lang/funcs: add "abspath" function

lang/funcs/filesystem.go

@@ -237,6 +237,21 @@ var DirnameFunc = function.New(&function.Spec{
 	},
 })
 
+// AbsPathFunc constructs a function that converts a filesystem path to an absolute path
+var AbsPathFunc = function.New(&function.Spec{
+	Params: []function.Parameter{
+		{
+			Name: "path",
+			Type: cty.String,
+		},
+	},
+	Type: function.StaticReturnType(cty.String),
+	Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
+		absPath, err := filepath.Abs(args[0].AsString())
+		return cty.StringVal(filepath.ToSlash(absPath)), err
+	},
+})
+
 // PathExpandFunc constructs a function that expands a leading ~ character to the current user's home directory.
 var PathExpandFunc = function.New(&function.Spec{
 	Params: []function.Parameter{

lang/functions.go

@@ -30,6 +30,7 @@ func (s *Scope) Functions() map[string]function.Function {
 
 		s.funcs = map[string]function.Function{
 			"abs":              stdlib.AbsoluteFunc,
+			"abspath":          funcs.AbsPathFunc,
 			"basename":         funcs.BasenameFunc,
 			"base64decode":     funcs.Base64DecodeFunc,
 			"base64encode":     funcs.Base64EncodeFunc,

lang/functions_test.go

@@ -2,6 +2,7 @@ package lang
 
 import (
 	"fmt"
+	"os"
 	"path/filepath"
 	"testing"
 
@@ -53,6 +54,19 @@ func TestFunctions(t *testing.T) {
 			},
 		},
 
+		"abspath": {
+			{
+				`abspath(".")`,
+				cty.StringVal((func() string {
+					cwd, err := os.Getwd()
+					if err != nil {
+						panic(err)
+					}
+					return cwd
+				})()),
+			},
+		},
+
 		"base64decode": {
 			{
 				`base64decode("YWJjMTIzIT8kKiYoKSctPUB+")`,

website/docs/configuration/functions/abspath.html.md

@@ -0,0 +1,30 @@
+---
+layout: "functions"
+page_title: "abspath - Functions - Configuration Language"
+sidebar_current: "docs-funcs-file-abspath"
+description: |-
+  The abspath function converts the argument to an absolute filesystem path.
+---
+
+# `abspath` Function
+
+-> **Note:** This page is about Terraform 0.12 and later. For Terraform 0.11 and
+earlier, see
+[0.11 Configuration Language: Interpolation Syntax](../../configuration-0-11/interpolation.html).
+
+`abspath` takes a string containing a filesystem path and converts it
+to an absolute path. That is, if the path is not absolute, it will be joined
+with the current working directory.
+
+Referring directly to filesystem paths in resource arguments may cause
+spurious diffs if the same configuration is applied from multiple systems or on
+different host operating systems. We recommend using filesystem paths only
+for transient values, such as the argument to [`file`](./file.html) (where
+only the contents are then stored) or in `connection` and `provisioner` blocks.
+
+## Examples
+
+```
+> abspath(path.root)
+/home/user/some/terraform/root
+```

website/layouts/functions.erb

@@ -260,6 +260,10 @@
             <a href="#docs-funcs-file">Filesystem Functions</a>
             <ul class="nav">
 
+              <li>
+                <a href="/docs/configuration/functions/abspath.html">abspath</a>
+              </li>
+
               <li>
                 <a href="/docs/configuration/functions/dirname.html">dirname</a>
               </li>