thread.c (thread_join_m): handle negative timeouts correctly

Users may subtract and round into negative values when using
Thread#join, so clamp the timeout to zero to avoid infinite/long
timeouts.

Note: other methods such as Kernel#sleep and IO.select will
raise on negative values, but Thread#join is an outlier *shrug*

This restores Ruby 2.5 (and earlier) behavior.

Fixes: r62182 (commit c915390b95)
       ("thread.c: avoid FP for Thread#join")

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62462 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_thread.rb

@@ -234,6 +234,13 @@ class TestThread < Test::Unit::TestCase
       t = Thread.new {}
       assert_same t, t.join(limit), "limit=#{limit.inspect}"
     end
+    t = Thread.new { sleep }
+    [ -1, -0.1, RbConfig::LIMITS['FIXNUM_MIN'], RbConfig::LIMITS['INT64_MIN'],
+      -Float::INFINITY
+    ].each do |limit|
+      assert_nil t.join(limit), "limit=#{limit.inspect}"
+    end
+    t.kill
   end
 
   def test_kill_main_thread

thread.c

@@ -1061,6 +1061,8 @@ thread_join_m(int argc, VALUE *argv, VALUE self)
       case T_NIL: break;
       case T_FIXNUM:
         timespec.tv_sec = NUM2TIMET(limit);
+        if (timespec.tv_sec < 0)
+            timespec.tv_sec = 0;
         timespec.tv_nsec = 0;
         ts = &timespec;
         break;
@@ -1116,8 +1118,8 @@ double2timespec(struct timespec *ts, double d)
     if (TIMESPEC_SEC_MAX_PLUS_ONE <= d) {
         return NULL;
     }
-    else if (d <= TIMESPEC_SEC_MIN) {
-        ts->tv_sec = TIMESPEC_SEC_MIN;
+    else if (d <= 0) {
+        ts->tv_sec = 0;
         ts->tv_nsec = 0;
     }
     else {