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.

gdb/ChangeLog

@@ -1,3 +1,23 @@
+2018-04-07  Simon Marchi  <simon.marchi@polymtl.ca>
+
+	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.
+
 2018-04-07  Simon Marchi  <simon.marchi@polymtl.ca>
 
 	* regformats/i386/i386-avx.dat: Remove.

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);

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;

gdb/ui-file.c

@@ -52,7 +52,7 @@ ui_file::putstr (const char *str, int quoter)
 void
 ui_file::putstrn (const char *str, int n, int quoter)
 {
-  fputstrn_unfiltered (str, n, quoter, this);
+  fputstrn_unfiltered (str, n, quoter, fputc_unfiltered, this);
 }
 
 int

gdb/utils.c

@@ -1170,9 +1170,7 @@ parse_escape (struct gdbarch *gdbarch, const char **string_ptr)
    character. */
 
 static void
-printchar (int c, void (*do_fputs) (const char *, struct ui_file *),
-	   void (*do_fprintf) (struct ui_file *, const char *, ...)
-	   ATTRIBUTE_FPTR_PRINTF_2, struct ui_file *stream, int quoter)
+printchar (int c, do_fputc_ftype do_fputc, ui_file *stream, int quoter)
 {
   c &= 0xFF;			/* Avoid sign bit follies */
 
@@ -1180,39 +1178,45 @@ printchar (int c, void (*do_fputs) (const char *, struct ui_file *),
       (c >= 0x7F && c < 0xA0) ||	/* DEL, High controls */
       (sevenbit_strings && c >= 0x80))
     {				/* high order bit set */
+      do_fputc ('\\', stream);
+
       switch (c)
 	{
 	case '\n':
-	  do_fputs ("\\n", stream);
+	  do_fputc ('n', stream);
 	  break;
 	case '\b':
-	  do_fputs ("\\b", stream);
+	  do_fputc ('b', stream);
 	  break;
 	case '\t':
-	  do_fputs ("\\t", stream);
+	  do_fputc ('t', stream);
 	  break;
 	case '\f':
-	  do_fputs ("\\f", stream);
+	  do_fputc ('f', stream);
 	  break;
 	case '\r':
-	  do_fputs ("\\r", stream);
+	  do_fputc ('r', stream);
 	  break;
 	case '\033':
-	  do_fputs ("\\e", stream);
+	  do_fputc ('e', stream);
 	  break;
 	case '\007':
-	  do_fputs ("\\a", stream);
+	  do_fputc ('a', stream);
 	  break;
 	default:
-	  do_fprintf (stream, "\\%.3o", (unsigned int) c);
+	  {
+	    do_fputc ('0' + ((c >> 6) & 0x7), stream);
+	    do_fputc ('0' + ((c >> 3) & 0x7), stream);
+	    do_fputc ('0' + ((c >> 0) & 0x7), stream);
 	    break;
 	  }
 	}
+    }
   else
     {
       if (quoter != 0 && (c == '\\' || c == quoter))
-	do_fputs ("\\", stream);
-      do_fprintf (stream, "%c", c);
+	do_fputc ('\\', stream);
+      do_fputc (c, stream);
     }
 }
 
@@ -1225,34 +1229,30 @@ void
 fputstr_filtered (const char *str, int quoter, struct ui_file *stream)
 {
   while (*str)
-    printchar (*str++, fputs_filtered, fprintf_filtered, stream, quoter);
+    printchar (*str++, fputc_filtered, stream, quoter);
 }
 
 void
 fputstr_unfiltered (const char *str, int quoter, struct ui_file *stream)
 {
   while (*str)
-    printchar (*str++, fputs_unfiltered, fprintf_unfiltered, stream, quoter);
+    printchar (*str++, fputc_unfiltered, stream, quoter);
 }
 
 void
 fputstrn_filtered (const char *str, int n, int quoter,
 		   struct ui_file *stream)
 {
-  int i;
-
-  for (i = 0; i < n; i++)
-    printchar (str[i], fputs_filtered, fprintf_filtered, stream, quoter);
+  for (int i = 0; i < n; i++)
+    printchar (str[i], fputc_filtered, stream, quoter);
 }
 
 void
 fputstrn_unfiltered (const char *str, int n, int quoter,
-		     struct ui_file *stream)
+		     do_fputc_ftype do_fputc, struct ui_file *stream)
 {
-  int i;
-
-  for (i = 0; i < n; i++)
-    printchar (str[i], fputs_unfiltered, fprintf_unfiltered, stream, quoter);
+  for (int i = 0; i < n; i++)
+    printchar (str[i], do_fputc, stream, quoter);
 }
 
 

gdb/utils.h

@@ -418,7 +418,10 @@ extern void fputstr_unfiltered (const char *str, int quotr,
 extern void fputstrn_filtered (const char *str, int n, int quotr,
 			       struct ui_file * stream);
 
+typedef int (*do_fputc_ftype) (int c, ui_file *stream);
+
 extern void fputstrn_unfiltered (const char *str, int n, int quotr,
+				 do_fputc_ftype do_fputc,
 				 struct ui_file * stream);
 
 /* Return nonzero if filtered printing is initialized.  */