Make gdb.base/corefile.exp work on terminals with few rows

When creating a pty to spawn a subprocess (such as gdb), Expect
copies the settings of its own controlling terminal, including the
number of rows and columns.  If you "make check" on a terminal with just
a few rows (e.g. 4), GDB will paginate before reaching the initial
prompt.  In default_gdb_start, used by most tests, this is already
handled: if we see the pagination prompt, we sent \n to continue.

Philippe reported that gdb.base/corefile.exp didn't work in terminals
with just a few rows.  This test spawns GDB by hand, because it needs to
check things before the initial prompt, which it couldn't do if it used
default_gdb_start.

In this case I think it's not safe to use the same technique as in
default_gdb_start.  Even if we could send a \n if we see a pagination
prompt, we match some multiline regexes in there.  So if a pagination
slips in there, it might make the regexes not match and fail the test.

It's also not possible to use -ex "set height 0" or -iex "set height 0",
it is handled after the introduction text is shown.

The simplest way I found to avoid showing the pagination completely is
to set stty_init (documented in expect's man page) to initialize gdb's
pty with a fixed number of rows.

And actually, if we set stty_init in gdb_init, it works nicely as a
general solution applicable to all tests.  We can therefore remove the
solution introduced in e882ef3cfc3 ("testsuite: expect possible
pagination when starting gdb") where we matched the pagination prompt
during startup.

gdb/testsuite/ChangeLog:

	* lib/gdb.exp (default_gdb_start): Don't match pagination
	prompt.
	(gdb_init): Set stty_init.

gdb/testsuite/ChangeLog

@@ -1,3 +1,9 @@
+2019-02-07  Simon Marchi  <simon.marchi@polymtl.ca>
+
+	* lib/gdb.exp (default_gdb_start): Don't match pagination
+	prompt.
+	(gdb_init): Set stty_init.
+
 2019-01-27  Tom Tromey  <tom@tromey.com>
 
 	* gdb.python/py-finish-breakpoint.exp: Remove duplicate call to

gdb/testsuite/lib/gdb.exp

@@ -1638,7 +1638,7 @@ proc default_gdb_spawn { } {
 # Default gdb_start procedure.
 
 proc default_gdb_start { } {
-    global gdb_prompt pagination_prompt
+    global gdb_prompt
     global gdb_spawn_id
     global inferior_spawn_id
 
@@ -1659,29 +1659,20 @@ proc default_gdb_start { } {
     # When running over NFS, particularly if running many simultaneous
     # tests on different hosts all using the same server, things can
     # get really slow.  Give gdb at least 3 minutes to start up.
-    set loop_again 1
-    while { $loop_again } {
-	set loop_again 0
-	gdb_expect 360 {
-	    -re "$pagination_prompt" {
-		verbose "Hit pagination during startup. Pressing enter to continue."
-		send_gdb "\n"
-		set loop_again 1
-	    }
-	    -re "\[\r\n\]$gdb_prompt $" {
-		verbose "GDB initialized."
-	    }
-	    -re "$gdb_prompt $"	{
-		perror "GDB never initialized."
-		unset gdb_spawn_id
-		return -1
-	    }
-	    timeout	{
-		perror "(timeout) GDB never initialized after 10 seconds."
-		remote_close host
-		unset gdb_spawn_id
-		return -1
-	    }
+    gdb_expect 360 {
+	-re "\[\r\n\]$gdb_prompt $" {
+	    verbose "GDB initialized."
+	}
+	-re "$gdb_prompt $"	{
+	    perror "GDB never initialized."
+	    unset gdb_spawn_id
+	    return -1
+	}
+	timeout	{
+	    perror "(timeout) GDB never initialized after 10 seconds."
+	    remote_close host
+	    unset gdb_spawn_id
+	    return -1
 	}
     }
 
@@ -4752,6 +4743,11 @@ proc gdb_init { test_file_name } {
     # tests.
     setenv TERM "dumb"
 
+    # Initialize GDB's pty with a fixed size, to make sure we avoid pagination
+    # during startup.  See "man expect" for details about stty_init.
+    global stty_init
+    set stty_init "rows 25 cols 80"
+
     # Some tests (for example gdb.base/maint.exp) shell out from gdb to use
     # grep.  Clear GREP_OPTIONS to make the behavior predictable,
     # especially having color output turned on can cause tests to fail.