[PATCHv2] Fix the crash at the end of the runtest

[Andrew Burgess <andrew.burgess@embecosm.com>]

Thanks for the feedback.  Below is the patch I intend to push first
thing tomorrow unless anyone objects.

Thanks,
Andrew

---

commit 41424acf01830d3cafb838a7d865985182edf47a
Author: Andrew Burgess <andrew.burgess@embecosm.com>
Date:   Thu Jul 22 14:07:15 2021 +0100

    gdb/testsuite: don't error when trying to unset last_spawn_tty_name
    
    In spawn_capture_tty_name (lib/gdb.exp) we either set or unset
    last_spawn_tty_name depending on whether spawn_out(slave,name) exists
    or not.
    
    One situation that might cause spawn_out(slave,name) to not exists is
    if the spawn function is called with the argument -leaveopen, which is
    how it is called when processes are created as part of a pipeline, the
    created process has no tty, instead its output is written to a file
    descriptor.
    
    If a pipeline is created consisting of multiple processes then there
    will be multiple sequential calls to spawn, all using -leaveopen.  The
    first of these calls is fine, spawn_out(slave,name) is not set, and so
    in spawn_capture_tty_name we unset last_spawn_tty_name.  However, on
    the second call to spawn, spawn_out(slave,name) is still not set and
    so in spawn_capture_tty_name we again try to unset
    last_spawn_tty_name, this now throws an error (as last_spawn_tty_name
    is already unset).
    
    Fix this issue by using -nocomplain with the call to unset in
    spawn_capture_tty_name.
    
    Before this commit I was seeing gdb.base/gnu-debugdata.exp report 1
    pass, and 1 unsupported test.  After this commit I now see 16 passes
    from this test script.
    
    I have also improved the code that used to do this:
    
        if { [info exists spawn_out] } {
            set ::last_spawn_tty_name $spawn_out(slave,name)
        } else {
            ...
        }
    
    The problem here is that we check for the existence of spawn_out, and
    then unconditionally read spawn_out(slave,name).  A situation could
    arise where some other element of spawn_out is set,
    e.g. spawn_out(foo), in which case we would enter the if block and try
    to read a non-existent variable.  After this commit we now check
    specifically for spawn_out(slave,name).
    
    Finally, it is worth noting that before this issue was fixed runtest
    itself, or rather the expect process behind runtest, would segfault
    while exiting.  I haven't looked at all into what the problem is here
    that caused expect to crash, as fixing the bug in GDB's testing
    scripts made the segfault go away.

diff --git a/gdb/testsuite/lib/gdb.exp b/gdb/testsuite/lib/gdb.exp
index e79e0622f9d..aeb8585bab9 100644
--- a/gdb/testsuite/lib/gdb.exp
+++ b/gdb/testsuite/lib/gdb.exp
@@ -2029,10 +2029,20 @@ proc gdb_file_cmd { arg } {
 proc spawn_capture_tty_name { args } {
     set result [uplevel builtin_spawn $args]
     upvar spawn_out spawn_out
-    if { [info exists spawn_out] } {
+    if { [info exists spawn_out(slave,name)] } {
 	set ::last_spawn_tty_name $spawn_out(slave,name)
     } else {
-	unset ::last_spawn_tty_name
+	# If a process is spawned as part of a pipe line (e.g. passing
+	# -leaveopen to the spawn proc) then the spawned process is no
+	# assigned a tty and spawn_out(slave,name) will not be set.
+	# In that case we want to ensure that last_spawn_tty_name is
+	# not set.
+	#
+	# If the previous process spawned was also not assigned a tty
+	# (e.g. multiple processed chained in a pipeline) then
+	# last_spawn_tty_name will already be unset, so, if we don't
+	# use -nocomplain here we would otherwise get an error.
+	unset -nocomplain ::last_spawn_tty_name
     }
     return $result
 }