Adding datasourceExists function

README.md

@@ -55,6 +55,7 @@ Gomplate is an alternative that will let you process templates which also includ
 					- [Basic usage](#basic-usage)
 					- [Usage with HTTP data](#usage-with-http-data)
 					- [Usage with Vault data](#usage-with-vault-data)
+			- [`datasourceExists`](#datasourceexists)
 			- [`ec2meta`](#ec2meta)
 				- [Example](#example)
 			- [`ec2dynamic`](#ec2dynamic)
@@ -368,6 +369,22 @@ $ echo 'db_password={{(datasource "vault" "db/pass").value}}' \
 db_password=prodsecret
 ```
 
+#### `datasourceExists`
+
+Tests whether or not a given datasource was defined on the commandline (with the
+[`--datasource/-d`](#--datasource-d) argument). This is intended mainly to allow
+a template to be rendered differently whether or not a given datasource was
+defined.
+
+Note: this does _not_ verify if the datasource is reachable.
+
+Useful when used in an `if`/`else` block
+
+```console
+$ echo '{{if (datasourceExists "test")}}{{datasource "test"}}{{else}}no worries{{end}}' | gomplate
+no worries
+```
+
 #### `ec2meta`
 
 Queries AWS [EC2 Instance Metadata](http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-instance-metadata.html) for information. This only retrieves data in the `meta-data` path -- for data in the `dynamic` path use `ec2dynamic`.

data.go

@@ -152,9 +152,18 @@ func absURL(value string) *url.URL {
 	return baseURL.ResolveReference(relURL)
 }
 
+// DatasourceExists -
+func (d *Data) DatasourceExists(alias string) bool {
+	_, ok := d.Sources[alias]
+	return ok
+}
+
 // Datasource -
 func (d *Data) Datasource(alias string, args ...string) map[string]interface{} {
-	source := d.Sources[alias]
+	source, ok := d.Sources[alias]
+	if !ok {
+		log.Fatalf("Undefined datasource '%s'", alias)
+	}
 	b, err := d.ReadSource(source.FS, source, args...)
 	if err != nil {
 		log.Fatalf("Couldn't read datasource '%s': %s", alias, err)

data_test.go

@@ -101,6 +101,15 @@ func TestDatasource(t *testing.T) {
 	test("yml", "application/yaml", `hello: world`)
 }
 
+func TestDatasourceExists(t *testing.T) {
+	sources := map[string]*Source{
+		"foo": {Alias: "foo"},
+	}
+	data := &Data{Sources: sources}
+	assert.True(t, data.DatasourceExists("foo"))
+	assert.False(t, data.DatasourceExists("bar"))
+}
+
 func setupHTTP(code int, mimetype string, body string) (*httptest.Server, *http.Client) {
 	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		w.Header().Set("Content-Type", mimetype)

main.go

@@ -48,22 +48,23 @@ func NewGomplate(data *Data) *Gomplate {
 	ec2info := aws.NewEc2Info()
 	return &Gomplate{
 		funcMap: template.FuncMap{
-			"getenv":     env.Getenv,
-			"bool":       typeconv.Bool,
-			"json":       typeconv.JSON,
-			"jsonArray":  typeconv.JSONArray,
-			"yaml":       typeconv.YAML,
-			"yamlArray":  typeconv.YAMLArray,
-			"slice":      typeconv.Slice,
-			"join":       typeconv.Join,
-			"ec2meta":    ec2meta.Meta,
-			"ec2dynamic": ec2meta.Dynamic,
-			"ec2tag":     ec2info.Tag,
-			"ec2region":  ec2meta.Region,
-			"title":      strings.Title,
-			"toUpper":    strings.ToUpper,
-			"toLower":    strings.ToLower,
-			"datasource": data.Datasource,
+			"getenv":           env.Getenv,
+			"bool":             typeconv.Bool,
+			"json":             typeconv.JSON,
+			"jsonArray":        typeconv.JSONArray,
+			"yaml":             typeconv.YAML,
+			"yamlArray":        typeconv.YAMLArray,
+			"slice":            typeconv.Slice,
+			"join":             typeconv.Join,
+			"ec2meta":          ec2meta.Meta,
+			"ec2dynamic":       ec2meta.Dynamic,
+			"ec2tag":           ec2info.Tag,
+			"ec2region":        ec2meta.Region,
+			"title":            strings.Title,
+			"toUpper":          strings.ToUpper,
+			"toLower":          strings.ToLower,
+			"datasource":       data.Datasource,
+			"datasourceExists": data.DatasourceExists,
 		},
 	}
 }