Add 'vm' template function

builder/azure/chroot/builder.go

@@ -115,6 +115,7 @@ type Builder struct {
 
 func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 	b.config.ctx.Funcs = azcommon.TemplateFuncs
+	b.config.ctx.Funcs["vm"] = CreateVMMetadataTemplateFunc()
 	err := config.Decode(&b.config, &config.DecodeOpts{
 		Interpolate:        true,
 		InterpolateContext: &b.config.ctx,
@@ -129,6 +130,9 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 			},
 		},
 	}, raws...)
+	if err != nil {
+		return nil, err
+	}
 
 	var errs *packer.MultiError
 	var warns []string

builder/azure/chroot/template_funcs.go

@@ -0,0 +1,37 @@
+package chroot
+
+import (
+	"fmt"
+	"sync"
+
+	"github.com/hashicorp/packer/builder/azure/common/client"
+)
+
+// CreateVMMetadataTemplateFunc returns a template function that retrieves VM metadata. VM metadata is retrieved only once and reused for all executions of the function.
+func CreateVMMetadataTemplateFunc() func(string) (string, error) {
+	var data *client.ComputeInfo
+	var dataErr error
+	once := sync.Once{}
+	return func(key string) (string, error) {
+		once.Do(func() {
+			data, dataErr = client.DefaultMetadataClient.GetComputeInfo()
+		})
+		if dataErr != nil {
+			return "", dataErr
+		}
+		switch key {
+		case "name":
+			return data.Name, nil
+		case "subscription_id":
+			return data.SubscriptionID, nil
+		case "resource_group":
+			return data.ResourceGroupName, nil
+		case "location":
+			return data.Location, nil
+		case "resource_id":
+			return data.ResourceID(), nil
+		default:
+			return "", fmt.Errorf("unknown metadata key: %s (supported: name, subscription_id, resource_group, location, resource_id)", key)
+		}
+	}
+}

website/source/docs/builders/azure-chroot.html.md.erb

@@ -101,7 +101,52 @@ mounts `/prod` and `/dev`:
 
 -   The mount directory.
 
-## Example
+## Additional template function
+
+Because this builder runs on an Azure VM, there is an additional template function
+available called `vm`, which returns the following VM metadata:
+
+-   name
+-   subscription_id
+-   resource_group
+-   location
+-   resource_id
+
+This function can be used in the configuration templates, for example, use
+```
+"{{ vm `subscription_id` }}"
+```
+to fill in the subscription ID of the VM in any of the configuration options.
+
+## Examples
+Here are some examples using this builder.
+
+### Using a VM with a Managed Identity
+On a VM with a system-assigned managed identity that has the contributor role
+on its own resource group, the following config can be used to create an
+updated Debian image:
+``` json
+{
+  "builders": [{
+    "type": "azure-chroot",
+
+    "command_wrapper": "sudo {{.Command}}",
+
+    "image_resource_id": "/subscriptions/{{vm `subscription_id`}}/resourceGroups/{{vm `resource_group`}}/providers/Microsoft.Compute/images/MyDebianOSImage-{{timestamp}}",
+    "source": "credativ:Debian:9:latest"
+  }],
+  "provisioners": [{
+    "inline": [
+      "apt-get update",
+      "apt-get upgrade -y"
+    ],
+    "inline_shebang": "/bin/sh -x",
+    "type": "shell"
+  }]
+}
+```
+
+### Using a Service Principal
 Here is an example that creates a Debian image with updated packages. Specify
 all environment variables (`ARM_CLIENT_ID`, `ARM_CLIENT_SECRET`, 
 `ARM_SUBSCRIPTION_ID`) to use a service principal.