providers: add template provider

Fixes hashicorp#215.

builtin/bins/provider-template/main.go

@@ -0,0 +1,12 @@
+package main
+
+import (
+	"github.com/hashicorp/terraform/builtin/providers/template"
+	"github.com/hashicorp/terraform/plugin"
+)
+
+func main() {
+	plugin.Serve(&plugin.ServeOpts{
+		ProviderFunc: template.Provider,
+	})
+}

builtin/providers/template/provider.go

@@ -0,0 +1,14 @@
+package template
+
+import (
+	"github.com/hashicorp/terraform/helper/schema"
+	"github.com/hashicorp/terraform/terraform"
+)
+
+func Provider() terraform.ResourceProvider {
+	return &schema.Provider{
+		ResourcesMap: map[string]*schema.Resource{
+			"template_file": resource(),
+		},
+	}
+}

builtin/providers/template/provider_test.go

@@ -0,0 +1,13 @@
+package template
+
+import (
+	"testing"
+
+	"github.com/hashicorp/terraform/helper/schema"
+)
+
+func TestProvider(t *testing.T) {
+	if err := Provider().(*schema.Provider).InternalValidate(); err != nil {
+		t.Fatalf("err: %s", err)
+	}
+}

builtin/providers/template/resource.go

@@ -0,0 +1,119 @@
+package template
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"fmt"
+	"io/ioutil"
+
+	"github.com/hashicorp/terraform/config"
+	"github.com/hashicorp/terraform/config/lang"
+	"github.com/hashicorp/terraform/config/lang/ast"
+	"github.com/hashicorp/terraform/helper/schema"
+)
+
+func resource() *schema.Resource {
+	return &schema.Resource{
+		Create: Create,
+		Read:   Read,
+		Update: Update,
+		Delete: Delete,
+		Exists: Exists,
+
+		Schema: map[string]*schema.Schema{
+			"filename": &schema.Schema{
+				Type:        schema.TypeString,
+				Required:    true,
+				Description: "file to read template from",
+			},
+			"vars": &schema.Schema{
+				Type:        schema.TypeMap,
+				Optional:    true,
+				Default:     make(map[string]interface{}),
+				Description: "variables to substitute",
+			},
+			"rendered": &schema.Schema{
+				Type:        schema.TypeString,
+				Computed:    true,
+				Default:     nil,
+				Description: "rendered template",
+			},
+		},
+	}
+}
+
+func Create(d *schema.ResourceData, meta interface{}) error { return eval(d) }
+func Update(d *schema.ResourceData, meta interface{}) error { return eval(d) }
+func Read(d *schema.ResourceData, meta interface{}) error   { return nil }
+func Delete(d *schema.ResourceData, meta interface{}) error {
+	d.SetId("")
+	return nil
+}
+func Exists(d *schema.ResourceData, meta interface{}) (bool, error) {
+	// Reload every time in case something has changed.
+	// This should be cheap, and cache invalidation is hard.
+	return false, nil
+}
+
+func eval(d *schema.ResourceData) error {
+	filename := d.Get("filename").(string)
+	vars := d.Get("vars").(map[string]interface{})
+
+	buf, err := ioutil.ReadFile(filename)
+	if err != nil {
+		return err
+	}
+
+	rendered, err := execute(string(buf), vars)
+	if err != nil {
+		return fmt.Errorf("failed to render %v: %v", filename, err)
+	}
+
+	d.Set("rendered", rendered)
+	d.SetId(hash(rendered))
+	return nil
+}
+
+// execute parses and executes a template using vars.
+func execute(s string, vars map[string]interface{}) (string, error) {
+	root, err := lang.Parse(s)
+	if err != nil {
+		return "", err
+	}
+
+	varmap := make(map[string]ast.Variable)
+	for k, v := range vars {
+		// As far as I can tell, v is always a string.
+		// If it's not, tell the user gracefully.
+		s, ok := v.(string)
+		if !ok {
+			return "", fmt.Errorf("unexpected type for variable %q: %T", k, v)
+		}
+		varmap[k] = ast.Variable{
+			Value: s,
+			Type:  ast.TypeString,
+		}
+	}
+
+	cfg := lang.EvalConfig{
+		GlobalScope: &ast.BasicScope{
+			VarMap:  varmap,
+			FuncMap: config.Funcs,
+		},
+	}
+
+	out, typ, err := lang.Eval(root, &cfg)
+	if err != nil {
+		return "", err
+	}
+	if typ != ast.TypeString {
+		return "", fmt.Errorf("unexpected output ast.Type: %v", typ)
+	}
+
+	return out.(string), nil
+}
+
+func hash(s string) string {
+	sha := sha256.Sum256([]byte(s))
+	return hex.EncodeToString(sha[:])[:20]
+}