[Bug mi/29002] Console input broken in MI mode

["eliz at gnu dot org" <sourceware-bugzilla@sourceware.org>]

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

--- Comment #3 from Eli Zaretskii <eliz at gnu dot org> ---
But this is not the whole story.  That commit also causes the Emacs gdb-mi
front-end to GDB to become broken, at least on MS-Windows.

What happens is that gdb-mi sends, via the GDB/MI interface, several commands
in a sequence when the debugging session begins.  Those commands are:

  1-gdb-set new-console off
  2-gdb-set interactive-mode on
  3-gdb-set height 0
  4-enable-pretty-printing
  5-enable-frame-filters
  6-file-list-exec-source-file
  7-gdb-show prompt
  8-stack-info-frame
  9-thread-info
  10-break-list
  11-thread-info
  12-break-list

But when GDB responds to those commands, some of the responses have the first
character of the TOKEN part stripped.  Here's what Emacs receives in response
(this is a single string sent to Emacs in one go):

1^done
(gdb) 
2^done
(gdb) 
3^done
(gdb) 
^done
(gdb) 
5^done
(gdb) 
^done,line=\"1189\",file=\"emacs.c\",fullname=\"d:\\\\gnu\\\\git\\\\emacs\\\\branch\\\\src\\\\emacs.c\",macro-info=\"1\"
(gdb) 
7^done,value=\"(gdb) \"
(gdb) 
8^error,msg=\"No registers.\"
(gdb) 
^done,threads=[]
(gdb) 
0^done,BreakpointTable={nr_rows=\"1\",nr_cols=\"6\",hdr=[{width=\"7\",alignment=\"-1\",col_name=\"number\",colhdr=\"Num\"},{width=\"14\",alignment=\"-1\",col_name=\"type\",colhdr=\"Type\"},{width=\"4\",alignment=\"-1\",col_name=\"disp\",colhdr=\"Disp\"},{width=\"3\",alignment=\"-1\",col_name=\"enabled\",colhdr=\"Enb\"},{width=\"10\",alignment=\"-1\",col_name=\"addr\",colhdr=\"Address\"},{width=\"40\",alignment=\"2\",col_name=\"what\",colhdr=\"What\"}],body=[bkpt={number=\"1\",type=\"breakpoint\",disp=\"keep\",enabled=\"y\",addr=\"0x0115dc0c\",func=\"terminate_due_to_signal\",file=\"emacs.c\",fullname=\"d:\\\\gnu\\\\git\\\\emacs\\\\branch\\\\src\\\\emacs.c\",line=\"400\",thread-groups=[\"i1\"],times=\"0\",original-location=\"terminate_due_to_signal\"}]}
(gdb) 
1^done,threads=[]
(gdb) 
12^done,BreakpointTable={nr_rows=\"1\",nr_cols=\"6\",hdr=[{width=\"7\",alignment=\"-1\",col_name=\"number\",colhdr=\"Num\"},{width=\"14\",alignment=\"-1\",col_name=\"type\",colhdr=\"Type\"},{width=\"4\",alignment=\"-1\",col_name=\"disp\",colhdr=\"Disp\"},{width=\"3\",alignment=\"-1\",col_name=\"enabled\",colhdr=\"Enb\"},{width=\"10\",alignment=\"-1\",col_name=\"addr\",colhdr=\"Address\"},{width=\"40\",alignment=\"2\",col_name=\"what\",colhdr=\"What\"}],body=[bkpt={number=\"1\",type=\"breakpoint\",disp=\"keep\",enabled=\"y\",addr=\"0x0115dc0c\",func=\"terminate_due_to_signal\",file=\"emacs.c\",fullname=\"d:\\\\gnu\\\\git\\\\emacs\\\\branch\\\\src\\\\emacs.c\",line=\"400\",thread-groups=[\"i1\"],times=\"0\",original-location=\"terminate_due_to_signal\"}]}

Note that responses to 4, 6, and 9 don't have the TOKEN part, and responses to
10 and 11 have the first digit, 1, removed from TOKEN, so they appear as if the
TOKEN were 0 (which is invalid) and 1 (which was already received, and so is
not expected at this time).

It turns out that restoring the "done_once" part of the offending commit fixes
this problem as well.  I'm guessing that the repeated calls to `setbuf` cause
those characters to be lost, perhaps because `setbuf` flushes the stream, which
for an input stream means it empties the buffer.  And we do call `setbuf` in
this scenario, because interaction between Emacs and GDB in this case is via
pipes.

In general, AFAIK the C standard allows to call `setvbuf` (and thus `setbuf`)
only once, before any I/O operation was done for a given stream.  But after the
commit mentioned above, we call `setbuf` multiple times.

So the proposed patch now changes to an almost full revert of the offending
commit, see the attachment.

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