fix: throw error on missing import (#1725)

src/root.js

@@ -94,12 +94,19 @@ Root.prototype.load = function load(filename, options, callback) {
     var sync = callback === SYNC; // undocumented
 
     // Finishes loading by calling the callback (exactly once)
+    var throwing = null;
     function finish(err, root) {
+        // If we get here, and we are throwing then something must have
+        // incorrectly caught the error, so we need to rethrow it.
+        if (throwing)
+            throw throwing;
         /* istanbul ignore if */
         if (!callback)
             return;
-        if (sync)
+        if (sync) {
+            throwing = err;
             throw err;
+        }
         var cb = callback;
         callback = null;
         cb(err, root);

tests/data/missing-import.proto

@@ -0,0 +1,7 @@
+syntax = "proto3";
+
+import "no/such/file.proto";
+
+message Something {
+  ImportedType field = 1;
+}

tests/node/api_load-sync.js

@@ -46,3 +46,17 @@ tape.test("should load bundled definitions even if resolvePath method was overri
     test.ok(root.lookup("Something"), "should parse message Something");
     test.end();
 });
+
+tape.test("should raise error if import is missing", function(test) {
+    var protoFilePath = "tests/data/missing-import.proto";
+    var root = new protobuf.Root();
+
+    test.throws(
+        () => {
+            root.loadSync(protoFilePath);
+        },
+        /ENOENT: no such file or directory, open 'tests\/data\/no\/such\/file.proto'/,
+        "should throw an error with the path to the missing file"
+    );
+    test.end();
+});