let QMakeVfs::readFile() report ENOFILE explicitly

when the QFile object is already constructed, querying whether the file
exists is actually cheap, so do it right away instead of later on
demand. that makes the calling code a bit cleaner.

fwiw, that we need to explicitly query the file's existence at all is a
result of QFile's completely useless error "codes" (which merely say
which function failed, as if the caller would not know).

Change-Id: Ifec39d05b1713d8128046f679287e510f10e45dc
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/shared/proparser/qmakeparser.cpp

@@ -242,8 +242,9 @@ bool QMakeParser::read(ProFile *pro, ParseFlags flags)
 {
     QString content;
     QString errStr;
-    if (!m_vfs->readFile(pro->fileName(), &content, &errStr)) {
-        if (m_handler && ((flags & ParseReportMissing) || m_vfs->exists(pro->fileName())))
+    QMakeVfs::ReadResult result = m_vfs->readFile(pro->fileName(), &content, &errStr);
+    if (result != QMakeVfs::ReadOk) {
+        if (m_handler && ((flags & ParseReportMissing) || result != QMakeVfs::ReadNotFound))
             m_handler->message(QMakeParserHandler::ParserIoError,
                                fL1S("Cannot read %1: %2").arg(pro->fileName(), errStr));
         return false;

src/shared/proparser/qmakevfs.cpp

@@ -114,7 +114,7 @@ bool QMakeVfs::readVirtualFile(const QString &fn, QString *contents)
 }
 #endif
 
-bool QMakeVfs::readFile(const QString &fn, QString *contents, QString *errStr)
+QMakeVfs::ReadResult QMakeVfs::readFile(const QString &fn, QString *contents, QString *errStr)
 {
 #ifndef PROEVALUATOR_FULL
 # ifdef PROEVALUATOR_THREAD_SAFE
@@ -124,25 +124,26 @@ bool QMakeVfs::readFile(const QString &fn, QString *contents, QString *errStr)
     if (it != m_files.constEnd()) {
         if (it->constData() == m_magicMissing.constData()) {
             *errStr = fL1S("No such file or directory");
-            return false;
+            return ReadNotFound;
         }
         if (it->constData() != m_magicExisting.constData()) {
             *contents = *it;
-            return true;
+            return ReadOk;
         }
     }
 #endif
 
     QFile file(fn);
     if (!file.open(QIODevice::ReadOnly)) {
+        if (!file.exists()) {
 #ifndef PROEVALUATOR_FULL
-        if (!IoUtils::exists(fn)) {
             m_files[fn] = m_magicMissing;
-            *errStr = fL1S("No such file or directory");
-        } else
 #endif
+            *errStr = fL1S("No such file or directory");
+            return ReadNotFound;
+        }
         *errStr = file.errorString();
-        return false;
+        return ReadOtherError;
     }
 #ifndef PROEVALUATOR_FULL
     m_files[fn] = m_magicExisting;
@@ -152,10 +153,10 @@ bool QMakeVfs::readFile(const QString &fn, QString *contents, QString *errStr)
     if (bcont.startsWith("\xef\xbb\xbf")) {
         // UTF-8 BOM will cause subtle errors
         *errStr = fL1S("Unexpected UTF-8 BOM");
-        return false;
+        return ReadOtherError;
     }
     *contents = QString::fromLocal8Bit(bcont);
-    return true;
+    return ReadOk;
 }
 
 bool QMakeVfs::exists(const QString &fn)

src/shared/proparser/qmakevfs.h

@@ -41,10 +41,16 @@ QT_BEGIN_NAMESPACE
 class QMAKE_EXPORT QMakeVfs
 {
 public:
+    enum ReadResult {
+        ReadOk,
+        ReadNotFound,
+        ReadOtherError
+    };
+
     QMakeVfs();
 
     bool writeFile(const QString &fn, QIODevice::OpenMode mode, bool exe, const QString &contents, QString *errStr);
-    bool readFile(const QString &fn, QString *contents, QString *errStr);
+    ReadResult readFile(const QString &fn, QString *contents, QString *errStr);
     bool exists(const QString &fn);
 
 #ifndef PROEVALUATOR_FULL