core: Add uuid() interpolate function.

Utilizes hashicorp's go-uuid library for proper random seeding setup.

config/interpolate_funcs.go

@@ -17,6 +17,7 @@ import (
 	"strings"
 
 	"github.com/apparentlymart/go-cidr/cidr"
+	"github.com/hashicorp/go-uuid"
 	"github.com/hashicorp/hil/ast"
 	"github.com/mitchellh/go-homedir"
 )
@@ -42,6 +43,7 @@ func Funcs() map[string]ast.Function {
 		"length":       interpolationFuncLength(),
 		"lower":        interpolationFuncLower(),
 		"md5":          interpolationFuncMd5(),
+		"uuid":         interpolationFuncUUID(),
 		"replace":      interpolationFuncReplace(),
 		"sha1":         interpolationFuncSha1(),
 		"sha256":       interpolationFuncSha256(),
@@ -682,3 +684,13 @@ func interpolationFuncBase64Sha256() ast.Function {
 		},
 	}
 }
+
+func interpolationFuncUUID() ast.Function {
+	return ast.Function{
+		ArgTypes:   []ast.Type{},
+		ReturnType: ast.TypeString,
+		Callback: func(args []interface{}) (interface{}, error) {
+			return uuid.GenerateUUID()
+		},
+	}
+}

config/interpolate_funcs_test.go

@@ -945,6 +945,28 @@ func TestInterpolateFuncMd5(t *testing.T) {
 	})
 }
 
+func TestInterpolateFuncUUID(t *testing.T) {
+	results := make(map[string]bool)
+
+	for i := 0; i < 100; i++ {
+		ast, err := hil.Parse("${uuid()}")
+		if err != nil {
+			t.Fatalf("err: %s", err)
+		}
+
+		out, _, err := hil.Eval(ast, langEvalConfig(nil))
+		if err != nil {
+			t.Fatalf("err: %s", err)
+		}
+
+		if results[out.(string)] {
+			t.Fatalf("Got unexpected duplicate uuid: %s", out)
+		}
+
+		results[out.(string)] = true
+	}
+}
+
 type testFunctionConfig struct {
 	Cases []testFunctionCase
 	Vars  map[string]ast.Variable

website/source/docs/configuration/interpolation.html.md

@@ -198,6 +198,8 @@ The supported built-in functions are:
 
   * `upper(string)` - Returns a copy of the string with all Unicode letters mapped to their upper case.
 
+  * `uuid()` - Returns a UUID string in RFC 4122 v4 format. This string will change with every invocation of the function, so in order to prevent diffs on every plan & apply, it must be used with the [`ignore_changes`](/docs/configuration/resources.html#ignore-changes) lifecycle attribute.
+
 ## Templates
 
 Long strings can be managed using templates. [Templates](/docs/providers/template/index.html) are [resources](/docs/configuration/resources.html) defined by a filename and some variables to use during interpolation. They have a computed `rendered` attribute containing the result.

website/source/docs/configuration/resources.html.md

@@ -68,6 +68,8 @@ The `lifecycle` block allows the following keys to be set:
       destruction of a given resource. When this is set to `true`, any plan
       that includes a destroy of this resource will return an error message.
 
+<a id="ignore-changes"></a>
+
   * `ignore_changes` (list of strings) - Customizes how diffs are evaluated for
       resources, allowing individual attributes to be ignored through changes.
       As an example, this can be used to ignore dynamic changes to the