported GOOGLE_APPLICATION_CREDENTIALS as an environment
variable (it comes free with our OAuth2 client) but it has never been
documented. Documenting it now to resolve#12626.
The default value for `source_dest_check` needs to remain the same, so as not to break any backwards compatibility, however, adding a new `network_interface` parameter with a pre-configured network_interface that has `source_dest_check` set to false throws a diff after initial apply. Since we don't want to change `source_dest_check` to computed in order to not break sane defaults, ignore the diff thrown if `network_interface` attributes are configured on an instance.
```
$ make testacc TEST=./builtin/providers/aws TESTARGS="-run=TestAccAWSInstance_primaryNetworkInterfaceSourceDestCheck"
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/04/28 16:26:02 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSInstance_primaryNetworkInterfaceSourceDestCheck -timeout 120m
=== RUN TestAccAWSInstance_primaryNetworkInterfaceSourceDestCheck
--- PASS: TestAccAWSInstance_primaryNetworkInterfaceSourceDestCheck (134.20s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/aws 134.211s
```
```
$ make testacc TEST=./builtin/providers/aws TESTARGS="-run=TestAccAWSInstance_sourceDestCheck"
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/04/28 16:15:14 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSInstance_sourceDestCheck -timeout 120m
=== RUN TestAccAWSInstance_sourceDestCheck
--- PASS: TestAccAWSInstance_sourceDestCheck (179.81s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/aws 179.815s
```
Fixes: #14068
Jake Champlin