From 1d638d9700dbf454f40c66499f6fc0471f177aab Mon Sep 17 00:00:00 2001
From: zrj <rimvydas.jasinskas@gmail.com>
Date: Sat, 23 Dec 2017 10:29:47 +0200
Subject: [PATCH] fflush(3): Return success on a read-only stream.

Loosen up the requirements for opened streams. It is needed to support c++
runtime lib after ext/basic_file_stdio.c s/this->sync()/fflush()/ change.
It likely needed for libc++ too.
Adjust the manpage for behaviour change and only set _w to 0 when stream is
not currently reading in fpurge(4) to avoid failures in fflush(4).

Taken-from: FreeBSD
---
 lib/libc/stdio/fflush.3 | 14 ++++++++------
 lib/libc/stdio/fflush.c | 25 +++++++++++++++----------
 lib/libc/stdio/fpurge.c |  2 +-
 3 files changed, 24 insertions(+), 17 deletions(-)

diff --git a/lib/libc/stdio/fflush.3 b/lib/libc/stdio/fflush.3
index 42e8e17fbf..ffbe40f7ae 100644
--- a/lib/libc/stdio/fflush.3
+++ b/lib/libc/stdio/fflush.3
@@ -31,9 +31,8 @@
 .\"
 .\"     @(#)fflush.3	8.1 (Berkeley) 6/4/93
 .\" $FreeBSD: src/lib/libc/stdio/fflush.3,v 1.11 2007/01/09 00:28:06 imp Exp $
-.\" $DragonFly: src/lib/libc/stdio/fflush.3,v 1.3 2004/06/08 00:29:47 hmp Exp $
 .\"
-.Dd June 4, 1993
+.Dd December 23, 2017
 .Dt FFLUSH 3
 .Os
 .Sh NAME
@@ -88,9 +87,7 @@ is set to indicate the error.
 The
 .Fa stream
 argument
-is not an open stream, or, in the case of
-.Fn fflush ,
-not a stream open for writing.
+is not an open stream.
 .El
 .Pp
 The function
@@ -98,7 +95,12 @@ The function
 may also fail and set
 .Va errno
 for any of the errors specified for the routine
-.Xr write 2 .
+.Xr write 2 ,
+except that in case of
+.Fa stream
+being a read-only descriptor,
+.Fn fflush
+returns 0.
 .Sh SEE ALSO
 .Xr write 2 ,
 .Xr fclose 3 ,
diff --git a/lib/libc/stdio/fflush.c b/lib/libc/stdio/fflush.c
index 9905662d55..93e2ff7bce 100644
--- a/lib/libc/stdio/fflush.c
+++ b/lib/libc/stdio/fflush.c
@@ -66,13 +66,12 @@ fflush(FILE *fp)
 	 * Given that applications may be written with the expectation of
 	 * either of these two behaviours, the only safe (non-astonishing)
 	 * option is to return EBADF and ask that applications be fixed.
+	 * But for c++ compatibility do not enforce it and match FreeBSD.
 	 */
-	if ((fp->pub._flags & (__SWR | __SRW)) == 0) {
-		errno = EBADF;
-		retval = EOF;
-	} else {
+	if ((fp->pub._flags & (__SWR | __SRW)) == 0)
+		retval = 0;
+	else
 		retval = __sflush(fp);
-	}
 	FUNLOCKFILE(fp);
 	return (retval);
 }
@@ -88,12 +87,10 @@ __fflush(FILE *fp)
 
 	if (fp == NULL)
 		return (_fwalk(sflush_locked));
-	if ((fp->pub._flags & (__SWR | __SRW)) == 0) {
-		errno = EBADF;
-		retval = EOF;
-	} else {
+	if ((fp->pub._flags & (__SWR | __SRW)) == 0)
+		retval = 0;
+	else
 		retval = __sflush(fp);
-	}
 	return (retval);
 }
 
@@ -122,6 +119,14 @@ __sflush(FILE *fp)
 	for (; n > 0; n -= t, p += t) {
 		t = _swrite(fp, (char *)p, n);
 		if (t <= 0) {
+			/* Reset _p and _w. */
+			if (p > fp->pub._p) {
+				/* Some was written. */
+				memmove(fp->pub._p, p, n);
+				fp->pub._p += n;
+				if ((fp->pub._flags & (__SLBF | __SNBF)) == 0)
+					fp->pub._w -= n;
+			}
 			fp->pub._flags |= __SERR;
 			return (EOF);
 		}
diff --git a/lib/libc/stdio/fpurge.c b/lib/libc/stdio/fpurge.c
index c9d4b7be21..08ae2b5b18 100644
--- a/lib/libc/stdio/fpurge.c
+++ b/lib/libc/stdio/fpurge.c
@@ -58,7 +58,7 @@ fpurge(FILE *fp)
 			FREEUB(fp);
 		fp->pub._p = fp->_bf._base;
 		fp->pub._r = 0;
-		fp->pub._w = fp->pub._flags & (__SLBF|__SNBF) ? 0 : fp->_bf._size;
+		fp->pub._w = fp->pub._flags & (__SLBF|__SNBF|__SRD) ? 0 : fp->_bf._size;
 		retval = 0;
 	}
 	FUNLOCKFILE(fp);
-- 
2.11.4.GIT