Implement write_async_safe for mi_console_file (PR 22299)

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.

gdb/ChangeLog:

	PR mi/22299
	* mi/mi-console.c (do_fputc_async_safe): New.
	(mi_console_file::write_async_safe): New.
	(mi_console_file::flush): Adjust calls to fputstrn_unfiltered.
	* mi/mi-console.h (class mi_console_file) <write_async_safe>:
	New.
	* ui-file.c (ui_file::putstrn): Adjust call to
	fputstrn_unfiltered.
	* utils.c (printchar): Replace do_fputs and do_fprintf
	parameters by do_fputc.
	(fputstr_filtered): Adjust call to printchar.
	(fputstr_unfiltered): Likewise.
	(fputstrn_filtered): Likewise.
	(fputstrn_unfiltered): Add do_fputc parameter, pass to
	printchar.
	* utils.h (do_fputc_ftype): New typedef.
	(fputstrn_unfiltered): Add do_fputc parameter.

2 files changed, 33 insertions, 2 deletions

diff --git a/gdb/mi/mi-console.c b/gdb/mi/mi-console.c
index 43d5388..248d070 100644
--- a/gdb/mi/mi-console.c
+++ b/gdb/mi/mi-console.c
@@ -48,6 +48,34 @@ mi_console_file::write (const char *buf, long length_buf)
     this->flush ();
 }
 
+/* Write C to STREAM's in an async-safe way.  */
+
+static int
+do_fputc_async_safe (int c, ui_file *stream)
+{
+  char ch = c;
+  stream->write_async_safe (&ch, 1);
+  return c;
+}
+
+void
+mi_console_file::write_async_safe (const char *buf, long length_buf)
+{
+  m_raw->write_async_safe (m_prefix, strlen (m_prefix));
+  if (m_quote)
+    {
+      m_raw->write_async_safe (&m_quote, 1);
+      fputstrn_unfiltered (buf, length_buf, m_quote, do_fputc_async_safe,
+			   m_raw);
+      m_raw->write_async_safe (&m_quote, 1);
+    }
+  else
+    fputstrn_unfiltered (buf, length_buf, 0, do_fputc_async_safe, m_raw);
+
+  char nl = '\n';
+  m_raw->write_async_safe (&nl, 1);
+}
+
 void
 mi_console_file::flush ()
 {
@@ -63,13 +91,14 @@ mi_console_file::flush ()
       if (m_quote)
 	{
 	  fputc_unfiltered (m_quote, m_raw);
-	  fputstrn_unfiltered (buf, length_buf, m_quote, m_raw);
+	  fputstrn_unfiltered (buf, length_buf, m_quote, fputc_unfiltered,
+			       m_raw);
 	  fputc_unfiltered (m_quote, m_raw);
 	  fputc_unfiltered ('\n', m_raw);
 	}
       else
 	{
-	  fputstrn_unfiltered (buf, length_buf, 0, m_raw);
+	  fputstrn_unfiltered (buf, length_buf, 0, fputc_unfiltered, m_raw);
 	  fputc_unfiltered ('\n', m_raw);
 	}
       gdb_flush (m_raw);
diff --git a/gdb/mi/mi-console.h b/gdb/mi/mi-console.h
index fbfa03b..d98a0c8 100644
--- a/gdb/mi/mi-console.h
+++ b/gdb/mi/mi-console.h
@@ -39,6 +39,8 @@ public:
 
   void write (const char *buf, long length_buf) override;
 
+  void write_async_safe (const char *buf, long length_buf) override;
+
 private:
   /* The wrapped raw output stream.  */
   ui_file *m_raw;