core: Add pathexpand interpolation function

Adds the `pathexpand` interpolation function to allow users to expand `~` to the home directory in filepath strings.

config/interpolate_funcs.go

@@ -79,6 +79,7 @@ func Funcs() map[string]ast.Function {
 		"md5":          interpolationFuncMd5(),
 		"merge":        interpolationFuncMerge(),
 		"min":          interpolationFuncMin(),
+		"pathexpand":   interpolationFuncPathExpand(),
 		"uuid":         interpolationFuncUUID(),
 		"replace":      interpolationFuncReplace(),
 		"sha1":         interpolationFuncSha1(),
@@ -431,6 +432,17 @@ func interpolationFuncMin() ast.Function {
 	}
 }
 
+// interpolationFuncPathExpand will expand any `~`'s found with the full file path
+func interpolationFuncPathExpand() ast.Function {
+	return ast.Function{
+		ArgTypes:   []ast.Type{ast.TypeString},
+		ReturnType: ast.TypeString,
+		Callback: func(args []interface{}) (interface{}, error) {
+			return homedir.Expand(args[0].(string))
+		},
+	}
+}
+
 // interpolationFuncCeil returns the the least integer value greater than or equal to the argument
 func interpolationFuncCeil() ast.Function {
 	return ast.Function{

config/interpolate_funcs_test.go

@@ -8,8 +8,11 @@ import (
 	"testing"
 	"time"
 
+	"path/filepath"
+
 	"github.com/hashicorp/hil"
 	"github.com/hashicorp/hil/ast"
+	"github.com/mitchellh/go-homedir"
 )
 
 func TestInterpolateFuncZipMap(t *testing.T) {
@@ -1972,3 +1975,39 @@ func testFunction(t *testing.T, config testFunctionConfig) {
 		}
 	}
 }
+
+func TestInterpolateFuncPathExpand(t *testing.T) {
+	homePath, err := homedir.Dir()
+	if err != nil {
+		t.Fatalf("Error getting home directory: %v", err)
+	}
+	testFunction(t, testFunctionConfig{
+		Cases: []testFunctionCase{
+			{
+				`${pathexpand("~/test-file")}`,
+				filepath.Join(homePath, "test-file"),
+				false,
+			},
+			{
+				`${pathexpand("~/another/test/file")}`,
+				filepath.Join(homePath, "another/test/file"),
+				false,
+			},
+			{
+				`${pathexpand("/root/file")}`,
+				"/root/file",
+				false,
+			},
+			{
+				`${pathexpand("/")}`,
+				"/",
+				false,
+			},
+			{
+				`${pathexpand()}`,
+				nil,
+				true,
+			},
+		},
+	})
+}

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

@@ -273,6 +273,8 @@ The supported built-in functions are:
   * `md5(string)` - Returns a (conventional) hexadecimal representation of the
     MD5 hash of the given string.
 
+  * `pathexpand(string)` - Returns a filepath string with `~` expanded to the home directory.
+  
   * `replace(string, search, replace)` - Does a search and replace on the
       given string. All instances of `search` are replaced with the value
       of `replace`. If `search` is wrapped in forward slashes, it is treated