moc: error out when the Q_PLUGIN_METADATA file can't be opened

Task-number: QTBUG-56045
Change-Id: Ib058791036a2728dcd6215009a4ff206278bed14
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/tools/moc/moc.cpp

@@ -1294,7 +1294,12 @@ void Moc::parsePluginData(ClassDef *def)
                 return;
             }
             QFile file(fi.canonicalFilePath());
-            file.open(QFile::ReadOnly);
+            if (!file.open(QFile::ReadOnly)) {
+                QByteArray msg = "Plugin Metadata file " + lexem() + " could not be opened: "
+                    + file.errorString().toUtf8();
+                error(msg.constData());
+                return;
+            }
             metaData = file.readAll();
         }
     }

tests/auto/tools/moc/tst_moc.cpp

@@ -2145,6 +2145,22 @@ void tst_Moc::warnings_data()
         << 0
         << QString()
         << QString("standard input:1: Note: No relevant classes found. No output generated.");
+
+    QTest::newRow("Q_PLUGIN_METADATA: invalid file")
+        << QByteArray("class X { \n Q_PLUGIN_METADATA(FILE \"does.not.exists\") \n };")
+        << QStringList()
+        << 1
+        << QString()
+        << QString("standard input:2: Error: Plugin Metadata file \"does.not.exists\" does not exist. Declaration will be ignored");
+
+#ifdef Q_OS_LINUX  // Limit to Linux because the error message is platform-dependent
+    QTest::newRow("Q_PLUGIN_METADATA: unreadable file")
+        << QByteArray("class X { \n Q_PLUGIN_METADATA(FILE \".\") \n };")
+        << QStringList()
+        << 1
+        << QString()
+        << QString("standard input:2: Error: Plugin Metadata file \".\" could not be opened: file to open is a directory");
+#endif
 }
 
 void tst_Moc::warnings()