provider/template: fix race causing panic in template_file

The render code path in `template_file` was doing unsynchronized access
to a shared mapping of functions in `config.Func`.

This caused a race condition that was most often triggered when a
`template_file` had a `count` of more than one, and expressed itself as
a panic in the plugin followed by a cascade of "unexpected EOF" errors
through the plugin system.

Here, we simply turn the FuncMap from shared state into a generated
value, which avoids the race. We do more re-initialization of the data
structure, but the performance implications are minimal, and we can
always revisit with a perf pass later now that the race is fixed.

builtin/providers/template/resource_template_file.go

@@ -155,7 +155,7 @@ func execute(s string, vars map[string]interface{}) (string, error) {
 	cfg := lang.EvalConfig{
 		GlobalScope: &ast.BasicScope{
 			VarMap:  varmap,
-			FuncMap: config.Funcs,
+			FuncMap: config.Funcs(),
 		},
 	}
 

builtin/providers/template/resource_template_file_test.go

@@ -2,6 +2,7 @@ package template
 
 import (
 	"fmt"
+	"sync"
 	"testing"
 
 	r "github.com/hashicorp/terraform/helper/resource"
@@ -76,6 +77,29 @@ func TestTemplateVariableChange(t *testing.T) {
 	})
 }
 
+// This test covers a panic due to config.Func formerly being a
+// shared map, causing multiple template_file resources to try and
+// accessing it parallel during their lang.Eval() runs.
+//
+// Before fix, test fails under `go test -race`
+func TestTemplateSharedMemoryRace(t *testing.T) {
+	var wg sync.WaitGroup
+	for i := 0; i < 100; i++ {
+		go func(wg sync.WaitGroup, t *testing.T, i int) {
+			wg.Add(1)
+			out, err := execute("don't panic!", map[string]interface{}{})
+			if err != nil {
+				t.Fatalf("err: %s", err)
+			}
+			if out != "don't panic!" {
+				t.Fatalf("bad output: %s", out)
+			}
+			wg.Done()
+		}(wg, t, i)
+	}
+	wg.Wait()
+}
+
 func testTemplateConfig(template, vars string) string {
 	return fmt.Sprintf(`
 		resource "template_file" "t0" {

config/interpolate_funcs.go

@@ -20,10 +20,8 @@ import (
 )
 
 // Funcs is the mapping of built-in functions for configuration.
-var Funcs map[string]ast.Function
-
-func init() {
-	Funcs = map[string]ast.Function{
+func Funcs() map[string]ast.Function {
+	return map[string]ast.Function{
 		"cidrhost":     interpolationFuncCidrHost(),
 		"cidrnetmask":  interpolationFuncCidrNetmask(),
 		"cidrsubnet":   interpolationFuncCidrSubnet(),

config/raw_config.go

@@ -300,7 +300,7 @@ type gobRawConfig struct {
 // langEvalConfig returns the evaluation configuration we use to execute.
 func langEvalConfig(vs map[string]ast.Variable) *lang.EvalConfig {
 	funcMap := make(map[string]ast.Function)
-	for k, v := range Funcs {
+	for k, v := range Funcs() {
 		funcMap[k] = v
 	}
 	funcMap["lookup"] = interpolationFuncLookup(vs)