Enabling "set debug lin-lwp 1" with the MI interpreter doesn't work.
When the sigchld_handler function wants to print a debug output
("sigchld\n"), it uses ui_file_write_async_safe. This ends up in the
default implementation of ui_file::write_async_safe, which aborts GDB.
This patch implements the write_async_safe method for mi_console_file.
The "normal" MI output is line buffered, which means the output
accumulates in m_buffer until a \n is written, at which point it's
flushed in m_raw. The implementation of write_async_safe provided by
this patch bypasses this buffer and writes directly to m_raw. There
are
two reasons for this:
(1) Appending to m_buffer (therefore to an std::string) is probably not
async-safe, as it may allocate memory.
(2) We may have a partial output already in m_buffer, so that would
lead
to some nested MI output, not so great.
There is probably still a chance to have bad MI output, if
sigchld_handler is invoked in the middle of mi_console_file's flush,
and
the line being flushed is only partially sent to m_raw. The solution
would probably be to block signals during flushing. Since this is only
used for debug output, I don't know if it's worth the effort to do
that.
To implement write_async_safe, I needed to use the fputstrn_unfiltered,
which does the necessary escaping (e.g. replace \n with \\n). I
started
by adding printchar's callback parameters to fputstrn_unfiltered, to be
able to pass async-safe versions of them. It's not easy to provide an
async-safe version of do_fprintf, but it turns out that we can easily
replace printchar's callbacks with a single do_fputc quite easily. The
async-safe version of do_fputc simply calls the underlying ui_file's
write_async_safe method.