Add Signal#trap_handler?

spec/std/signal_spec.cr

@@ -29,13 +29,35 @@ describe "Signal" do
         sleep 0.1
       end
       ran.should be_true
+    ensure
+      Signal::USR1.reset
     end
 
     it "ignores a signal" do
       Signal::USR2.ignore
       Process.signal Signal::USR2, Process.pid
     end
 
+    it "allows chaining of signals" do
+      ran_first = false
+      ran_second = false
+
+      Signal::USR1.trap { ran_first = true }
+      existing = Signal::USR1.trap_handler?
+
+      Signal::USR1.trap do |signal|
+        existing.try &.call(signal)
+        ran_second = true
+      end
+
+      Process.signal Signal::USR1, Process.pid
+      sleep 0.1
+      ran_first.should be_true
+      ran_second.should be_true
+    ensure
+      Signal::USR1.reset
+    end
+
     it "CHLD.reset sets default Crystal child handler" do
       Signal::CHLD.reset
       child = Process.new("true", shell: true)

src/crystal/system/signal.cr

@@ -2,6 +2,9 @@ module Crystal::System::Signal
   # Sets the handler for this signal to the passed function.
   # def self.trap(signal, handler) : Nil
 
+  # Returns any existing handler set on the signal
+  # def self.trap_handler?(signal)
+
   # Resets the handler for this signal to the OS default.
   # def self.reset(signal) : Nil
 

src/crystal/system/unix/signal.cr

@@ -15,7 +15,7 @@ module Crystal::System::Signal
   @@pipe = IO.pipe(read_blocking: false, write_blocking: true)
   @@handlers = {} of ::Signal => Handler
   @@sigset = Sigset.new
-  class_setter child_handler : Handler?
+  class_property child_handler : Handler?
   @@mutex = Mutex.new(:unchecked)
 
   def self.trap(signal, handler) : Nil
@@ -30,6 +30,10 @@ module Crystal::System::Signal
     end
   end
 
+  def self.trap_handler?(signal)
+    @@mutex.synchronize { @@handlers[signal]? }
+  end
+
   def self.reset(signal) : Nil
     set(signal, LibC::SIG_DFL)
   end

src/crystal/system/wasi/signal.cr

@@ -3,6 +3,10 @@ module Crystal::System::Signal
     raise NotImplementedError.new("Crystal::System::Signal.trap")
   end
 
+  def self.trap_handler?(signal)
+    raise NotImplementedError.new("Crystal::System::Signal.trap_handler?")
+  end
+
   def self.reset(signal) : Nil
     raise NotImplementedError.new("Crystal::System::Signal.reset")
   end

src/crystal/system/win32/signal.cr

@@ -5,6 +5,10 @@ module Crystal::System::Signal
     raise NotImplementedError.new("Crystal::System::Signal.trap")
   end
 
+  def self.trap_handler?(signal)
+    raise NotImplementedError.new("Crystal::System::Signal.trap_handler?")
+  end
+
   def self.reset(signal) : Nil
     raise NotImplementedError.new("Crystal::System::Signal.reset")
   end

src/signal.cr

@@ -117,6 +117,24 @@ enum Signal : Int32
     Crystal::System::Signal.trap(self, handler)
   end
 
+  # Returns any existing handler for this signal
+  #
+  # ```
+  # Signal::USR1.trap { }
+  # prev_handler = Signal::USR1.trap_handler?
+  #
+  # Signal::USR1.trap do |signal|
+  #   prev_handler.try &.call(signal)
+  #   # ...
+  # end
+  # ```
+  def trap_handler?
+    {% if @type.has_constant?("CHLD") %}
+      return Crystal::System::Signal.child_handler if self == CHLD
+    {% end %}
+    Crystal::System::Signal.trap_handler?(self)
+  end
+
   # Resets the handler for this signal to the OS default.
   #
   # Note that trying to reset `CHLD` will actually set the default crystal