Fix try/catch catches more than it should #1859

Close #1885, #2140, #2011, #2220, #2485, #2073

Rename the FORK_OPT opcode to TRY_BEGIN, add a TRY_END opcode, and wrap
errors when raising through a TRY_END so that they will not be caught by
the matching TRY_BEGIN.

Now a `try exp catch handler` expression generates code like:

    TRY_BEGIN handler
    <exp>
    TRY_END
    JUMP past_handler
    handler: <handler>
    past_handler:
    ...

On backtrack through TRY_BEGIN it just backtracks.

If anything past the whole thing raises when <exp> produced a value,
then the TRY_END will catch the error, wrap it in another, and
backtrack.  The TRY_BEGIN will see a wrapped error and then it will
unwrap and re-raise the error.

If <exp> raises, then TRY_BEGIN will catch the error and jump to the
handler, but the TRY_BEGIN will not stack_save() in that case, so on
raise/backtrack the TRY_BEGIN will not execute again (nor will the
TRY_END).

src/execute.c

@@ -348,6 +348,7 @@ jv jq_next(jq_state *jq) {
 
   int raising;
   int backtracking = !jq->initial_execution;
+
   jq->initial_execution = 0;
   assert(jv_get_kind(jq->error) == JV_KIND_NULL);
   while (1) {
@@ -808,7 +809,59 @@ jv jq_next(jq_state *jq) {
       break;
     }
 
-    case FORK_OPT:
+    case TRY_BEGIN:
+      stack_save(jq, pc - 1, stack_get_pos(jq));
+      pc++; // skip handler offset this time
+      break;
+
+    case TRY_END:
+      stack_save(jq, pc - 1, stack_get_pos(jq));
+      break;
+
+    case ON_BACKTRACK(TRY_BEGIN): {
+      if (!raising) {
+        /*
+         * `try EXP ...` -- EXP backtracked (e.g., EXP was `empty`), so we
+         * backtrack more:
+         */
+        jv_free(stack_pop(jq));
+        goto do_backtrack;
+      }
+
+      /*
+       * Else `(try EXP ... ) | EXP2` raised an error.
+       *
+       * If the error was wrapped in another error, then that means EXP2 raised
+       * the error.  We unwrap it and re-raise it as it wasn't raised by EXP.
+       *
+       * See commentary in gen_try().
+       */
+      jv e = jv_invalid_get_msg(jv_copy(jq->error));
+      if (!jv_is_valid(e) && jv_invalid_has_msg(jv_copy(e))) {
+        set_error(jq, e);
+        goto do_backtrack;
+      }
+      jv_free(e);
+
+      /*
+       * Else we caught an error containing a non-error value, so we jump to
+       * the handler.
+       *
+       * See commentary in gen_try().
+       */
+      uint16_t offset = *pc++;
+      jv_free(stack_pop(jq)); // free the input
+      stack_push(jq, jv_invalid_get_msg(jq->error));  // push the error's message
+      jq->error = jv_null();
+      pc += offset;
+      break;
+    }
+    case ON_BACKTRACK(TRY_END):
+      // Wrap the error so the matching TRY_BEGIN doesn't catch it
+      if (raising)
+        set_error(jq, jv_invalid_with_msg(jv_copy(jq->error)));
+      goto do_backtrack;
+
     case DESTRUCTURE_ALT:
     case FORK: {
       stack_save(jq, pc - 1, stack_get_pos(jq));
@@ -816,7 +869,6 @@ jv jq_next(jq_state *jq) {
       break;
     }
 
-    case ON_BACKTRACK(FORK_OPT):
     case ON_BACKTRACK(DESTRUCTURE_ALT): {
       if (jv_is_valid(jq->error)) {
         // `try EXP ...` backtracked here (no value, `empty`), so we backtrack more