Refactor signal handlers (#5730)

Breaking change:
- Harness the SIGCHLD handling, which is required by Process#wait.
  Now we always handle SIGCHLD using SignalChildHandler. Trying to
  reset or ignore SIGCHLD will actually set the default handler,
  trying to trap SIGCHLD will wrap the custom handler instead.

Fixes:
- Synchronize some accesses using a Mutex and an Atomic to further
  enhance potential concurrency issues —probably impossible until
  parallelism is implemented.
- No longer closes the file descriptor at exit, which prevents an
  unhandled exception when receiving a signal while the program is
  exiting.
- Restore STDIN/OUT/ERR blocking state on exit.

Simplify implementation:
- Move private types (SignalHandler, SignalChildHandler) to the
  private Crystal namespace.
- Rename SignalHandler to Crystal::Signal.
- No more singleton classes.
- Using a Channel directly instead of a Concurrent::Future.
- Using macros under enum (it wasn't possible before).
- Introduce LibC::SIG_DFL and LibC::SIG_IGN definitions.

spec/std/signal_spec.cr

@@ -24,5 +24,31 @@ describe "Signal" do
     Process.kill Signal::USR2, Process.pid
   end
 
+  it "CHLD.reset sets default Crystal child handler" do
+    Signal::CHLD.reset
+    child = Process.new("true", shell: true)
+    child.wait # doesn't block forever
+  end
+
+  it "CHLD.ignore sets default Crystal child handler" do
+    Signal::CHLD.ignore
+    child = Process.new("true", shell: true)
+    child.wait # doesn't block forever
+  end
+
+  it "CHLD.trap is called after default Crystal child handler" do
+    called = false
+    child = nil
+
+    Signal::CHLD.trap do
+      called = true
+      Process.exists?(child.not_nil!.pid).should be_false
+    end
+
+    child = Process.new("true", shell: true)
+    child.not_nil!.wait # doesn't block forever
+    called.should be_true
+  end
+
   # TODO: test Signal::X.reset
 end

src/compiler/crystal/command.cr

@@ -211,7 +211,7 @@ class Crystal::Command
           Process.run(output_filename, args: run_args, input: Process::Redirect::Inherit, output: Process::Redirect::Inherit, error: Process::Redirect::Inherit) do |process|
             # Ignore the signal so we don't exit the running process
             # (the running process can still handle this signal)
-            Signal::INT.ignore # do
+            ::Signal::INT.ignore # do
           end
         end
         {$?, elapsed}
@@ -235,11 +235,11 @@ class Crystal::Command
       exit status.exit_code
     else
       case status.exit_signal
-      when Signal::KILL
+      when ::Signal::KILL
         STDERR.puts "Program was killed"
-      when Signal::SEGV
+      when ::Signal::SEGV
         STDERR.puts "Program exited because of a segmentation fault (11)"
-      when Signal::INT
+      when ::Signal::INT
         # OK, bubbled from the sub-program
       else
         STDERR.puts "Program received and didn't handle signal #{status.exit_signal} (#{status.exit_signal.value})"

src/io.cr

@@ -145,7 +145,7 @@ abstract class IO
     # reader.gets # => "hello"
     # reader.gets # => "world"
     # ```
-    def self.pipe(read_blocking = false, write_blocking = false)
+    def self.pipe(read_blocking = false, write_blocking = false) : {IO::FileDescriptor, IO::FileDescriptor}
       pipe_fds = uninitialized StaticArray(LibC::Int, 2)
       if LibC.pipe(pipe_fds) != 0
         raise Errno.new("Could not create pipe")

src/kernel.cr

@@ -189,23 +189,22 @@ class Process
   def self.after_fork_child_callbacks
     @@after_fork_child_callbacks ||= [
       ->Scheduler.after_fork,
-      ->Event::SignalHandler.after_fork,
-      ->{ Event::SignalChildHandler.instance.after_fork },
+      ->Crystal::Signal.after_fork,
+      ->Crystal::SignalChildHandler.after_fork,
       ->Random::DEFAULT.new_seed,
     ] of -> Nil
   end
 end
 
 {% unless flag?(:win32) %}
-  Signal.setup_default_handlers
-
-  at_exit { Event::SignalHandler.close }
-
   # Background loop to cleanup unused fiber stacks.
   spawn do
     loop do
       sleep 5
       Fiber.stack_pool_collect
     end
   end
+
+  Signal.setup_default_handlers
+  LibExt.setup_sigfault_handler
 {% end %}

src/lib_c/aarch64-linux-gnu/c/signal.cr

@@ -36,6 +36,10 @@ lib LibC
   SIGSTKFLT = 16
   SIGUNUSED = 31
 
+  alias SighandlerT = Int ->
+  SIG_DFL = SighandlerT.new(Pointer(Void).new(0_u64), Pointer(Void).null)
+  SIG_IGN = SighandlerT.new(Pointer(Void).new(1_u64), Pointer(Void).null)
+
   fun kill(pid : PidT, sig : Int) : Int
   fun signal(sig : Int, handler : Int -> Void) : Int -> Void
 end

src/lib_c/amd64-unknown-openbsd/c/signal.cr

@@ -34,6 +34,10 @@ lib LibC
   SIGINFO   = 29
   SIGWINCH  = 28
 
+  alias SighandlerT = Int ->
+  SIG_DFL = SighandlerT.new(Pointer(Void).new(0_u64), Pointer(Void).null)
+  SIG_IGN = SighandlerT.new(Pointer(Void).new(1_u64), Pointer(Void).null)
+
   fun kill(x0 : PidT, x1 : Int) : Int
   fun signal(x0 : Int, x1 : Int -> Void) : Int -> Void
 end

src/lib_c/arm-linux-gnueabihf/c/signal.cr

@@ -36,6 +36,10 @@ lib LibC
   SIGSTKFLT = 16
   SIGUNUSED = 31
 
+  alias SighandlerT = Int ->
+  SIG_DFL = SighandlerT.new(Pointer(Void).new(0_u64), Pointer(Void).null)
+  SIG_IGN = SighandlerT.new(Pointer(Void).new(1_u64), Pointer(Void).null)
+
   fun kill(pid : PidT, sig : Int) : Int
   fun signal(sig : Int, handler : Int -> Void) : Int -> Void
 end

src/lib_c/i686-linux-gnu/c/signal.cr

@@ -36,6 +36,10 @@ lib LibC
   SIGSTKFLT = 16
   SIGUNUSED = 31
 
+  alias SighandlerT = Int ->
+  SIG_DFL = SighandlerT.new(Pointer(Void).new(0_u64), Pointer(Void).null)
+  SIG_IGN = SighandlerT.new(Pointer(Void).new(1_u64), Pointer(Void).null)
+
   fun kill(pid : PidT, sig : Int) : Int
   fun signal(sig : Int, handler : Int -> Void) : Int -> Void
 end

src/lib_c/i686-linux-musl/c/signal.cr

@@ -35,6 +35,10 @@ lib LibC
   SIGSTKFLT = 16
   SIGUNUSED = LibC::SIGSYS
 
+  alias SighandlerT = Int ->
+  SIG_DFL = SighandlerT.new(Pointer(Void).new(0_u64), Pointer(Void).null)
+  SIG_IGN = SighandlerT.new(Pointer(Void).new(1_u64), Pointer(Void).null)
+
   fun kill(x0 : PidT, x1 : Int) : Int
   fun signal(x0 : Int, x1 : Int -> Void) : Int -> Void
 end

src/lib_c/x86_64-linux-gnu/c/signal.cr

@@ -36,6 +36,10 @@ lib LibC
   SIGSTKFLT = 16
   SIGUNUSED = 31
 
+  alias SighandlerT = Int ->
+  SIG_DFL = SighandlerT.new(Pointer(Void).new(0_u64), Pointer(Void).null)
+  SIG_IGN = SighandlerT.new(Pointer(Void).new(1_u64), Pointer(Void).null)
+
   fun kill(pid : PidT, sig : Int) : Int
   fun signal(sig : Int, handler : Int -> Void) : Int -> Void
 end

src/lib_c/x86_64-linux-musl/c/signal.cr

@@ -35,6 +35,10 @@ lib LibC
   SIGSTKFLT = 16
   SIGUNUSED = LibC::SIGSYS
 
+  alias SighandlerT = Int ->
+  SIG_DFL = SighandlerT.new(Pointer(Void).new(0_u64), Pointer(Void).null)
+  SIG_IGN = SighandlerT.new(Pointer(Void).new(1_u64), Pointer(Void).null)
+
   fun kill(x0 : PidT, x1 : Int) : Int
   fun signal(x0 : Int, x1 : Int -> Void) : Int -> Void
 end

src/lib_c/x86_64-macosx-darwin/c/signal.cr

@@ -34,6 +34,10 @@ lib LibC
   SIGINFO   = 29
   SIGWINCH  = 28
 
+  alias SighandlerT = Int ->
+  SIG_DFL = SighandlerT.new(Pointer(Void).new(0_u64), Pointer(Void).null)
+  SIG_IGN = SighandlerT.new(Pointer(Void).new(1_u64), Pointer(Void).null)
+
   fun kill(x0 : PidT, x1 : Int) : Int
   fun signal(x0 : Int, x1 : Int -> Void) : Int -> Void
 end

src/lib_c/x86_64-portbld-freebsd/c/signal.cr

@@ -34,6 +34,10 @@ lib LibC
   SIGINFO   = 29
   SIGWINCH  = 28
 
+  alias SighandlerT = Int ->
+  SIG_DFL = SighandlerT.new(Pointer(Void).new(0_u64), Pointer(Void).null)
+  SIG_IGN = SighandlerT.new(Pointer(Void).new(1_u64), Pointer(Void).null)
+
   fun kill(x0 : PidT, x1 : Int) : Int
   fun signal(x0 : Int, x1 : Int -> Void) : Int -> Void
 end