Document the file descriptrs for struct process_io_request

They make sense for local processes, but for remote processes, stdin_fd
and stdout_fd are backwards, and stderr_fd is -1.

Author: DemiMarie

libqrexec/libqrexec-utils.h

@@ -276,6 +276,9 @@ struct process_io_request {
     libvchan_t *vchan;
     struct buffer *stdin_buf;
 
+    /* Note that stdin_fd, stdout_fd, and stderr_fd are named assuming a
+     * *local* process.  For a *remote* process, stdin_fd is the standard
+     * *output*, stdout_fd is the standard *input*, and stderr_fd must be -1. */
     // stderr_fd can be -1
     int stdin_fd, stdout_fd, stderr_fd;
     // 0 if no child process
@@ -284,12 +287,14 @@ struct process_io_request {
     /*
       is_service true (this is a service):
         - send local data as MSG_DATA_STDOUT
+        - send local stderr as MSG_DATA_STDERR, unless in dom0
         - send exit code
         - wait just for local end
         - return local exit code
 
       is_service false (this is a client):
         - send local data as MSG_DATA_STDIN
+        - stderr_fd is always -1
         - don't send exit code
         - wait for local and remote end
         - return remote exit code

libqrexec/process_io.c

@@ -19,6 +19,7 @@
  *
  */
 
+#include <assert.h>
 #include <signal.h>
 #include <sys/wait.h>
 #include <sys/socket.h>
@@ -110,8 +111,10 @@ int process_io(const struct process_io_request *req) {
     set_nonblock(stdin_fd);
     if (stdout_fd != stdin_fd)
         set_nonblock(stdout_fd);
-    if (stderr_fd >= 0)
+    if (stderr_fd >= 0) {
+        assert(is_service); // if this is a client, stderr_fd is *always* -1
         set_nonblock(stderr_fd);
+    }
 
     /* Convenience macros that eliminate a ton of error-prone boilerplate */
 #define close_stdin() do {                      \