hcl2: avoid panic on unset variable

Variables that are unset return the correct diagnostic but throw a panic when
we later parse the job body. Return early if there are any variable parsing
errors instead of continuing in a potentially invalid state.

jobspec2/parse_test.go

@@ -196,6 +196,28 @@ job "example" {
 	require.Equal(t, meta, out.Meta)
 }
 
+// TestParse_UnsetVariables asserts that variables that have neither types nor
+// values return early instead of panicking.
+func TestParse_UnsetVariables(t *testing.T) {
+	hcl := `
+variable "region_var" {}
+job "example" {
+  datacenters = [for s in ["dc1", "dc2"] : upper(s)]
+  region      = var.region_var
+}
+`
+
+	_, err := ParseWithConfig(&ParseConfig{
+		Path:    "input.hcl",
+		Body:    []byte(hcl),
+		ArgVars: []string{},
+		AllowFS: true,
+	})
+
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "Unset variable")
+}
+
 func TestParse_Locals(t *testing.T) {
 	hcl := `
 variables {

jobspec2/types.config.go

@@ -71,6 +71,13 @@ func (c *jobConfig) decodeBody(body hcl.Body) hcl.Diagnostics {
 	diags = append(diags, moreDiags...)
 	diags = append(diags, c.evaluateLocalVariables(c.LocalBlocks)...)
 
+	// Errors at this point are likely syntax errors which can result in
+	// invalid state when we try to decode the rest of the job. If we continue
+	// we may panic and that will obscure the error, so return early so the
+	// user can be told how to fix their jobspec.
+	if diags.HasErrors() {
+		return diags
+	}
 	nctx := c.EvalContext()
 
 	diags = append(diags, c.decodeJob(content, nctx)...)