Fixed bug in QIODevice::read after first reading 0 bytes.

Change 02532ec80375c686503c4250c6ad6bb211515ec8 removed the early-exit
for 0 byte reads, causing us to hit code that assumed the buffer was
empty since nothing was read. It would thus read more into the end of
the buffer, causing the buffer to grow bigger than QIODEVICE_BUFFERSIZE.
Next, if the actual number of bytes we wanted to read was bigger than
the original buffer size we'd read the same data twice.

Reviewed-by: João Abecasis
Reviewed-by: Thiago Macieira

2 files changed, 29 insertions, 0 deletions

diff --git a/src/corelib/io/qiodevice.cpp b/src/corelib/io/qiodevice.cpp
index bb11d6b..223df9b 100644
--- a/src/corelib/io/qiodevice.cpp
+++ b/src/corelib/io/qiodevice.cpp
@@ -810,6 +810,9 @@ qint64 QIODevice::read(char *data, qint64 maxSize)
                 }
             }
 
+            if (!maxSize)
+                return readSoFar;
+
             if ((d->openMode & Unbuffered) == 0 && maxSize < QIODEVICE_BUFFERSIZE) {
                 // In buffered mode, we try to fill up the QIODevice buffer before
                 // we do anything else.
diff --git a/tests/auto/qbuffer/tst_qbuffer.cpp b/tests/auto/qbuffer/tst_qbuffer.cpp
index fcef6a3..dd5ca91 100644
--- a/tests/auto/qbuffer/tst_qbuffer.cpp
+++ b/tests/auto/qbuffer/tst_qbuffer.cpp
@@ -76,6 +76,7 @@ private slots:
     void atEnd();
     void readLineBoundaries();
     void writeAfterQByteArrayResize();
+    void read_null();
 
 protected slots:
     void readyReadSlot();
@@ -529,5 +530,30 @@ void tst_QBuffer::writeAfterQByteArrayResize()
     QCOMPARE(buffer.buffer().size(), 1000);
 }
 
+void tst_QBuffer::read_null()
+{
+    QByteArray buffer;
+    buffer.resize(32000);
+    for (int i = 0; i < buffer.size(); ++i)
+        buffer[i] = char(i & 0xff);
+
+    QBuffer in(&buffer);
+    in.open(QIODevice::ReadOnly);
+
+    QByteArray chunk;
+
+    chunk.resize(16380);
+    in.read(chunk.data(), 16380);
+
+    QCOMPARE(chunk, buffer.mid(0, chunk.size()));
+
+    in.read(chunk.data(), 0);
+
+    chunk.resize(8);
+    in.read(chunk.data(), chunk.size());
+
+    QCOMPARE(chunk, buffer.mid(16380, chunk.size()));
+}
+
 QTEST_MAIN(tst_QBuffer)
 #include "tst_qbuffer.moc"