Add provisioner interface and request types

This adds the interface for the new resource provisioners.
The methods closely follow the grpc service methods, using the type
expected within terraform core.

provisioner/doc.go

@@ -0,0 +1,3 @@
+// Package provisioner contains the interface and primary types to implement a
+// Terraform resource provisioner.
+package provisioner

provisioner/provisioner.go

@@ -0,0 +1,67 @@
+package provisioner
+
+import (
+	"github.com/hashicorp/terraform/tfdiags"
+	"github.com/zclconf/go-cty/cty"
+)
+
+// Interface is the set of methods required for a resource provisioner plugin.
+type Interface interface {
+	// ValidateProvisionerConfig allows the provisioner to validate the
+	// configuration values.
+	ValidateProvisionerConfig(ValidateProvisionerConfigRequest) ValidateProvisionerConfigResponse
+
+	// ProvisionResource runs the provisioner with provided configuration.
+	// ProvisionResource blocks until the execution is complete.
+	// If the returned diagnostics contain any errors, the resource will be
+	// left in a tainted state.
+	ProvisionResource(ProvisionResourceRequest) ProvisionResourceResponse
+
+	// Stop is called to interrupt the provisioner.
+	//
+	// Stop should not block waiting for in-flight actions to complete. It
+	// should take any action it wants and return immediately acknowledging it
+	// has received the stop request. Terraform will not make any further API
+	// calls to the provisioner after Stop is called.
+	//
+	// The error returned, if non-nil, is assumed to mean that signaling the
+	// stop somehow failed and that the user should expect potentially waiting
+	// a longer period of time.
+	Stop() error
+}
+
+// UIOutput provides the Output method for resource provisioner
+// plugins to write any output to the UI.
+//
+// Provisioners may call the Output method multiple times while Apply is in
+// progress. It is invalid to call Output after Apply returns.
+type UIOutput interface {
+	Output(string)
+}
+
+type ValidateProvisionerConfigRequest struct {
+	// Config is the complete configuration to be used for the provisioner.
+	Config cty.Value
+}
+
+type ValidateProvisionerConfigResponse struct {
+	// Diagnostics contains any warnings or errors from the method call.
+	Diagnostics tfdiags.Diagnostics
+}
+
+type ProvisionResourceRequest struct {
+	// Config is the complete provisioner configuration.
+	Config cty.Value
+
+	// Connection is a map of string values containing any information required
+	// to access the resource instance.
+	Connection map[string]string
+
+	// UIOutput is used to return output during the Apply operation.
+	UIOutput UIOutput
+}
+
+type ProvisionResourceResponse struct {
+	// Diagnostics contains any warnings or errors from the method call.
+	Diagnostics tfdiags.Diagnostics
+}