Fix reorder Process.lock_write outside of block_signals (#16431)

Signals shouldn't be blocked while the process potentially waits to acquire the write lock. So `lock_write` must be the outermost block.

Author: straight-shoota

src/crystal/system/unix/process.cr

@@ -197,22 +197,30 @@ struct Crystal::System::Process
   def self.fork
     {% raise("Process fork is unsupported with multithreaded mode") if flag?(:preview_mt) %}
 
-    block_signals do
-      case pid = lock_write { LibC.fork }
-      when 0
-        # forked process
-
-        ::Process.after_fork_child_callbacks.each(&.call)
-
-        nil
-      when -1
-        # forking process: error
-        raise RuntimeError.from_errno("fork")
-      else
-        # forking process: success
-        pid
+    result = lock_write do
+      block_signals do
+        case pid = LibC.fork
+        when 0
+          # forked process
+
+          ::Process.after_fork_child_callbacks.each(&.call)
+
+          nil
+        when -1
+          # forking process: error
+          Errno.value
+        else
+          # forking process: success
+          pid
+        end
       end
     end
+
+    if result.is_a?(Errno)
+      raise RuntimeError.from_os_error("fork", result)
+    else
+      result
+    end
   end
 
   private def self.block_signals(&)

src/crystal/system/unix/spawn.cr

@@ -59,25 +59,33 @@ struct Crystal::System::Process
   end
 
   private def self.fork_for_exec
-    block_signals do |sigmask|
-      case pid = lock_write { LibC.fork }
-      when 0
-        # forked process
+    result = lock_write do
+      block_signals do |sigmask|
+        case pid = LibC.fork
+        when 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)
 
-        nil
-      when -1
-        # forking process: error
-        raise RuntimeError.from_errno("fork")
-      else
-        # forking process: success
-        pid
+          nil
+        when -1
+          # forking process: error
+          Errno.value
+        else
+          # forking process: success
+          pid
+        end
       end
     end
+
+    if result.is_a?(Errno)
+      raise RuntimeError.from_os_error("fork", result)
+    else
+      result
+    end
   end
 
   # This method is similar to `.replace` (used for `Process.exec`) with some