QXpmHandler: fix re-entrancy bug in xpm_color_name

The xpm_color_name() function returned a pointer to a function-static
buffer. This is infamously non-reentrant, and an actual problem,
because we explicitly allow QImage operations (incl. saving to an
.xpm) from non-GUI-threads.

Fix by using the CSS pattern (Caller-Supplied Storage; also used in
the QAnyStringView(char32_t) and QAnyStringView(QStringBuilder) ctors)
to force the caller to allocate storage in its own stack frame. As a
consequence, we re-gain re-entrancy, but the returned pointer is now
only valid until the end of the full-expression, which isn't a problem,
as all callers immediately pass the result to a consumer (asprintf() or
QByteArray).

[ChangeLog][QtGui][QImage] Fixed a race condition when concurrently
writing .xpm files.

Change-Id: I36d7173d53839a52f5cdf58324474c1b32c71f33
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
(cherry picked from commit 73fabadcee71af858388fb245fccf4e96d4ead4e)

1 files changed, 12 insertions, 3 deletions

diff --git a/src/gui/image/qxpmhandler.cpp b/src/gui/image/qxpmhandler.cpp
index f5fe6cb7ea..1bd9531e9b 100644
--- a/src/gui/image/qxpmhandler.cpp
+++ b/src/gui/image/qxpmhandler.cpp
@@ -49,6 +49,7 @@
 #include <qvariant.h>
 
 #include <algorithm>
+#include <array>
 
 QT_BEGIN_NAMESPACE
 
@@ -1055,15 +1056,23 @@ bool qt_read_xpm_image_or_array(QIODevice *device, const char * const * source,
     return read_xpm_body(device, source, index, state, cpp, ncols, w, h, image);
 }
 
-static const char* xpm_color_name(int cpp, int index)
+namespace {
+template <size_t N>
+struct CharBuffer : std::array<char, N>
+{
+    CharBuffer() {} // avoid value-initializing the whole array
+};
+}
+
+static const char* xpm_color_name(int cpp, int index, CharBuffer<5> && returnable = {})
 {
-    static char returnable[5];
     static const char code[] = ".#abcdefghijklmnopqrstuvwxyzABCD"
                                "EFGHIJKLMNOPQRSTUVWXYZ0123456789";
     // cpp is limited to 4 and index is limited to 64^cpp
     if (cpp > 1) {
         if (cpp > 2) {
             if (cpp > 3) {
+                returnable[4] = '\0';
                 returnable[3] = code[index % 64];
                 index /= 64;
             } else
@@ -1083,7 +1092,7 @@ static const char* xpm_color_name(int cpp, int index)
         returnable[1] = '\0';
     returnable[0] = code[index];
 
-    return returnable;
+    return returnable.data();
 }