ash: fix `trap`

function                                             old     new   delta
forkshell                                            738     810     +72
popstring                                            134     140      +6
parse_command                                       1460    1463      +3
evalvar                                             1373    1371      -2
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 3/1 up/down: 81/-2)              Total: 79 bytes

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

4 files changed, 68 insertions, 8 deletions

diff --git a/shell/ash.c b/shell/ash.c
index 4360770d4..68aa675e5 100644
--- a/shell/ash.c
+++ b/shell/ash.c
@@ -4553,7 +4553,52 @@ forkchild(struct job *jp, union node *n, int mode)
          * Do we do it correctly? */
 
         closescript();
-        clear_traps();
+
+        if (mode == FORK_NOJOB          /* is it `xxx` ? */
+         && n && n->type == NCMD        /* is it single cmd? */
+        /* && n->ncmd.args->type == NARG - always true? */
+         && strcmp(n->ncmd.args->narg.text, "trap") == 0
+         && n->ncmd.args->narg.next == NULL /* "trap" with no arguments */
+        /* && n->ncmd.args->narg.backquote == NULL - do we need to check this? */
+        ) {
+                TRACE(("Trap hack\n"));
+                /* 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.
+                 */
+        } else {
+                clear_traps();
+        }
 #if JOBS
         /* do job control only in root shell */
         doing_jobctl = 0;
@@ -4597,8 +4642,14 @@ forkchild(struct job *jp, union node *n, int mode)
                 setsignal(SIGQUIT);
         }
 #if JOBS
-        if (n && n->type == NCMD && strcmp(n->ncmd.args->narg.text, "jobs") == 0) {
+        if (n && n->type == NCMD
+         && strcmp(n->ncmd.args->narg.text, "jobs") == 0
+        ) {
                 TRACE(("Job hack\n"));
+                /* "jobs": we do not want to clear job list for it,
+                 * instead we remove only _its_ own_ job from job list.
+                 * This makes "jobs .... | cat" more useful.
+                 */
                 freejob(curjob);
                 return;
         }
@@ -12208,7 +12259,7 @@ trapcmd(int argc UNUSED_PARAM, char **argv UNUSED_PARAM)
         if (!*ap) {
                 for (signo = 0; signo < NSIG; signo++) {
                         if (trap[signo] != NULL) {
-                                out1fmt("trap -- %s %s\n",
+                                out1fmt("trap -- %s SIG%s\n",
                                                 single_quote(trap[signo]),
                                                 get_signame(signo));
                         }
diff --git a/shell/ash_test/ash-signals/savetrap.right b/shell/ash_test/ash-signals/savetrap.right
new file mode 100644
index 000000000..2d33427aa
--- /dev/null
+++ b/shell/ash_test/ash-signals/savetrap.right
@@ -0,0 +1,3 @@
+trap -- 'echo WINCH!' SIGWINCH
+trap -- 'echo WINCH!' SIGWINCH
+Done
diff --git a/shell/ash_test/ash-signals/savetrap.tests b/shell/ash_test/ash-signals/savetrap.tests
new file mode 100755
index 000000000..6492e86a2
--- /dev/null
+++ b/shell/ash_test/ash-signals/savetrap.tests
@@ -0,0 +1,6 @@
+trap 'echo WINCH!' SIGWINCH
+v=` trap   `
+echo $v
+v=`trap`
+echo $v
+echo Done
diff --git a/shell/ash_test/ash-signals/signal1.right b/shell/ash_test/ash-signals/signal1.right
index cf403ac62..beb0a988e 100644
--- a/shell/ash_test/ash-signals/signal1.right
+++ b/shell/ash_test/ash-signals/signal1.right
@@ -1,20 +1,20 @@
 got signal
-trap -- 'echo got signal' USR1
+trap -- 'echo got signal' SIGUSR1
 sent 1 signal
 got signal
 wait interrupted
-trap -- 'echo got signal' USR1
+trap -- 'echo got signal' SIGUSR1
 sent 2 signal
 got signal
 wait interrupted
-trap -- 'echo got signal' USR1
+trap -- 'echo got signal' SIGUSR1
 sent 3 signal
 got signal
 wait interrupted
-trap -- 'echo got signal' USR1
+trap -- 'echo got signal' SIGUSR1
 sent 4 signal
 got signal
 wait interrupted
-trap -- 'echo got signal' USR1
+trap -- 'echo got signal' SIGUSR1
 sent 5 signal
 sleep completed