gdb: don't print backtrace when dumping core after an internal error

Currently, when GDB hits an internal error, and the user selects to
dump core, the recently added feature to write a backtrace to the
console will kick in, and print a backtrace as well as dumping the
core.

This was certainly not my intention when adding the backtrace on fatal
signal functionality, this feature was intended to produce a backtrace
when GDB crashes due to some fatal signal, internal errors should have
continued to behave as they did before, unchanged.

In this commit I set the signal disposition of SIGABRT back to SIG_DFL
just prior to the call to abort() that GDB uses to trigger the core
dump, this prevents GDB reaching the code that writes the backtrace to
the console.

I've also added a test that checks we don't see a backtrace on the
console after an internal error.

gdb/testsuite/gdb.base/bt-on-fatal-signal.exp

@@ -135,3 +135,39 @@ foreach test_data {{SEGV "Segmentation fault"} \
 	gdb_exit
     }
 }
+
+# Check that when we get an internal error and choose to dump core, we
+# don't print a backtrace to the console.
+with_test_prefix "internal-error" {
+    # Restart GDB.
+    clean_restart $binfile
+
+    set saw_bt_start false
+
+    gdb_test_multiple "maint internal-error foo" "" {
+	-early -re "internal-error: foo\r\n" {
+	    exp_continue
+	}
+	-early -re "^A problem internal to GDB has been detected,\r\n" {
+	    exp_continue
+	}
+	-early -re "^further debugging may prove unreliable\\.\r\n" {
+	    exp_continue
+	}
+	-early -re "^Quit this debugging session\\? \\(y or n\\)" {
+	    send_gdb "y\n"
+	    exp_continue
+	}
+	-early -re "^Create a core file of GDB\\? \\(y or n\\)" {
+	    send_gdb "y\n"
+	    exp_continue
+	}
+	-early -re "----- Backtrace -----\r\n" {
+	    set saw_bt_start true
+	    exp_continue
+	}
+	eof {
+	    gdb_assert { [expr ! $saw_bt_start] }
+	}
+    }
+}

gdb/utils.c

@@ -201,6 +201,11 @@ dump_core (void)
   setrlimit (RLIMIT_CORE, &rlim);
 #endif /* HAVE_SETRLIMIT */
 
+  /* Ensure that the SIGABRT we're about to raise will immediately cause
+     GDB to exit and dump core, we don't want to trigger GDB's printing of
+     a backtrace to the console here.  */
+  signal (SIGABRT, SIG_DFL);
+
   abort ();		/* ARI: abort */
 }