Fix testsuite hangs when gdb_test_multiple body errors out

Fix testsuite hangs when gdb_test_multiple body errors out

This commit fixes a regression in the testsuite itself, triggered by
errors being raised from within gdb_test_multiple, originally reported
by Pedro Franco de Carvalho's at
<https://sourceware.org/ml/gdb-patches/2019-03/msg00160.html>.  Parts
of the commit message are based on his report.

This started happening due to a commit that was introduced recently,
and it can cause the testsuite to hang.

The commit that triggers this is:

 fe1a5cad302b5535030cdf62895e79512713d738
 [gdb/testsuite] Log wait status on process no longer exists error

That commit introduces a new "eof" block in gdb_test_multiple.  That
is not incorrect itself, but dejagnu's remote_expect is picking that
block as the "default" action when an error is raised from within the
commands inside a call to gdb_test_multiple:

  # remote_expect works basically the same as standard expect, but it
  # also takes care of getting the file descriptor from the specified
  # host and also calling the timeout/eof/default section if there is an
  # error on the expect call.
  #
  proc remote_expect { board timeout args } {

I find that "feature" surprising, and I don't really know why it
exists, but this means that the eof section that remote_expect picks
as the error block can be executed even when there was no actual eof
and the GDB process is still running, so the wait introduced in the
commit that tries to get the exit status of GDB hangs forever, while
GDB itself waits for input.

This only happens when there are internal testsuite errors (not
testcase failures).  This can be reproduced easily with a testcase
such as:

  gdb_start
  gdb_test_multiple "show version" "show version" {
    -re ".*" {
       error "forced error"
    }
  }

I think that working around this in GDB is useful so that the
testsuite doesn't hang in these cases.

Adding an empty "default" block at the end of the expect body in
gdb_test_multiple doesn't work, because dejagnu gives preference to
"eof" blocks:

	    if { $x eq "eof" } {
		set save_next 1
	    } elseif { $x eq "default" || $x eq "timeout" } {
		if { $error_sect eq "" } {
		    set save_next 1
		}
	    }

And we do have "eof" blocks.  So we need to make sure that the last
"eof" block is safe to use as the default error block.  It's also
pedantically incorrect to print

 "ERROR: Process no longer exists"

which is what we'd get if the last eof block we have was selected
(more below on this).

So this commit solves this by appending an "eof" with an empty
spawn_id list, so that it won't ever match.

Now, why is the first "eof" block selected today as the error block,
instead of the last one?

The reason is that remote_expect, while parsing the body to select the
default block to execute after an error, is affected by the comments
in the body (since they are also parsed).

If this comment in gdb_test_multiple

 # patterns below apply to any spawn id specified.

is changed to

 # The patterns below apply to any spawn id specified.

then the second eof block is selected and there is no hang.

Any comment at that same place with an even number of tokens also
works.

This is IMO a coincidence caused by how comments work in TCL.
Comments should only appear in places where a command can appear.  And
here, remote_expect is parsing a list of options, not commands, so
it's not unreasonable to not parse comments, similarly to how this:

  set a_list {
     an_element
     # another_element
  }

results in a list with three elements, not one element.

The fact that comments with an even number of tokens work is just a
coincidence of how remote_expect's little state machine is
implemented.

I thought we could solve this by stripping out comment lines in
gdb_expect, but I didn't find an easy way to do that.  Particularly, a
couple naive approaches I tried run into complications.  For example,
we have gdb_test calls with regular expressions that include sequences
like "\r\n#", and by the time we get to gdb_expect, the \r\n have
already been expanded to a real newline, so just splitting the whole
body at newline boundaries, looking for lines that start with #
results in incorrectly stripping out half of the gdb_text regexp.  I
think it's better (at least in this commit), to move the comments out
of the list, because it's much simpler and risk free.

gdb/testsuite/ChangeLog:
2019-03-25  Pedro Alves  <palves@redhat.com>

	* lib/gdb.exp (gdb_test_multiple): Split appends to $code and
	move comments outside list.  Append '-i "" eof' section.