hush: fix trap

function                                             old     new   delta
expand_variables                                    2217    2280     +63
static.argv                                            -       8      +8

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

shell/hush.c

@@ -5180,6 +5180,47 @@ static FILE *generate_stream_from_string(const char *s)
 		xmove_fd(channel[1], 1);
 		/* Prevent it from trying to handle ctrl-z etc */
 		IF_HUSH_JOB(G.run_list_level = 1;)
+		/* Awful hack for `trap` or $(trap).
+		 *
+		 * http://www.opengroup.org/onlinepubs/009695399/utilities/trap.html
+		 * contains an example where "trap" is executed in a subshell:
+		 *
+		 * save_traps=$(trap)
+		 * ...
+		 * eval "$save_traps"
+		 *
+		 * Standard does not say that "trap" in subshell shall print
+		 * parent shell's traps. It only says that its output
+		 * must have suitable form, but then, in the above example
+		 * (which is not supposed to be normative), it implies that.
+		 *
+		 * bash (and probably other shell) does implement it
+		 * (traps are reset to defaults, but "trap" still shows them),
+		 * but as a result, "trap" logic is hopelessly messed up:
+		 *
+		 * # trap
+		 * trap -- 'echo Ho' SIGWINCH  <--- we have a handler
+		 * # (trap)        <--- trap is in subshell - no output (correct, traps are reset)
+		 * # true | trap   <--- trap is in subshell - no output (ditto)
+		 * # echo `true | trap`    <--- in subshell - output (but traps are reset!)
+		 * trap -- 'echo Ho' SIGWINCH
+		 * # echo `(trap)`         <--- in subshell in subshell - output
+		 * trap -- 'echo Ho' SIGWINCH
+		 * # echo `true | (trap)`  <--- in subshell in subshell in subshell - output!
+		 * trap -- 'echo Ho' SIGWINCH
+		 *
+		 * The rules when to forget and when to not forget traps
+		 * get really complex and nonsensical.
+		 *
+		 * Our solution: ONLY bare $(trap) or `trap` is special.
+		 */
+		s = skip_whitespace(s);
+		if (strncmp(s, "trap", 4) == 0 && (*skip_whitespace(s + 4) == '\0'))
+		{
+			static const char *const argv[] = { NULL, NULL };
+			builtin_trap((char**)argv);
+			exit(0); /* not _exit() - we need to fflush */
+		}
 #if BB_MMU
 		reset_traps_to_defaults();
 		parse_and_run_string(s);
@@ -7057,7 +7098,8 @@ static int FAST_FUNC builtin_trap(char **argv)
 			if (G.traps[i]) {
 				printf("trap -- ");
 				print_escaped(G.traps[i]);
-				printf(" %s\n", get_signame(i));
+				/* bash compat: it says SIGxxx, not just xxx */
+				printf(" SIG%s\n", get_signame(i));
 			}
 		}
 		/*fflush(stdout); - done after each builtin anyway */

shell/hush_test/hush-trap/savetrap.right

@@ -0,0 +1,3 @@
+trap -- 'echo WINCH!' SIGWINCH
+trap -- 'echo WINCH!' SIGWINCH
+Done

shell/hush_test/hush-trap/savetrap.tests

@@ -0,0 +1,6 @@
+trap 'echo WINCH!' SIGWINCH
+v=` trap   `
+echo $v
+v=`trap`
+echo $v
+echo Done