Fix fragile specs (#1041)

* Use monotonic time if possible to guard against clock skew
* Relax time comparisons to make test failures under load less likely
* Fix memory corruption segfaults on macOS due to mixing Threads and Timeout

Without the timeout thread safety fix, the specs always crash in the
"threaded queries should be supported" spec for various reasons on
macOS 10.14.4 with ruby 2.6.1 / MySQL 8.0.15.

Some observed errors:

    malloc: *** error for object 0x7fcbba90ef80: pointer being freed was not allocated
    malloc: *** set a breakpoint in malloc_error_break to debug

    lib/mysql2/client.rb:131:in `_query': Bad file descriptor (Errno::EBADF)

Now specs run fine, even during Prime95 load test on i9-8950HK (6c/12t).

spec/mysql2/client_spec.rb

@@ -607,29 +607,29 @@ def run_gc
       # XXX this test is not deterministic (because Unix signal handling is not)
       # and may fail on a loaded system
       it "should run signal handlers while waiting for a response" do
-        kill_time = 0.1
-        query_time = 2 * kill_time
+        kill_time = 0.25
+        query_time = 4 * kill_time
 
         mark = {}
 
         begin
-          trap(:USR1) { mark.store(:USR1, Time.now) }
+          trap(:USR1) { mark.store(:USR1, clock_time) }
           pid = fork do
             sleep kill_time # wait for client query to start
             Process.kill(:USR1, Process.ppid)
             sleep # wait for explicit kill to prevent GC disconnect
           end
-          mark.store(:QUERY_START, Time.now)
+          mark.store(:QUERY_START, clock_time)
           @client.query("SELECT SLEEP(#{query_time})")
-          mark.store(:QUERY_END, Time.now)
+          mark.store(:QUERY_END, clock_time)
         ensure
           Process.kill(:TERM, pid)
           Process.waitpid2(pid)
           trap(:USR1, 'DEFAULT')
         end
 
         # the query ran uninterrupted
-        expect(mark.fetch(:QUERY_END) - mark.fetch(:QUERY_START)).to be_within(0.02).of(query_time)
+        expect(mark.fetch(:QUERY_END) - mark.fetch(:QUERY_START)).to be_within(0.1).of(query_time)
         # signals fired while the query was running
         expect(mark.fetch(:USR1)).to be_between(mark.fetch(:QUERY_START), mark.fetch(:QUERY_END))
       end
@@ -694,6 +694,7 @@ def run_gc
         sleep_time = 0.5
 
         # Note that each thread opens its own database connection
+        start = clock_time
         threads = Array.new(5) do
           Thread.new do
             new_client do |client|
@@ -702,10 +703,12 @@ def run_gc
             Thread.current.object_id
           end
         end
+        values = threads.map(&:value)
+        stop = clock_time
 
-        # This timeout demonstrates that the threads are sleeping concurrently:
-        # In the serial case, the timeout would fire and the test would fail
-        values = Timeout.timeout(sleep_time * 1.1) { threads.map(&:value) }
+        # This check demonstrates that the threads are sleeping concurrently:
+        # In the serial case, the difference would be a multiple of sleep time
+        expect(stop - start).to be_within(0.1).of(sleep_time)
 
         expect(values).to match_array(threads.map(&:object_id))
       end

spec/spec_helper.rb

@@ -40,6 +40,17 @@ def num_classes
     # rubocop:enable Lint/UnifiedInteger
   end
 
+  # Use monotonic time if possible (ruby >= 2.1.0)
+  if defined?(Process::CLOCK_MONOTONIC)
+    def clock_time
+      Process.clock_gettime Process::CLOCK_MONOTONIC
+    end
+  else
+    def clock_time
+      Time.now.to_f
+    end
+  end
+
   config.before :each do
     @client = new_client
   end