Optimize handle_interrupt(Exception => ..) as a common case

When interrupt behavior is configured for all possible exceptions using
'Exception', there's no need to iterate the pending exception's
ancestors for hash lookups.

More significantly, by storing the catch-all timing symbol directly in
the mask stack, we can skip allocating the hash we would otherwise need.

thread.c

@@ -1891,6 +1891,23 @@ enum handle_interrupt_timing {
     INTERRUPT_NEVER
 };
 
+static enum handle_interrupt_timing
+rb_threadptr_pending_interrupt_from_symbol(rb_thread_t *th, VALUE sym)
+{
+    if (sym == sym_immediate) {
+        return INTERRUPT_IMMEDIATE;
+    }
+    else if (sym == sym_on_blocking) {
+        return INTERRUPT_ON_BLOCKING;
+    }
+    else if (sym == sym_never) {
+        return INTERRUPT_NEVER;
+    }
+    else {
+        rb_raise(rb_eThreadError, "unknown mask signature");
+    }
+}
+
 static enum handle_interrupt_timing
 rb_threadptr_pending_interrupt_check_mask(rb_thread_t *th, VALUE err)
 {
@@ -1903,6 +1920,16 @@ rb_threadptr_pending_interrupt_check_mask(rb_thread_t *th, VALUE err)
     for (i=0; i<mask_stack_len; i++) {
         mask = mask_stack[mask_stack_len-(i+1)];
 
+        if (SYMBOL_P(mask)) {
+            /* do not match RUBY_FATAL_THREAD_KILLED etc */
+            if (err != rb_cInteger) {
+                return rb_threadptr_pending_interrupt_from_symbol(th, mask);
+            }
+            else {
+                continue;
+            }
+        }
+
         for (mod = err; mod; mod = RCLASS_SUPER(mod)) {
             VALUE klass = mod;
             VALUE sym;
@@ -1915,18 +1942,7 @@ rb_threadptr_pending_interrupt_check_mask(rb_thread_t *th, VALUE err)
             }
 
             if ((sym = rb_hash_aref(mask, klass)) != Qnil) {
-                if (sym == sym_immediate) {
-                    return INTERRUPT_IMMEDIATE;
-                }
-                else if (sym == sym_on_blocking) {
-                    return INTERRUPT_ON_BLOCKING;
-                }
-                else if (sym == sym_never) {
-                    return INTERRUPT_NEVER;
-                }
-                else {
-                    rb_raise(rb_eThreadError, "unknown mask signature");
-                }
+                return rb_threadptr_pending_interrupt_from_symbol(th, sym);
             }
         }
         /* try to next mask */
@@ -2018,12 +2034,24 @@ handle_interrupt_arg_check_i(VALUE key, VALUE val, VALUE args)
         rb_raise(rb_eArgError, "unknown mask signature");
     }
 
-    if (maskp) {
-        if (!*maskp) {
+    if (key == rb_eException && (UNDEF_P(*maskp) || NIL_P(*maskp))) {
+        *maskp = val;
+        return ST_CONTINUE;
+    }
+
+    if (RTEST(*maskp)) {
+        if (!RB_TYPE_P(*maskp, T_HASH)) {
+            VALUE prev = *maskp;
             *maskp = rb_ident_hash_new();
+            if (SYMBOL_P(prev)) {
+                rb_hash_aset(*maskp, rb_eException, prev);
+            }
         }
         rb_hash_aset(*maskp, key, val);
     }
+    else {
+        *maskp = Qfalse;
+    }
 
     return ST_CONTINUE;
 }
@@ -2139,7 +2167,7 @@ handle_interrupt_arg_check_i(VALUE key, VALUE val, VALUE args)
 static VALUE
 rb_thread_s_handle_interrupt(VALUE self, VALUE mask_arg)
 {
-    VALUE mask;
+    VALUE mask = Qundef;
     rb_execution_context_t * volatile ec = GET_EC();
     rb_thread_t * volatile th = rb_ec_thread_ptr(ec);
     volatile VALUE r = Qnil;
@@ -2152,14 +2180,19 @@ rb_thread_s_handle_interrupt(VALUE self, VALUE mask_arg)
     mask_arg = rb_to_hash_type(mask_arg);
 
     if (OBJ_FROZEN(mask_arg) && rb_hash_compare_by_id_p(mask_arg)) {
-        rb_hash_foreach(mask_arg, handle_interrupt_arg_check_i, 0);
+        mask = Qnil;
+    }
+
+    rb_hash_foreach(mask_arg, handle_interrupt_arg_check_i, (VALUE)&mask);
+
+    if (UNDEF_P(mask)) {
+        return rb_yield(Qnil);
+    }
+
+    if (!RTEST(mask)) {
         mask = mask_arg;
-    } else {
-        mask = 0;
-        rb_hash_foreach(mask_arg, handle_interrupt_arg_check_i, (VALUE)&mask);
-        if (!mask) {
-            return rb_yield(Qnil);
-        }
+    }
+    else if (RB_TYPE_P(mask, T_HASH)) {
         OBJ_FREEZE_RAW(mask);
     }