[Bug server/29796] New: gdbserver should imply --once if connected via stdio

["pedro at palves dot net" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=29796

            Bug ID: 29796
           Summary: gdbserver should imply --once if connected via stdio
           Product: gdb
           Version: HEAD
            Status: NEW
          Severity: normal
          Priority: P2
         Component: server
          Assignee: unassigned at sourceware dot org
          Reporter: pedro at palves dot net
  Target Milestone: ---

(gdb) target extended-remote | gdbserver --multi -
Remote debugging using | gdbserver --multi -
Remote debugging using stdio
(gdb) quit
* hang for 5 seconds *

You get the hang plus the PIPE_CLOSE_TIMEOUT timeout, because gdbserver hangs
while writing:

  "Remote side has terminated connection.  GDBserver will reopen the
connection.\n"

to stderr.  Like so:

(gdb) bt
#0  0x00007fa124ff9077 in __GI___libc_write (fd=2, buf=0x560d25e89158,
nbytes=78) at ../sysdeps/unix/sysv/linux/write.c:26
#1  0x00007fa124f79e8d in _IO_new_file_write (f=0x7fa1250d85c0
<_IO_2_1_stderr_>, data=0x560d25e89158, n=78) at fileops.c:1176
#2  0x00007fa124f7a7a8 in new_do_write (to_do=<optimized out>,
data=0x560d25e89158 "Remote side has terminated connection.  GDBserver will
reopen the connection.\n", fp=0x7fa1250d85c0 <_IO_2_1_stderr_>) at libioP.h:948
#3  _IO_new_file_xsputn (n=78, data=<optimized out>, f=<optimized out>) at
fileops.c:1255
#4  _IO_new_file_xsputn (f=0x7fa1250d85c0 <_IO_2_1_stderr_>, data=<optimized
out>, n=78) at fileops.c:1197
#5  0x00007fa124f6e3c1 in __GI__IO_fwrite (buf=0x560d25e89158, size=1,
count=78, fp=0x7fa1250d85c0 <_IO_2_1_stderr_>) at libioP.h:948
#6  0x0000560d25e081c4 in captured_main (argc=3, argv=0x7ffcfd97ceb8) at
../../src/gdbserver/server.cc:4145
#7  0x0000560d25e0840f in main (argc=3, argv=0x7ffcfd97ceb8) at
../../src/gdbserver/server.cc:4211


If you pass --once to gdbserver, then the hang disappears, since gdbserver
exits as soon as the connection closes:

(gdb) target extended-remote | gdbserver --once --multi -
Remote debugging using | ../gdbserver/gdbserver --once --multi -
Remote debugging using stdio
(gdb) q
$ # got back the shell prompt immediately.


I'm thinking that gdbserver should auto-imply --once when connected via stdio.

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