Add QIODevice::NewOnly and QIODevice::ExistingOnly OpenMode flags

When QFile::open is called with the NewOnly flag, the call will
fail if the file already exists. As usual, if the file does not exist,
it will be created. Like QTemporaryFile, there is a guarantee from
the operating system that you are not accidentally creating a new file
on top of an older file. When QFile::open is called with the
ExistingOnly flag, the call will fail if the file does not exist. The
ExistingOnly flag only provides new functionality when used with the
WriteOnly flag. For ReadOnly it provides no change in functionality,
as ReadOnly by itself already never creates.

Task-number: QTBUG-52244
Change-Id: I8e3206728f245f95172c225bf297023fb078fc6d
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

9 files changed, 90 insertions, 36 deletions

diff --git a/src/corelib/io/qfile.cpp b/src/corelib/io/qfile.cpp
index e4888e9523..33b0b2eb66 100644
--- a/src/corelib/io/qfile.cpp
+++ b/src/corelib/io/qfile.cpp
@@ -893,9 +893,9 @@ bool QFile::open(OpenMode mode)
         qWarning("QFile::open: File (%s) already open", qPrintable(fileName()));
         return false;
     }
-    if (mode & Append)
+    // Either Append or NewOnly implies WriteOnly
+    if (mode & (Append | NewOnly))
         mode |= WriteOnly;
-
     unsetError();
     if ((mode & (ReadOnly | WriteOnly)) == 0) {
         qWarning("QIODevice::open: File access not specified");
@@ -965,7 +965,8 @@ bool QFile::open(FILE *fh, OpenMode mode, FileHandleFlags handleFlags)
         qWarning("QFile::open: File (%s) already open", qPrintable(fileName()));
         return false;
     }
-    if (mode & Append)
+    // Either Append or NewOnly implies WriteOnly
+    if (mode & (Append | NewOnly))
         mode |= WriteOnly;
     unsetError();
     if ((mode & (ReadOnly | WriteOnly)) == 0) {
@@ -1023,7 +1024,8 @@ bool QFile::open(int fd, OpenMode mode, FileHandleFlags handleFlags)
         qWarning("QFile::open: File (%s) already open", qPrintable(fileName()));
         return false;
     }
-    if (mode & Append)
+    // Either Append or NewOnly implies WriteOnly
+    if (mode & (Append | NewOnly))
         mode |= WriteOnly;
     unsetError();
     if ((mode & (ReadOnly | WriteOnly)) == 0) {
diff --git a/src/corelib/io/qfsfileengine.cpp b/src/corelib/io/qfsfileengine.cpp
index 538fcb9a37..387990ed79 100644
--- a/src/corelib/io/qfsfileengine.cpp
+++ b/src/corelib/io/qfsfileengine.cpp
@@ -165,6 +165,35 @@ QFSFileEngine::QFSFileEngine(QFSFileEnginePrivate &dd)
 }
 
 /*!
+    \internal
+*/
+bool QFSFileEngine::processOpenModeFlags(QIODevice::OpenMode *mode)
+{
+    QIODevice::OpenMode &openMode = *mode;
+    if ((openMode & QFile::NewOnly) && (openMode & QFile::ExistingOnly)) {
+        qWarning("NewOnly and ExistingOnly are mutually exclusive");
+        setError(QFile::OpenError, QLatin1String("NewOnly and ExistingOnly are mutually exclusive"));
+        return false;
+    }
+
+    if ((openMode & QFile::ExistingOnly) && !(openMode & (QFile::ReadOnly | QFile::WriteOnly))) {
+        qWarning("ExistingOnly must be specified alongside ReadOnly, WriteOnly, or ReadWrite");
+        setError(QFile::OpenError, QLatin1String("ExistingOnly must be specified alongside ReadOnly, WriteOnly, or ReadWrite"));
+        return false;
+    }
+
+    // Either Append or NewOnly implies WriteOnly
+    if (openMode & (QFile::Append | QFile::NewOnly))
+        openMode |= QFile::WriteOnly;
+
+    // WriteOnly implies Truncate when ReadOnly, Append, and NewOnly are not set.
+    if ((openMode & QFile::WriteOnly) && !(openMode & (QFile::ReadOnly | QFile::Append | QFile::NewOnly)))
+        openMode |= QFile::Truncate;
+
+    return true;
+}
+
+/*!
     Destructs the QFSFileEngine.
 */
 QFSFileEngine::~QFSFileEngine()
@@ -205,13 +234,8 @@ bool QFSFileEngine::open(QIODevice::OpenMode openMode)
         return false;
     }
 
-    // Append implies WriteOnly.
-    if (openMode & QFile::Append)
-        openMode |= QFile::WriteOnly;
-
-    // WriteOnly implies Truncate if neither ReadOnly nor Append are sent.
-    if ((openMode & QFile::WriteOnly) && !(openMode & (QFile::ReadOnly | QFile::Append)))
-        openMode |= QFile::Truncate;
+    if (!processOpenModeFlags(&openMode))
+        return false;
 
     d->openMode = openMode;
     d->lastFlushFailed = false;
@@ -238,13 +262,8 @@ bool QFSFileEngine::open(QIODevice::OpenMode openMode, FILE *fh, QFile::FileHand
 
     Q_D(QFSFileEngine);
 
-    // Append implies WriteOnly.
-    if (openMode & QFile::Append)
-        openMode |= QFile::WriteOnly;
-
-    // WriteOnly implies Truncate if neither ReadOnly nor Append are sent.
-    if ((openMode & QFile::WriteOnly) && !(openMode & (QFile::ReadOnly | QFile::Append)))
-        openMode |= QFile::Truncate;
+    if (!processOpenModeFlags(&openMode))
+        return false;
 
     d->openMode = openMode;
     d->lastFlushFailed = false;
@@ -302,13 +321,8 @@ bool QFSFileEngine::open(QIODevice::OpenMode openMode, int fd, QFile::FileHandle
 {
     Q_D(QFSFileEngine);
 
-    // Append implies WriteOnly.
-    if (openMode & QFile::Append)
-        openMode |= QFile::WriteOnly;
-
-    // WriteOnly implies Truncate if neither ReadOnly nor Append are sent.
-    if ((openMode & QFile::WriteOnly) && !(openMode & (QFile::ReadOnly | QFile::Append)))
-        openMode |= QFile::Truncate;
+    if (!processOpenModeFlags(&openMode))
+        return false;
 
     d->openMode = openMode;
     d->lastFlushFailed = false;
diff --git a/src/corelib/io/qfsfileengine_p.h b/src/corelib/io/qfsfileengine_p.h
index 16ba161b75..6b091a8eef 100644
--- a/src/corelib/io/qfsfileengine_p.h
+++ b/src/corelib/io/qfsfileengine_p.h
@@ -131,6 +131,9 @@ public:
 
 protected:
     QFSFileEngine(QFSFileEnginePrivate &dd);
+
+private:
+    inline bool processOpenModeFlags(QIODevice::OpenMode *mode);
 };
 
 class Q_AUTOTEST_EXPORT QFSFileEnginePrivate : public QAbstractFileEnginePrivate
@@ -219,6 +222,12 @@ public:
     int sysOpen(const QString &, int flags);
 #endif
 
+    static bool openModeCanCreate(QIODevice::OpenMode openMode)
+    {
+        // WriteOnly can create, but only when ExistingOnly isn't specified.
+        // ReadOnly by itself never creates.
+        return (openMode & QFile::WriteOnly) && !(openMode & QFile::ExistingOnly);
+    }
 protected:
     QFSFileEnginePrivate();
 
diff --git a/src/corelib/io/qfsfileengine_unix.cpp b/src/corelib/io/qfsfileengine_unix.cpp
index c040d67862..7dd4f6556d 100644
--- a/src/corelib/io/qfsfileengine_unix.cpp
+++ b/src/corelib/io/qfsfileengine_unix.cpp
@@ -74,11 +74,13 @@ static inline int openModeToOpenFlags(QIODevice::OpenMode mode)
     oflags |= QT_OPEN_LARGEFILE;
 #endif
 
-    if ((mode & QFile::ReadWrite) == QFile::ReadWrite) {
-        oflags = QT_OPEN_RDWR | QT_OPEN_CREAT;
-    } else if (mode & QFile::WriteOnly) {
-        oflags = QT_OPEN_WRONLY | QT_OPEN_CREAT;
-    }
+    if ((mode & QFile::ReadWrite) == QFile::ReadWrite)
+        oflags = QT_OPEN_RDWR;
+    else if (mode & QFile::WriteOnly)
+        oflags = QT_OPEN_WRONLY;
+
+    if (QFSFileEnginePrivate::openModeCanCreate(mode))
+        oflags |= QT_OPEN_CREAT;
 
     if (mode & QFile::Append) {
         oflags |= QT_OPEN_APPEND;
@@ -87,6 +89,9 @@ static inline int openModeToOpenFlags(QIODevice::OpenMode mode)
             oflags |= QT_OPEN_TRUNC;
     }
 
+    if (mode & QFile::NewOnly)
+        oflags |= QT_OPEN_EXCL;
+
     return oflags;
 }
 
diff --git a/src/corelib/io/qfsfileengine_win.cpp b/src/corelib/io/qfsfileengine_win.cpp
index 759effe632..8199f6a846 100644
--- a/src/corelib/io/qfsfileengine_win.cpp
+++ b/src/corelib/io/qfsfileengine_win.cpp
@@ -117,9 +117,12 @@ bool QFSFileEnginePrivate::nativeOpen(QIODevice::OpenMode openMode)
     if (openMode & QIODevice::WriteOnly)
         accessRights |= GENERIC_WRITE;
 
-
     // WriteOnly can create files, ReadOnly cannot.
-    DWORD creationDisp = (openMode & QIODevice::WriteOnly) ? OPEN_ALWAYS : OPEN_EXISTING;
+    DWORD creationDisp = (openMode & QIODevice::NewOnly)
+                            ? CREATE_NEW
+                            : openModeCanCreate(openMode)
+                                ? OPEN_ALWAYS
+                                : OPEN_EXISTING;
     // Create the file handle.
 #ifndef Q_OS_WINRT
     SECURITY_ATTRIBUTES securityAtts = { sizeof(SECURITY_ATTRIBUTES), NULL, FALSE };
diff --git a/src/corelib/io/qiodevice.cpp b/src/corelib/io/qiodevice.cpp
index 7d46898911..95a5fb27cf 100644
--- a/src/corelib/io/qiodevice.cpp
+++ b/src/corelib/io/qiodevice.cpp
@@ -324,6 +324,23 @@ QIODevicePrivate::~QIODevicePrivate()
                      terminators are translated to the local encoding, for
                      example '\\r\\n' for Win32.
     \value Unbuffered Any buffer in the device is bypassed.
+    \value NewOnly   Fail if the file to be opened already exists. Create and
+                     open the file only if it does not exist. There is a
+                     guarantee from the operating system that you are the only
+                     one creating and opening the file. Note that this mode
+                     implies WriteOnly, and combining it with ReadWrite is
+                     allowed. This flag currently only affects QFile. Other
+                     classes might use this flag in the future, but until then
+                     using this flag with any classes other than QFile may
+                     result in undefined behavior.
+    \value ExistingOnly Fail if the file to be opened does not exist. This flag
+                     must be specified alongside ReadOnly, WriteOnly, or
+                     ReadWrite. Note that using this flag with ReadOnly alone
+                     is redundant, as ReadOnly already fails when the file does
+                     not exist. This flag currently only affects QFile. Other
+                     classes might use this flag in the future, but until then
+                     using this flag with any classes other than QFile may
+                     result in undefined behavior.
 
     Certain flags, such as \c Unbuffered and \c Truncate, are
     meaningless when used with some subclasses. Some of these
diff --git a/src/corelib/io/qiodevice.h b/src/corelib/io/qiodevice.h
index af37b3fd53..2e4debe339 100644
--- a/src/corelib/io/qiodevice.h
+++ b/src/corelib/io/qiodevice.h
@@ -76,7 +76,9 @@ public:
         Append = 0x0004,
         Truncate = 0x0008,
         Text = 0x0010,
-        Unbuffered = 0x0020
+        Unbuffered = 0x0020,
+        NewOnly = 0x0040,
+        ExistingOnly = 0x0080
     };
     Q_DECLARE_FLAGS(OpenMode, OpenModeFlag)
 
diff --git a/src/corelib/io/qsavefile.cpp b/src/corelib/io/qsavefile.cpp
index 63f2284ef5..56934a9a0f 100644
--- a/src/corelib/io/qsavefile.cpp
+++ b/src/corelib/io/qsavefile.cpp
@@ -184,7 +184,8 @@ void QSaveFile::setFileName(const QString &name)
     Important: the \a mode must include QIODevice::WriteOnly.
     It may also have additional flags, such as QIODevice::Text and QIODevice::Unbuffered.
 
-    QIODevice::ReadWrite and QIODevice::Append are not supported at the moment.
+    QIODevice::ReadWrite, QIODevice::Append, QIODevice::NewOnly and
+    QIODevice::ExistingOnly are not supported at the moment.
 
     \sa QIODevice::OpenMode, setFileName()
 */
@@ -201,7 +202,8 @@ bool QSaveFile::open(OpenMode mode)
         return false;
     }
     // In the future we could implement ReadWrite by copying from the existing file to the temp file...
-    if ((mode & ReadOnly) || (mode & Append)) {
+    // The implications of NewOnly and ExistingOnly when used with QSaveFile need to be considered carefully...
+    if (mode & (ReadOnly | Append | NewOnly | ExistingOnly)) {
         qWarning("QSaveFile::open: Unsupported open mode 0x%x", int(mode));
         return false;
     }
diff --git a/src/corelib/io/qtemporaryfile.cpp b/src/corelib/io/qtemporaryfile.cpp
index 35699d52df..73249d7df8 100644
--- a/src/corelib/io/qtemporaryfile.cpp
+++ b/src/corelib/io/qtemporaryfile.cpp
@@ -248,7 +248,7 @@ static bool createFileFromTemplate(NativeFileHandle &file, QTemporaryFileName &t
         }
 #else // POSIX
         file = QT_OPEN(path.constData(),
-                QT_OPEN_CREAT | O_EXCL | QT_OPEN_RDWR | QT_OPEN_LARGEFILE,
+                QT_OPEN_CREAT | QT_OPEN_EXCL | QT_OPEN_RDWR | QT_OPEN_LARGEFILE,
                 static_cast<mode_t>(mode));
 
         if (file != -1)