mdocml: Fix a process group race.

[schwarze@mdocml.bsd.lv]

Log Message:
-----------
Fix a process group race.

It could occasionally happen that the child process spawned less(1)
before the parent process passed the control of the terminal to the
child, and in that case, less(1) sometimes complained "Stopped (tty 
output)".  Issue reported by naddy@.

Modified Files:
--------------
    mdocml:
        main.c

Revision Data
-------------
Index: main.c
===================================================================
RCS file: /home/cvs/mdocml/mdocml/main.c,v
retrieving revision 1.264
retrieving revision 1.265
diff -Lmain.c -Lmain.c -u -p -r1.264 -r1.265
--- main.c
+++ main.c
@@ -35,6 +35,7 @@
 #include <stdint.h>
 #include <stdlib.h>
 #include <string.h>
+#include <time.h>
 #include <unistd.h>
 
 #include "mandoc_aux.h"
@@ -1020,6 +1021,7 @@ mmsg(enum mandocerr t, enum mandoclevel 
 static pid_t
 spawn_pager(struct tag_files *tag_files)
 {
+	const struct timespec timeout = { 0, 100000000 };  /* 0.1s */
 #define MAX_PAGER_ARGS 16
 	char		*argv[MAX_PAGER_ARGS];
 	const char	*pager;
@@ -1069,8 +1071,6 @@ spawn_pager(struct tag_files *tag_files)
 	case -1:
 		err((int)MANDOCLEVEL_SYSERR, "fork");
 	case 0:
-		/* Set pgrp in both parent and child to avoid racing exec. */
-		(void)setpgid(0, 0);
 		break;
 	default:
 		(void)setpgid(pager_pid, 0);
@@ -1089,6 +1089,12 @@ spawn_pager(struct tag_files *tag_files)
 		err((int)MANDOCLEVEL_SYSERR, "pager stdout");
 	close(tag_files->ofd);
 	close(tag_files->tfd);
+
+	/* Do not start the pager before controlling the terminal. */
+
+	while (tcgetpgrp(STDIN_FILENO) != getpid())
+		nanosleep(&timeout, NULL);
+
 	execvp(argv[0], argv);
 	err((int)MANDOCLEVEL_SYSERR, "exec %s", argv[0]);
 }
--
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