add mustEnv function

docs/templating-language.md

@@ -47,6 +47,7 @@ provides the following functions:
   - [containsNone](#containsnone)
   - [containsNotAll](#containsnotall)
   - [env](#env)
+  - [mustEnv](#mustEnv)
   - [envOrDefault](#envOrDefault)
   - [executeTemplate](#executetemplate)
   - [explode](#explode)
@@ -60,7 +61,7 @@ provides the following functions:
   - [trimSpace](#trimspace)
   - [trim](#trim)
   - [trimPrefix](#trimprefix)
-  - [trimSuffix](#trimsuffix)  
+  - [trimSuffix](#trimsuffix)
   - [parseBool](#parsebool)
   - [parseFloat](#parsefloat)
   - [parseInt](#parseint)
@@ -1140,6 +1141,14 @@ Reads the given environment variable and if it does not exist or is blank use a
 {{ or (env "CLUSTER_ID") "12345" }}
 ```
 
+### `mustEnv`
+
+Reads the given environment variable accessible to the current process. If the variable is not defined or is empty the substitution will fail with error.
+
+```golang
+{{ mustEnv "CLUSTER_ID" }}
+```
+
 ### `envOrDefault`
 
 Reads the given environment variable accessible to the current process. If the environment variable is found, the value of that variable will be used. This includes empty values. Otherwise, the default will be used instead.

template/funcs.go

@@ -84,6 +84,31 @@ func envFunc(env []string) func(string) (string, error) {
 	}
 }
 
+// mustEnvFunc returns a function which checks the value of an environment variable
+// and returns an error if environment variable value is empty.
+// Invokers can specify their own environment, which takes precedences over any
+// real environment variables.
+func mustEnvFunc(env []string) func(string) (string, error) {
+	return func(s string) (string, error) {
+		val := ""
+		for _, e := range env {
+			split := strings.SplitN(e, "=", 2)
+			k, v := split[0], split[1]
+			if k == s {
+				val = v
+				break
+			}
+		}
+		if val == "" {
+			val = os.Getenv(s)
+		}
+		if val == "" {
+			return "", fmt.Errorf("required environment variable %s is empty", s)
+		}
+		return val, nil
+	}
+}
+
 // envWithDefaultFunc returns a function which checks the value of an environment variable.
 // Invokers can specify their own environment, which takes precedences over any
 // real environment variables.

template/template.go

@@ -317,6 +317,7 @@ func funcMap(i *funcMapInput) template.FuncMap {
 		"containsNone":          containsSomeFunc(true, false),
 		"containsNotAll":        containsSomeFunc(false, true),
 		"env":                   envFunc(i.env),
+		"mustEnv":               mustEnvFunc(i.env),
 		"envOrDefault":          envWithDefaultFunc(i.env),
 		"executeTemplate":       executeTemplateFunc(i.newTmpl),
 		"explode":               explode,

template/template_test.go

@@ -1183,6 +1183,34 @@ func TestTemplate_Execute(t *testing.T) {
 			"1",
 			false,
 		},
+		{
+			"helper_mustEnv",
+			&NewTemplateInput{
+				Contents: `{{ mustEnv "CT_TEST" }}`,
+			},
+			&ExecuteInput{
+				Brain: func() *Brain {
+					// Cheat and use the brain callback here to set the env.
+					if err := os.Setenv("CT_TEST", "1"); err != nil {
+						t.Fatal(err)
+					}
+					return NewBrain()
+				}(),
+			},
+			"1",
+			false,
+		},
+		{
+			"helper_mustEnv_negative",
+			&NewTemplateInput{
+				Contents: `{{ mustEnv "CT_TEST_NONEXISTENT" }}`,
+			},
+			&ExecuteInput{
+				Brain:  NewBrain(),
+			},
+			"",
+			true,
+		},
 		{
 			"helper_env__override",
 			&NewTemplateInput{