gdb: move setbuf calls out of gdb_readline_no_editing_callback

After this commit:

  commit d08cbc5d3203118da5583296e49273cf82378042
  Date:   Wed Dec 22 12:57:44 2021 +0000

      gdb: unbuffer all input streams when not using readline

Issues were reported with some MS-Windows hosts, see the thread
starting here:

  https://sourceware.org/pipermail/gdb-patches/2022-March/187004.html

Filed in bugzilla as: PR mi/29002

The problem seems to be that calling setbuf on terminal file handles
is not always acceptable, see this mail for more details:

  https://sourceware.org/pipermail/gdb-patches/2022-April/187310.html

This commit does two things, first moving the setbuf calls out of
gdb_readline_no_editing_callback so that we don't end up calling
setbuf so often.

Then, for MS-Windows hosts, we don't call setbuf for terminals, this
appears to resolve the issues that have been reported.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29002

1 files changed, 39 insertions, 0 deletions

diff --git a/gdb/top.c b/gdb/top.c
index 7831b4f..3b39b7d 100644
--- a/gdb/top.c
+++ b/gdb/top.c
@@ -260,6 +260,41 @@ void (*deprecated_context_hook) (int id);
 /* The highest UI number ever assigned.  */
 static int highest_ui_num;
 
+/* Unbuffer STREAM.  This is a wrapper around setbuf(STREAM, nullptr)
+   which applies some special rules for MS-Windows hosts.  */
+
+static void
+unbuffer_stream (FILE *stream)
+{
+  /* Unbuffer the input stream so that in gdb_readline_no_editing_callback,
+     the calls to fgetc fetch only one char at the time from STREAM.
+
+     This is important because gdb_readline_no_editing_callback will read
+     from STREAM up to the first '\n' character, after this GDB returns to
+     the event loop and relies on a select on STREAM indicating that more
+     input is pending.
+
+     If STREAM is buffered then the fgetc calls may have moved all the
+     pending input from the kernel into a local buffer, after which the
+     select will not indicate that more input is pending, and input after
+     the first '\n' will not be processed immediately.
+
+     Please ensure that any changes in this area run the MI tests with the
+     FORCE_SEPARATE_MI_TTY=1 flag being passed.  */
+
+#ifdef __MINGW32__
+  /* With MS-Windows runtime, making stdin unbuffered when it's
+     connected to the terminal causes it to misbehave.  */
+  if (!ISATTY (stream))
+    setbuf (stream, nullptr);
+#else
+  /* On GNU/Linux the issues described above can impact GDB even when
+     dealing with input from a terminal.  For now we unbuffer the input
+     stream for everyone except MS-Windows.  */
+  setbuf (stream, nullptr);
+#endif
+}
+
 /* See top.h.  */
 
 ui::ui (FILE *instream_, FILE *outstream_, FILE *errstream_)
@@ -286,6 +321,8 @@ ui::ui (FILE *instream_, FILE *outstream_, FILE *errstream_)
 {
   buffer_init (&line_buffer);
 
+  unbuffer_stream (instream_);
+
   if (ui_list == NULL)
     ui_list = this;
   else
@@ -415,6 +452,8 @@ read_command_file (FILE *stream)
 {
   struct ui *ui = current_ui;
 
+  unbuffer_stream (stream);
+
   scoped_restore save_instream
     = make_scoped_restore (&ui->instream, stream);