system/FileDescriptor: add method IsReadyForWriting()

src/system/FileDescriptor.cxx

@@ -222,4 +222,10 @@ FileDescriptor::WaitWritable(int timeout) const noexcept
 	return Poll(POLLOUT, timeout);
 }
 
+bool
+FileDescriptor::IsReadyForWriting() const noexcept
+{
+	return WaitWritable(0) > 0;
+}
+
 #endif

src/system/FileDescriptor.hxx

@@ -180,6 +180,9 @@ public:
 
 	int WaitReadable(int timeout) const noexcept;
 	int WaitWritable(int timeout) const noexcept;
+
+	gcc_pure
+	bool IsReadyForWriting() const noexcept;
 #endif
 };