testsuite: add print-stack.exp

I wrote this support file to help me debug Tcl issues in the
testsuite.

Adding a call to:

  print_stack_backtrace

somewhere in a .exp file (along with "load_lib print-stack.exp") leads
to the interpreter printing a backtrace in a form that e.g. Emacs can
consume, with filename:linenum: lines, and quoting the line of .exp
source code.

Fer example, adding a print_stack_backtrace to scansarif.exp in
run-sarif-pytest I get this output:

VVV START OF BACKTRACE VVV
  /home/david/coding/gcc-newgit/src/gcc/testsuite/lib/scansarif.exp:142: frame 16 in proc print_stack_backtrace
    142 |     print_stack_backtrace
  <proc>: frame 15 in proc run-sarif-pytest
  <eval>: frame 14 in proc dg-final-proc
  /usr/share/dejagnu/dg.exp:851: frame 13 in proc dg-final-proc
    851 | 	if {[catch "dg-final-proc $prog" errmsg]} {
  <eval>: frame 12 in proc saved-dg-test
  /home/david/coding/gcc-newgit/src/gcc/testsuite/lib/gcc-dg.exp:1080: frame 11 in proc saved-dg-test
    1080 | 	if { [ catch { eval saved-dg-test $args } errmsg ] } {
  /usr/share/dejagnu/dg.exp:559: frame 10 in proc dg-test
    559 | 	dg-test $testcase $options ${default-extra-options}
  /home/david/coding/gcc-newgit/src/gcc/testsuite/gcc.dg/sarif-output/sarif-output.exp:28: frame 9
    28 | dg-runtest [lsort [glob -nocomplain $srcdir/$subdir/*.c]] "" ""
  <eval>: frame 8
  <eval>: frame 7
  /usr/share/dejagnu/runtest.exp:1460: frame 6
    1460 | 	if { [catch "uplevel #0 source $test_file_name"] == 1 } {
  /usr/share/dejagnu/runtest.exp:1886: frame 5 in proc dg-runtest
    1886 | 			runtest $test_name
  /usr/share/dejagnu/runtest.exp:1845: frame 4 in proc dg-runtest
    1845 | 		    foreach test_name [lsort [find ${dir} *.exp]] {
  /usr/share/dejagnu/runtest.exp:1788: frame 3 in proc dg-runtest
    1788 | 	    foreach dir "${test_top_dirs}" {
  /usr/share/dejagnu/runtest.exp:1669: frame 2 in proc dg-runtest
    1669 |     foreach pass $multipass {
  /usr/share/dejagnu/runtest.exp:1619: frame 1 in proc dg-runtest
    1619 | foreach current_target $target_list {
^^^  END OF BACKTRACE  ^^^

and can click on the lines in Emacs's compilation buffer to take
me to the relevant places.

I found this made it *much* easier to debug my .exp files.  That
said, I'm uncomfortable with Tcl, and so
(a) there may be a better way of doing this
(b) I may have made mistakes

gcc/testsuite/ChangeLog:
	* lib/print-stack.exp: New file.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>

gcc/testsuite/lib/print-stack.exp

@@ -0,0 +1,62 @@
+# Copyright (C) 2024 Free Software Foundation, Inc.
+#  Contributed by David Malcolm <dmalcolm@redhat.com>.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with GCC; see the file COPYING3.  If not see
+# <http://www.gnu.org/licenses/>.
+
+# Get the 1-based line for LINENUM from FILENAME as a string
+
+proc get_line { filename linenum } {
+    set f [open $filename]
+    set lines [split [read $f] \n]
+    close $f
+    return [lindex $lines [expr $linenum - 1] ]
+}
+
+# Print a backtrace of the Tcl interpreter's stack, showing
+# frames, levels, source file and line where available.
+#
+# This isn't used anywhere, but is occasionally very helpful
+# to use when debugging.
+
+proc print_stack_backtrace {} {
+    set current_frame_level [info frame]
+    puts "VVV START OF BACKTRACE VVV"
+    for {set i [expr $current_frame_level - 1]} {$i > 0} {incr i -1} {
+	set frame [info frame $i]
+	if { [dict exists $frame "level"] } {
+	    set level_num [dict get $frame "level"]
+	    set relative_level_offset [expr 1 - $level_num]
+	    set level [info level $relative_level_offset]
+	    set procname [lindex $level 0]
+	    # TODO: args = rest of $level, but this can be very long
+	} else {
+	    set procname ""
+	}
+	set suffix ""
+	if { $procname != "" } {
+	    set suffix " in proc $procname"
+	}
+	if { [dict get $frame "type"] == "source" } {
+	    set fname [dict get $frame "file"]
+	    set line [dict get $frame "line"]
+	    puts "  $fname:$line: frame $i$suffix"
+	    puts "    $line | [get_line $fname $line]"
+	} else {
+	    set type [dict get $frame "type"]
+	    puts "  <$type>: frame $i$suffix"
+	}
+    }
+    puts "^^^  END OF BACKTRACE  ^^^"
+}