lang: Add fileexists function

lang/funcs/filesystem.go

@@ -63,6 +63,49 @@ func MakeFileFunc(baseDir string, encBase64 bool) function.Function {
 	})
 }
 
+// MakeFileExistsFunc constructs a function that takes a path
+// and determines whether a file exists at that path
+func MakeFileExistsFunc(baseDir string) function.Function {
+	return function.New(&function.Spec{
+		Params: []function.Parameter{
+			{
+				Name: "path",
+				Type: cty.String,
+			},
+		},
+		Type: function.StaticReturnType(cty.Bool),
+		Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
+			path := args[0].AsString()
+			path, err := homedir.Expand(path)
+			if err != nil {
+				return cty.UnknownVal(cty.Bool), fmt.Errorf("failed to expand ~: %s", err)
+			}
+
+			if !filepath.IsAbs(path) {
+				path = filepath.Join(baseDir, path)
+			}
+
+			// Ensure that the path is canonical for the host OS
+			path = filepath.Clean(path)
+
+			fi, err := os.Stat(path)
+			if err != nil {
+				if os.IsNotExist(err) {
+					return cty.False, nil
+				}
+				return cty.UnknownVal(cty.Bool), fmt.Errorf("failed to stat %s", path)
+			}
+
+			if fi.Mode().IsRegular() {
+				return cty.True, nil
+			}
+
+			return cty.False, fmt.Errorf("%s is not a regular file, but %q",
+				path, fi.Mode().String())
+		},
+	})
+}
+
 // BasenameFunc constructs a function that takes a string containing a filesystem path
 // and removes all except the last portion from it.
 var BasenameFunc = function.New(&function.Spec{
@@ -121,6 +164,16 @@ func File(baseDir string, path cty.Value) (cty.Value, error) {
 	return fn.Call([]cty.Value{path})
 }
 
+// FileExists determines whether a file exists at the given path.
+//
+// The underlying function implementation works relative to a particular base
+// directory, so this wrapper takes a base directory string and uses it to
+// construct the underlying function before calling it.
+func FileExists(baseDir string, path cty.Value) (cty.Value, error) {
+	fn := MakeFileExistsFunc(baseDir)
+	return fn.Call([]cty.Value{path})
+}
+
 // FileBase64 reads the contents of the file at the given path.
 //
 // The bytes from the file are encoded as base64 before returning.

lang/funcs/filesystem_test.go

@@ -52,6 +52,49 @@ func TestFile(t *testing.T) {
 	}
 }
 
+func TestFileExists(t *testing.T) {
+	tests := []struct {
+		Path cty.Value
+		Want cty.Value
+		Err  bool
+	}{
+		{
+			cty.StringVal("testdata/hello.txt"),
+			cty.BoolVal(true),
+			false,
+		},
+		{
+			cty.StringVal(""), // empty path
+			cty.BoolVal(false),
+			true,
+		},
+		{
+			cty.StringVal("testdata/missing"),
+			cty.BoolVal(false),
+			false, // no file exists
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(fmt.Sprintf("FileExists(\".\", %#v)", test.Path), func(t *testing.T) {
+			got, err := FileExists(".", test.Path)
+
+			if test.Err {
+				if err == nil {
+					t.Fatal("succeeded; want error")
+				}
+				return
+			} else if err != nil {
+				t.Fatalf("unexpected error: %s", err)
+			}
+
+			if !got.RawEquals(test.Want) {
+				t.Errorf("wrong result\ngot:  %#v\nwant: %#v", got, test.Want)
+			}
+		})
+	}
+}
+
 func TestFileBase64(t *testing.T) {
 	tests := []struct {
 		Path cty.Value

lang/functions.go

@@ -53,6 +53,7 @@ func (s *Scope) Functions() map[string]function.Function {
 			"element":      funcs.ElementFunc,
 			"chunklist":    funcs.ChunklistFunc,
 			"file":         funcs.MakeFileFunc(s.BaseDir, false),
+			"fileexists":   funcs.MakeFileExistsFunc(s.BaseDir),
 			"filebase64":   funcs.MakeFileFunc(s.BaseDir, true),
 			"flatten":      funcs.FlattenFunc,
 			"floor":        funcs.FloorFunc,

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

@@ -42,3 +42,5 @@ Hello World
 * [`filebase64`](./filebase64.html) also reads the contents of a given file,
   but returns the raw bytes in that file Base64-encoded, rather than
   interpreting the contents as UTF-8 text.
+* [`fileexists`](./fileexists.html) determines whether a file exists
+  at a given path.

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

@@ -0,0 +1,37 @@
+---
+layout: "functions"
+page_title: "fileexists function"
+sidebar_current: "docs-funcs-file-file-exists"
+description: |-
+  The fileexists function determines whether a file exists at a given path.
+---
+
+# `fileexists` Function
+
+`fileexists` determines whether a file exists at a given path.
+
+```hcl
+fileexists(path)
+```
+
+Functions are evaluated during configuration parsing rather than at apply time,
+so this function can only be used with files that are already present on disk
+before Terraform takes any actions.
+
+This function works only with regular files. If used with a directory, FIFO,
+or other special mode, it will return an error.
+
+## Examples
+
+```
+> fileexists("${path.module}/hello.txt")
+true
+```
+
+```hcl
+fileexists("custom-section.sh") ? file("custom-section.sh") : local.default_content
+```
+
+## Related Functions
+
+* [`file`](./file.html) reads the contents of a file at a given path

website/layouts/functions.erb

@@ -249,6 +249,10 @@
                 <a href="/docs/configuration/functions/file.html">file</a>
               </li>
 
+              <li<%= sidebar_current("docs-funcs-file-file-exists") %>>
+                <a href="/docs/configuration/functions/fileexists.html">fileexists</a>
+              </li>
+
               <li<%= sidebar_current("docs-funcs-file-filebase64") %>>
                 <a href="/docs/configuration/functions/filebase64.html">filebase64</a>
               </li>