templater: fail rendering if a variable is not set

previously the default behaviour of text/template was used which
would print the string "<no value>", this could lead to a situation
where an job file appears valid but would fail to run

levant/templater.go

@@ -85,7 +85,7 @@ func renderTFTemplte(src, variableFile string, flagVars *map[string]string) (tpl
 	mergedVars := helper.VariableMerge(&variables, flagVars)
 
 	// Setup the template file for rendering
-	t := template.New("jobTemplate").Delims("[[", "]]")
+	t := newTemplate()
 
 	if t, err = t.Parse(src); err != nil {
 		return
@@ -117,7 +117,7 @@ func renderYAMLVarsTemplate(src, variableFile string, flagVars *map[string]strin
 	mergedVars := helper.VariableMerge(&variables, flagVars)
 
 	// Setup the template file for rendering
-	t := template.New("jobTemplate").Delims("[[", "]]")
+	t := newTemplate()
 	if t, err = t.Parse(string(src)); err != nil {
 		return
 	}
@@ -134,7 +134,7 @@ func readJobFile(src string, flagVars *map[string]string) (tpl *bytes.Buffer, er
 	tpl = &bytes.Buffer{}
 
 	// Setup the template file for rendering
-	t := template.New("jobTemplate").Delims("[[", "]]")
+	t := newTemplate()
 	if t, err = t.Parse(src); err != nil {
 		return
 	}
@@ -145,3 +145,8 @@ func readJobFile(src string, flagVars *map[string]string) (tpl *bytes.Buffer, er
 
 	return tpl, nil
 }
+
+// newTemplate returns an empty template with default options set
+func newTemplate() *template.Template {
+	return template.New("jobTemplate").Delims("[[", "]]").Option("missingkey=error")
+}

levant/templater_test.go

@@ -1,6 +1,7 @@
 package levant
 
 import (
+	"strings"
 	"testing"
 
 	nomad "github.com/hashicorp/nomad/api"
@@ -69,4 +70,14 @@ func TestTemplater_RenderTemplate(t *testing.T) {
 	if job.Datacenters[0] != testDCName {
 		t.Fatalf("expected %s but got %v", testDCName, job.Datacenters[0])
 	}
+
+	// Test var-args only render.
+	fVars["job_name"] = testJobName
+	job, err = RenderTemplate("test-fixtures/missing_var.nomad", "", &fVars)
+	if err == nil {
+		t.Fatal("expected err to not be nil")
+	}
+	if !strings.Contains(err.Error(), "binary_url") {
+		t.Fatal("expected err to mention missing var (binary_url)")
+	}
 }

levant/test-fixtures/missing_var.nomad

@@ -0,0 +1,56 @@
+job "[[.job_name]]" {
+  datacenters = ["dc1"]
+  type = "service"
+  update {
+    max_parallel     = 1
+    min_healthy_time = "10s"
+    healthy_deadline = "1m"
+    auto_revert      = true
+  }
+
+  group "cache" {
+    count = 1
+    restart {
+      attempts = 10
+      interval = "5m"
+      delay = "25s"
+      mode = "delay"
+    }
+    ephemeral_disk {
+      size = 300
+    }
+    task "redis" {
+      artifact {
+        # .binary_url is not set
+        source = "[[ .binary_url ]]"
+      }
+
+      driver = "docker"
+      config {
+        image = "redis:3.2"
+        port_map {
+          db = 6379
+        }
+      }
+      resources {
+        cpu    = 500
+        memory = 256
+        network {
+          mbits = 10
+          port "db" {}
+        }
+      }
+      service {
+        name = "global-redis-check"
+        tags = ["global", "cache"]
+        port = "db"
+        check {
+          name     = "alive"
+          type     = "tcp"
+          interval = "10s"
+          timeout  = "2s"
+        }
+      }
+    }
+  }
+}