diff: Fix pagination leak

* Drop an unnecessary variable and rename pidfd to procd.

* Rewinding stdout serves no purpose, so stop doing it.

* Don't bother freeing memory or setting the global status right
  before erroring out.

* Error out if dup(2) or dup2(2) fail.

* In the unlikely case that our pipe is equal to stdout, we need to
  record that information so we don't close it when cleaning up.

* Don't bother closing a descriptor before dup2(2)ing to it.

* Don't forget to close the the process descriptor after reaping the
  child process.

PR:		266592
MFC after:	1 week
Sponsored by:	Klara, Inc.
Reviewed by:	kevans, markj
Differential Revision:	https://reviews.freebsd.org/D55112

usr.bin/diff/pr.c

@@ -45,7 +45,6 @@ struct pr *
 start_pr(char *file1, char *file2)
 {
 	int pfd[2];
-	int pr_pd;
 	pid_t pid;
 	char *header;
 	struct pr *pr;
@@ -55,13 +54,10 @@ start_pr(char *file1, char *file2)
 	xasprintf(&header, "%s %s %s", diffargs, file1, file2);
 	signal(SIGPIPE, SIG_IGN);
 	fflush(stdout);
-	rewind(stdout);
 	if (pipe(pfd) == -1)
 		err(2, "pipe");
-	switch ((pid = pdfork(&pr_pd, PD_CLOEXEC))) {
+	switch ((pid = pdfork(&pr->procd, PD_CLOEXEC))) {
 	case -1:
-		status |= 2;
-		free(header);
 		err(2, "No more processes");
 	case 0:
 		/* child */
@@ -73,15 +69,17 @@ start_pr(char *file1, char *file2)
 		execl(_PATH_PR, _PATH_PR, "-h", header, (char *)0);
 		_exit(127);
 	default:
-		pr->pidfd = pr_pd;
 		/* parent */
-		if (pfd[1] != STDOUT_FILENO) {
-			pr->ostdout = dup(STDOUT_FILENO);
-			dup2(pfd[1], STDOUT_FILENO);
+		if (pfd[1] == STDOUT_FILENO) {
+			pr->ostdout = STDOUT_FILENO;
+		} else {
+			if ((pr->ostdout = dup(STDOUT_FILENO)) < 0 ||
+			    dup2(pfd[1], STDOUT_FILENO) < 0) {
+				err(2, "stdout");
+			}
 			close(pfd[1]);
 		}
 		close(pfd[0]);
-		rewind(stdout);
 		free(header);
 	}
 	return (pr);
@@ -98,14 +96,14 @@ stop_pr(struct pr *pr)
 
 	fflush(stdout);
 	if (pr->ostdout != STDOUT_FILENO) {
-		close(STDOUT_FILENO);
 		dup2(pr->ostdout, STDOUT_FILENO);
 		close(pr->ostdout);
 	}
-	while (pdwait(pr->pidfd, &wstatus, WEXITED, NULL, NULL) == -1) {
+	while (pdwait(pr->procd, &wstatus, WEXITED, NULL, NULL) == -1) {
 		if (errno != EINTR)
 			err(2, "pdwait");
 	}
+	close(pr->procd);
 	free(pr);
 	if (WIFEXITED(wstatus) && WEXITSTATUS(wstatus) != 0)
 		errx(2, "pr exited abnormally");

usr.bin/diff/pr.h

@@ -26,7 +26,7 @@
 
 struct pr {
 	int ostdout;
-	int pidfd;
+	int procd;
 };
 
 struct pr *start_pr(char *file1, char *file2);