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 necessitated
simplifying one caller (sorry!).
To see why said simplification is valid, observe that xpm_color_name()
writes a (now-explicit) NUL into returnable[cpp] and the old code read
max(cpp, 4) characters from xpm_color_name()'s result.
NB: cpp can be 5, even though the code comments say otherwise! :(
[ChangeLog][QtGui][QImage] Fixed a race condition when concurrently
writing .xpm files.
Pick-to: 6.2 6.1 5.15 5.12
Change-Id: I36d7173d53839a52f5cdf58324474c1b32c71f33
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
73fabadcee