Introduce template function `requiredEnv`

The new template function `requiredEnv` ensures that the given env var is set and not empty.

README.md

@@ -52,9 +52,11 @@ releases:
       - name: address
         value: https://vault.example.com
       - name: db.password
-        value: {{ env "DB_PASSWORD" }}    # value taken from environment variable. Quotes are necessary. Will throw an error if the environment variable is not set. $DB_PASSWORD needs to be set in the calling environment ex: export DB_PASSWORD='password1'
+        value: {{ requiredEnv "DB_PASSWORD" }}    # value taken from environment variable. Quotes are necessary. Will throw an error if the environment variable is not set. $DB_PASSWORD needs to be set in the calling environment ex: export DB_PASSWORD='password1'
       - name: proxy.domain
-        value: {{ env "PLATFORM_ID" }}.my-domain.com # Interpolate environment variable with a fixed string
+        value: {{ requiredEnv "PLATFORM_ID" }}.my-domain.com # Interpolate environment variable with a fixed string
+      - name: proxy.scheme
+        value: {{ env "SCHEME" | default "https" }}
 
   # Local chart example
   - name: grafana                            # name of this release
@@ -62,10 +64,18 @@ releases:
     chart: ../my-charts/grafana              # the chart being installed to create this release, referenced by relative path to local chart
     values:
     - "../../my-values/grafana/values.yaml"             # Values file (relative path to manifest)
-    - ./values/{{ env "PLATFORM_ENV" }}/config.yaml # Values file taken from path with environment variable. $PLATFORM_ENV must be set in the calling environment.
+    - ./values/{{ requiredEnv "PLATFORM_ENV" }}/config.yaml # Values file taken from path with environment variable. $PLATFORM_ENV must be set in the calling environment.
 
 ```
 
+## Templating
+
+Helmfile uses [Go templates](https://godoc.org/text/template) for templating your helmfile.yaml. While go ships several built-in functions, we have added all of the functions in the [Sprig library](https://godoc.org/github.com/Masterminds/sprig).
+
+We also added one special template function: `requiredEnv`.  
+The `required_env` function allows you to declare a particular environment variable as required for template rendering.  
+If the environment variable is unset or empty, the template rendering will fail with an error message.
+
 ## Using environment variables
 
 Environment variables can be used in most places for templating the helmfile. Currently this is supported for `name`, `namespace`, `value` (in set) and `url` (in repositories).
@@ -75,17 +85,19 @@ Examples:
 ```yaml
 respositories:
 - name: your-private-git-repo-hosted-charts
-  url: https://{{ env "GITHUB_TOKEN"}}@raw.githubusercontent.com/kmzfs/helm-repo-in-github/master/
+  url: https://{{ requiredEnv "GITHUB_TOKEN"}}@raw.githubusercontent.com/kmzfs/helm-repo-in-github/master/
 ```
 
 ```yaml
 releases:
-  - name: {{ env "NAME" }}-vault
+  - name: {{ requiredEnv "NAME" }}-vault
-    namespace: {{ env "NAME" }}
+    namespace: {{ requiredEnv "NAME" }}
     chart: roboll/vault-secret-manager
     set:
       - name: proxy.domain
-        value: {{ env "PLATFORM_ID" }}.my-domain.com
+        value: {{ requiredEnv "PLATFORM_ID" }}.my-domain.com
+      - name: proxy.scheme
+        value: {{ env "SCHEME" | default "https" }}
 ```
 
 ## installation

state/state.go

@@ -107,7 +107,21 @@ func readFromYaml(content []byte, file string) (*HelmState, error) {
 }
 
 func stringTemplate() *template.Template {
-	return template.New("stringTemplate").Funcs(sprig.TxtFuncMap())
+	funcMap := sprig.TxtFuncMap()
+	alterFuncMap(&funcMap)
+	return template.New("stringTemplate").Funcs(funcMap)
+}
+
+func alterFuncMap(funcMap *template.FuncMap) {
+	(*funcMap)["requiredEnv"] = getRequiredEnv
+}
+
+func getRequiredEnv(name string) (string, error) {
+	if val, exists := os.LookupEnv(name); exists && len(val) > 0 {
+		return val, nil
+	}
+
+	return "", fmt.Errorf("required env var `%s` is not set", name)
 }
 
 func renderTemplateString(s string) (string, error) {

state/state_test.go

@@ -351,6 +351,25 @@ func Test_renderTemplateString(t *testing.T) {
 			},
 			wantErr: true,
 		},
+		{
+			name: "required env var",
+			args: args{
+				s: "{{ requiredEnv \"HF_TEST\" }}",
+				envs: map[string]string{
+					"HF_TEST": "value",
+				},
+			},
+			want:    "value",
+			wantErr: false,
+		},
+		{
+			name: "required env var not set",
+			args: args{
+				s:    "{{ requiredEnv \"HF_TEST_NONE\" }}",
+				envs: map[string]string{},
+			},
+			wantErr: true,
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {