libc/stdio: only roll FILE state back on EINTR

Reported by:	JunT <[email protected]>
PR:	266171
Reviewed by:	emaste, markj
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D36427

lib/libc/stdio/fflush.c

@@ -136,7 +136,8 @@ __sflush(FILE *fp)
 				fp->_p += n;
 				if ((fp->_flags & (__SLBF | __SNBF)) == 0)
 					fp->_w -= n;
-			} else if (p == fp->_p) { /* cond. to handle setvbuf */
+			/* conditional to handle setvbuf */
+			} else if (p == fp->_p && errno == EINTR) {
 				fp->_p = old_p;
 				fp->_w = old_w;
 			}

lib/libc/stdio/fvwrite.c

@@ -38,6 +38,7 @@ static char sccsid[] = "@(#)fvwrite.c	8.1 (Berkeley) 6/4/93";
 #include <sys/cdefs.h>
 __FBSDID("$FreeBSD$");
 
+#include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -140,7 +141,7 @@ __sfvwrite(FILE *fp, struct __suio *uio)
 				fp->_p += w;
 				old_p = fp->_p;
 				if (__fflush(fp) == EOF) {
-					if (old_p == fp->_p)
+					if (old_p == fp->_p && errno == EINTR)
 						fp->_p -= w;
 					goto err;
 				}
@@ -184,7 +185,7 @@ __sfvwrite(FILE *fp, struct __suio *uio)
 				fp->_p += w;
 				old_p = fp->_p;
 				if (__fflush(fp) == EOF) {
-					if (old_p == fp->_p)
+					if (old_p == fp->_p && errno == EINTR)
 						fp->_p -= w;
 					goto err;
 				}

lib/libc/stdio/wbuf.c

@@ -91,7 +91,7 @@ __swbuf(int c, FILE *fp)
 	old_p = fp->_p;
 	if (++n == fp->_bf._size || (fp->_flags & __SLBF && c == '\n')) {
 		if (__fflush(fp) != 0) {
-			if (fp->_p == old_p) {
+			if (fp->_p == old_p && errno == EINTR) {
 				fp->_p--;
 				fp->_w++;
 			}