Add Process.pthread_disable_cancelstate (#16446)

`fork` in a multi-threaded process is a very delicate matter. We must avoid cancelling the thread while performing a fork. Cleanup could cause problems while we're setting up the new process.

This would not be necessary if we were using `vfork` semantics. That's generally the better choice for spawning a new process, as there is no need to copy the entire process memory. `vfork` is more efficient, and we should ideally use that.

However, we're hoping to switch to `posix_spawn` eventually (#11337), which provides a safe and efficient implementation, and we don't need to deal with the details explicitly.
This currently blocked on the need to reopen stdio in the housekeeping stage (#16353) but we're expecting to resolve that soon.

Author: straight-shoota

src/crystal/system/unix/process.cr

@@ -209,9 +209,11 @@ struct Crystal::System::Process
     {% raise("Process fork is unsupported with multithreaded mode") if flag?(:preview_mt) %}
 
     pid, errno = lock_write do
-      block_signals do
-        pid = LibC.fork
-        {pid, Errno.value}
+      pthread_disable_cancelstate do
+        block_signals do
+          pid = LibC.fork
+          {pid, Errno.value}
+        end
       end
     end
 
@@ -252,6 +254,21 @@ struct Crystal::System::Process
     end
   end
 
+  private def self.pthread_disable_cancelstate(&)
+    # No thread cancellation on android
+    {% if flag?(:android) %}
+      yield
+    {% else %}
+      LibC.pthread_setcancelstate(LibC::PTHREAD_CANCEL_DISABLE, out cancel_state)
+
+      begin
+        yield
+      ensure
+        LibC.pthread_setcancelstate(cancel_state, nil)
+      end
+    {% end %}
+  end
+
   # Duplicates the current process.
   # Returns a `Process` representing the new child process in the current process
   # and `nil` inside the new child process.

src/crystal/system/unix/spawn.cr

@@ -60,17 +60,19 @@ struct Crystal::System::Process
 
   private def self.fork_for_exec
     pid, errno = lock_write do
-      block_signals do |sigmask|
-        pid = LibC.fork
-        if pid == 0
-          # forked process
+      pthread_disable_cancelstate do
+        block_signals do |sigmask|
+          pid = LibC.fork
+          if pid == 0
+            # forked process
 
-          Crystal::System::Signal.after_fork_before_exec
+            Crystal::System::Signal.after_fork_before_exec
 
-          # reset sigmask (inherited on exec)
-          LibC.sigemptyset(sigmask)
+            # reset sigmask (inherited on exec)
+            LibC.sigemptyset(sigmask)
+          end
+          {pid, Errno.value}
         end
-        {pid, Errno.value}
       end
     end
 

src/lib_c/aarch64-darwin/c/pthread.cr

@@ -3,6 +3,9 @@ require "./sys/types"
 lib LibC
   PTHREAD_MUTEX_ERRORCHECK = 1
 
+  PTHREAD_CANCEL_ENABLE  = 0
+  PTHREAD_CANCEL_DISABLE = 1
+
   fun pthread_condattr_destroy(x0 : PthreadCondattrT*) : Int
   fun pthread_condattr_init(x0 : PthreadCondattrT*) : Int
   fun pthread_cond_broadcast(x0 : PthreadCondT*) : Int
@@ -25,5 +28,6 @@ lib LibC
   fun pthread_mutex_trylock(x0 : PthreadMutexT*) : Int
   fun pthread_mutex_unlock(x0 : PthreadMutexT*) : Int
   fun pthread_self : PthreadT
+  fun pthread_setcancelstate(state : Int, oldstate : Int*) : Int
   fun pthread_setname_np(Char*) : Int
 end

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

@@ -11,6 +11,9 @@ require "./sys/types"
 lib LibC
   PTHREAD_MUTEX_ERRORCHECK = 2
 
+  PTHREAD_CANCEL_ENABLE  = 0
+  PTHREAD_CANCEL_DISABLE = 1
+
   fun pthread_attr_destroy(attr : PthreadAttrT*) : Int
   fun pthread_attr_getstack(addr : PthreadAttrT*, stackaddr : Void**, stacksize : SizeT*) : Int
   fun pthread_condattr_destroy(attr : PthreadCondattrT*) : Int
@@ -37,4 +40,8 @@ lib LibC
   fun pthread_mutex_unlock(mutex : PthreadMutexT*) : Int
   fun pthread_self : PthreadT
   fun pthread_setname_np(PthreadT, Char*) : Int
+
+  # Set cancellability state of current thread to STATE, returning old
+  # state in *OLDSTATE if OLDSTATE is not NULL.
+  fun pthread_setcancelstate(__state : Int, __oldstate : Int*) : Int
 end

src/lib_c/aarch64-linux-musl/c/pthread.cr

@@ -3,6 +3,9 @@ require "./sys/types"
 lib LibC
   PTHREAD_MUTEX_ERRORCHECK = 2
 
+  PTHREAD_CANCEL_ENABLE  = 0
+  PTHREAD_CANCEL_DISABLE = 1
+
   fun pthread_attr_destroy(x0 : PthreadAttrT*) : Int
   fun pthread_attr_getstack(x0 : PthreadAttrT*, x1 : Void**, x2 : SizeT*) : Int
   fun pthread_condattr_destroy(x0 : PthreadCondattrT*) : Int
@@ -27,5 +30,6 @@ lib LibC
   fun pthread_mutex_trylock(x0 : PthreadMutexT*) : Int
   fun pthread_mutex_unlock(x0 : PthreadMutexT*) : Int
   fun pthread_self : PthreadT
+  fun pthread_setcancelstate(Int, Int*) : Int
   fun pthread_setname_np(PthreadT, Char*) : Int
 end

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

@@ -11,6 +11,9 @@ require "./sys/types"
 lib LibC
   PTHREAD_MUTEX_ERRORCHECK = 2
 
+  PTHREAD_CANCEL_ENABLE  = 0
+  PTHREAD_CANCEL_DISABLE = 1
+
   fun pthread_attr_destroy(attr : PthreadAttrT*) : Int
   fun pthread_attr_getstack(addr : PthreadAttrT*, stackaddr : Void**, stacksize : SizeT*) : Int
   fun pthread_condattr_destroy(attr : PthreadCondattrT*) : Int
@@ -37,4 +40,8 @@ lib LibC
   fun pthread_mutex_unlock(mutex : PthreadMutexT*) : Int
   fun pthread_self : PthreadT
   fun pthread_setname_np(PthreadT, Char*) : Int
+
+  # Set cancellability state of current thread to STATE, returning old
+  # state in *OLDSTATE if OLDSTATE is not NULL.
+  fun pthread_setcancelstate(__state : Int, __oldstate : Int*) : Int
 end

src/lib_c/i386-linux-gnu/c/pthread.cr

@@ -11,6 +11,9 @@ require "./sys/types"
 lib LibC
   PTHREAD_MUTEX_ERRORCHECK = 2
 
+  PTHREAD_CANCEL_ENABLE  = 0
+  PTHREAD_CANCEL_DISABLE = 1
+
   fun pthread_attr_destroy(attr : PthreadAttrT*) : Int
   fun pthread_attr_getstack(addr : PthreadAttrT*, stackaddr : Void**, stacksize : SizeT*) : Int
   fun pthread_condattr_destroy(attr : PthreadCondattrT*) : Int
@@ -36,4 +39,8 @@ lib LibC
   fun pthread_mutex_unlock(mutex : PthreadMutexT*) : Int
   fun pthread_self : PthreadT
   fun pthread_setname_np(PthreadT, Char*) : Int
+
+  # Set cancellability state of current thread to STATE, returning old
+  # state in *OLDSTATE if OLDSTATE is not NULL.
+  fun pthread_setcancelstate(__state : Int, __oldstate : Int*) : Int
 end

src/lib_c/i386-linux-musl/c/pthread.cr

@@ -3,6 +3,9 @@ require "./sys/types"
 lib LibC
   PTHREAD_MUTEX_ERRORCHECK = 2
 
+  PTHREAD_CANCEL_ENABLE  = 0
+  PTHREAD_CANCEL_DISABLE = 1
+
   fun pthread_attr_destroy(x0 : PthreadAttrT*) : Int
   fun pthread_attr_getstack(x0 : PthreadAttrT*, x1 : Void**, x2 : SizeT*) : Int
   fun pthread_condattr_destroy(x0 : PthreadCondattrT*) : Int
@@ -27,5 +30,6 @@ lib LibC
   fun pthread_mutex_trylock(x0 : PthreadMutexT*) : Int
   fun pthread_mutex_unlock(x0 : PthreadMutexT*) : Int
   fun pthread_self : PthreadT
+  fun pthread_setcancelstate(Int, Int*) : Int
   fun pthread_setname_np(PthreadT, Char*) : Int
 end

src/lib_c/x86_64-darwin/c/pthread.cr

@@ -3,6 +3,9 @@ require "./sys/types"
 lib LibC
   PTHREAD_MUTEX_ERRORCHECK = 1
 
+  PTHREAD_CANCEL_ENABLE  = 0
+  PTHREAD_CANCEL_DISABLE = 1
+
   fun pthread_condattr_destroy(x0 : PthreadCondattrT*) : Int
   fun pthread_condattr_init(x0 : PthreadCondattrT*) : Int
   fun pthread_cond_broadcast(x0 : PthreadCondT*) : Int
@@ -25,5 +28,6 @@ lib LibC
   fun pthread_mutex_trylock(x0 : PthreadMutexT*) : Int
   fun pthread_mutex_unlock(x0 : PthreadMutexT*) : Int
   fun pthread_self : PthreadT
+  fun pthread_setcancelstate(state : Int, oldstate : Int*) : Int
   fun pthread_setname_np(Char*) : Int
 end

src/lib_c/x86_64-dragonfly/c/pthread.cr

@@ -4,6 +4,10 @@ require "./sys/types"
 lib LibC
   PTHREAD_MUTEX_ERRORCHECK = 1
 
+  # Flags for cancelling threads
+  PTHREAD_CANCEL_ENABLE  = 0
+  PTHREAD_CANCEL_DISABLE = 1
+
   fun pthread_attr_destroy(x0 : PthreadAttrT*) : Int
   fun pthread_attr_get_np(x0 : PthreadT, x1 : PthreadAttrT*) : Int
   fun pthread_attr_getstack(x0 : PthreadAttrT*, x1 : Void**, x2 : SizeT*) : Int
@@ -29,5 +33,6 @@ lib LibC
   fun pthread_mutex_trylock(x0 : PthreadMutexT*) : Int
   fun pthread_mutex_unlock(x0 : PthreadMutexT*) : Int
   fun pthread_self : PthreadT
+  fun pthread_setcancelstate(Int, Int*) : Int
   fun pthread_setname_np(PthreadT, Char*) : Int
 end