gdb/guile: don't try to print location for watchpoints

Currently, using the guile API, if a user tries to print a breakpoint
object that represents a watchpoint, then GDB will crash.  For
example:

  (gdb) guile (use-modules (gdb))
  (gdb) guile (define wp1 (make-breakpoint "some_variable" #:type BP_WATCHPOINT #:wp-class WP_WRITE))
  (gdb) guile (register-breakpoint! wp1)
  (gdb) guile (display wp1) (newline)
  Aborted (core dumped)

This turns out to be because GDB calls event_location_to_string on the
breakpoints location, and watchpoint breakpoints don't have a
location.

This commit resolves the crash by just skipping the printing of the
location if the breakpoint doesn't have one.

Potentially, we could improve on this by printing details about what
the watchpoint is watching, however, I'm considering this a possible
future enhancement, this commit focuses just on having GDB not crash.

gdb/ChangeLog:

	* guile/scm-breakpoint.c (bpscm_print_breakpoint_smob): Only print
	breakpoint locations when the breakpoint actually has a location.

gdb/testsuite/ChangeLog:

	* gdb.guile/scm-breakpoint.exp (test_watchpoints): Print the
	watchpoint object before and after registering it with GDB.

1 files changed, 5 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.guile/scm-breakpoint.exp b/gdb/testsuite/gdb.guile/scm-breakpoint.exp
index 9d27173..5605894 100644
--- a/gdb/testsuite/gdb.guile/scm-breakpoint.exp
+++ b/gdb/testsuite/gdb.guile/scm-breakpoint.exp
@@ -248,8 +248,13 @@ proc_with_prefix test_watchpoints { } {
 
     gdb_scm_test_silent_cmd  "guile (define wp1 (make-breakpoint \"result\" #:type BP_WATCHPOINT #:wp-class WP_WRITE))" \
 	"create watchpoint"
+    gdb_test "guile (display wp1) (newline)" "#<gdb:breakpoint #-1>" \
+	"print watchpoint before registering"
     gdb_scm_test_silent_cmd "guile (register-breakpoint! wp1)" \
 	"register wp1"
+    gdb_test "guile (display wp1) (newline)" \
+	"#<gdb:breakpoint #${decimal} BP_(?:HARDWARE_)?WATCHPOINT enabled noisy hit:0 ignore:0>" \
+	"print watchpoint after registering"
     gdb_test "continue" \
 	".*\[Ww\]atchpoint.*result.*Old value = 0.*New value = 25.*main.*" \
 	"test watchpoint write"