Add the timestamp interpolation function.

Adds the timestamp interpolation function, tests, and documentation to allow a user to insert an RFC 3339 formatted UTC timestamp.

config/interpolate_funcs.go

@@ -21,6 +21,7 @@ import (
 	"github.com/hashicorp/hil"
 	"github.com/hashicorp/hil/ast"
 	"github.com/mitchellh/go-homedir"
+	"time"
 )
 
 // stringSliceToVariableValue converts a string slice into the value
@@ -85,6 +86,7 @@ func Funcs() map[string]ast.Function {
 		"signum":       interpolationFuncSignum(),
 		"sort":         interpolationFuncSort(),
 		"split":        interpolationFuncSplit(),
+		"timestamp":    interpolationFuncTimestamp(),
 		"title":        interpolationFuncTitle(),
 		"trimspace":    interpolationFuncTrimSpace(),
 		"upper":        interpolationFuncUpper(),
@@ -1109,6 +1111,17 @@ func interpolationFuncUUID() ast.Function {
 	}
 }
 
+// interpolationFuncTimestamp
+func interpolationFuncTimestamp() ast.Function {
+	return ast.Function{
+		ArgTypes:   []ast.Type{},
+		ReturnType: ast.TypeString,
+		Callback: func(args []interface{}) (interface{}, error) {
+			return time.Now().UTC().Format(time.RFC3339), nil
+		},
+	}
+}
+
 // interpolationFuncTitle implements the "title" function that returns a copy of the
 // string in which first characters of all the words are capitalized.
 func interpolationFuncTitle() ast.Function {

config/interpolate_funcs_test.go

@@ -9,6 +9,7 @@ import (
 
 	"github.com/hashicorp/hil"
 	"github.com/hashicorp/hil/ast"
+	"time"
 )
 
 func TestInterpolateFuncZipMap(t *testing.T) {
@@ -1921,6 +1922,27 @@ func TestInterpolateFuncUUID(t *testing.T) {
 	}
 }
 
+func TestInterpolateFuncTimestamp(t *testing.T) {
+	currentTime := time.Now().UTC()
+	ast, err := hil.Parse("${timestamp()}")
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	result, err := hil.Eval(ast, langEvalConfig(nil))
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+	resultTime, err := time.Parse(time.RFC3339, result.Value.(string))
+	if err != nil {
+		t.Fatalf("Error parsing timestamp: %s", err)
+	}
+
+	if resultTime.Sub(currentTime).Seconds() > 10.0 {
+		t.Fatalf("Timestamp Diff too large. Expected: %s\nRecieved: %s", currentTime.Format(time.RFC3339), result.Value.(string))
+	}
+}
+
 type testFunctionConfig struct {
 	Cases []testFunctionCase
 	Vars  map[string]ast.Variable

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

@@ -267,6 +267,10 @@ The supported built-in functions are:
       `a_resource_param = ["${split(",", var.CSV_STRING)}"]`.
       Example: `split(",", module.amod.server_ids)`
       
+  * `timestamp()` - Returns a UTC timestamp string in RFC 3339 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.
+
   * `title(string)` - Returns a copy of the string with the first characters of all the words capitalized.
 
   * `trimspace(string)` - Returns a copy of the string with all leading and trailing white spaces removed.