Merge #108: doc: Add comments for socket descriptor handling when forking

b56bf21 doc: Add comments for socket descriptor handling when forking (TheCharlatan)

Pull request description:

  I find logic around forking child processes always a bit confusing, hopefully these comments can make it a bit easier for a future reader.

ACKs for top commit:
  ryanofsky:
    Code review ACK b56bf21

Tree-SHA512: ce7d783d3cfd4b925c12459568e621f74e86ba7f38e534dd3d4e3fb728d743a13c32e3467f9402b16afa16045ff3270ae986019ca9b901c4460e46a61a9cc706

Author: ryanofsky

src/mp/util.cpp

@@ -110,10 +110,12 @@ int SpawnProcess(int& pid, FdToArgsFn&& fd_to_args)
     if (pid == -1) {
         throw std::system_error(errno, std::system_category(), "fork");
     }
+    // Parent process closes the descriptor for socket 0, child closes the descriptor for socket 1.
     if (close(fds[pid ? 0 : 1]) != 0) {
         throw std::system_error(errno, std::system_category(), "close");
     }
     if (!pid) {
+        // Child process must close all potentially open descriptors, except socket 0.
         int maxFd = MaxFd();
         for (int fd = 3; fd < maxFd; ++fd) {
             if (fd != fds[0]) {