moc: Do not accept empty input files on --collect-json

... and also don't generate them on --output-json. That was an
invitation for build time race conditions.

Change-Id: I206ee47f08aa657f9b1cbec4e9e9b3ea97605cae
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/tools/moc/collectjson.cpp

@@ -12,8 +12,10 @@
 static bool readFromFile(QFile *device, QJsonArray *allMetaObjects)
 {
     const QByteArray contents = device->readAll();
-    if (contents.isEmpty())
-        return true;
+    if (contents.isEmpty()) {
+        fprintf(stderr, "%s:0: metatypes input file is empty\n", qPrintable(device->fileName()));
+        return false;
+    }
 
     QJsonParseError error {};
     QJsonDocument metaObjects = QJsonDocument::fromJson(contents, &error);

src/tools/moc/main.cpp

@@ -580,11 +580,17 @@ int runMoc(int argc, char **argv)
 
     if (pp.preprocessOnly) {
         fprintf(out.get(), "%s\n", composePreprocessorOutput(moc.symbols).constData());
+    } else if (moc.classList.isEmpty()) {
+        moc.note("No relevant classes found. No output generated.");
+        if (jsonOutput) {
+            const QJsonDocument jsonDoc(QJsonObject {
+                    { "outputRevision"_L1, mocOutputRevision },
+                    { "inputFile"_L1, QLatin1StringView(moc.strippedFileName()) }
+            });
+            fputs(jsonDoc.toJson().constData(), jsonOutput.get());
+        }
     } else {
-        if (moc.classList.isEmpty())
-            moc.note("No relevant classes found. No output generated.");
-        else
-            moc.generate(out.get(), jsonOutput.get());
+        moc.generate(out.get(), jsonOutput.get());
     }
 
     out.reset();

src/tools/moc/moc.cpp

@@ -1076,6 +1076,18 @@ void Moc::parse()
     }
 }
 
+QByteArrayView Moc::strippedFileName() const
+{
+    QByteArrayView fn = QByteArrayView(filename);
+
+    auto isSlash = [](char ch) { return ch == '/' || ch == '\\'; };
+    auto rit = std::find_if(fn.crbegin(), fn.crend(), isSlash);
+    if (rit != fn.crend())
+        fn = fn.last(rit - fn.crbegin());
+
+    return fn;
+}
+
 static bool any_type_contains(const QList<PropertyDef> &properties, const QByteArray &pattern)
 {
     for (const auto &p : properties) {
@@ -1143,12 +1155,7 @@ static QByteArrayList requiredQtContainers(const QList<ClassDef> &classes)
 
 void Moc::generate(FILE *out, FILE *jsonOutput)
 {
-    QByteArrayView fn = QByteArrayView(filename);
-
-    auto isSlash = [](char ch) { return ch == '/' || ch == '\\'; };
-    auto rit = std::find_if(fn.crbegin(), fn.crend(), isSlash);
-    if (rit != fn.crend())
-        fn = fn.last(rit - fn.crbegin());
+    QByteArrayView fn = strippedFileName();
 
     fprintf(out, "/****************************************************************************\n"
             "** Meta object code from reading C++ file '%s'\n**\n" , fn.constData());

src/tools/moc/moc.h

@@ -242,6 +242,7 @@ public:
     QList<QString> parsedPluginMetadataFiles;
 
     void parse();
+    QByteArrayView strippedFileName() const;
     void generate(FILE *out, FILE *jsonOutput);
 
     bool parseClassHead(ClassDef *def);

tests/auto/tools/moc/allmocs_baseline_in.json

@@ -3381,5 +3381,9 @@
         ],
         "inputFile": "trigraphs.h",
         "outputRevision": 69
+    },
+    {
+        "inputFile": "using-namespaces.h",
+        "outputRevision": 69
     }
 ]