nice(): Put back old return value, keeping [EPERM] error.

Commit r279154 changed the API and ABI significantly, and {NZERO} is still
wrong.

Also, preserve errno on success instead of setting it to 0.

PR:		189821
Reported by:	bde
Relnotes:	yes

2 files changed, 19 insertions, 12 deletions

diff --git a/lib/libc/gen/nice.3 b/lib/libc/gen/nice.3
index 3bad5f7..8ce13af 100644
--- a/lib/libc/gen/nice.3
+++ b/lib/libc/gen/nice.3
@@ -28,7 +28,7 @@
 .\"     @(#)nice.3	8.1 (Berkeley) 6/4/93
 .\" $FreeBSD$
 .\"
-.Dd February 22, 2015
+.Dd February 28, 2015
 .Dt NICE 3
 .Os
 .Sh NAME
@@ -48,9 +48,9 @@ This interface is obsoleted by
 .Pp
 The
 .Fn nice
-function obtains the scheduling priority of the process
-from the system and sets it to the priority value specified in
-.Fa incr .
+function adds
+.Fa incr
+to the scheduling priority of the process.
 The priority is a value in the range -20 to 20.
 The default priority is 0; lower priorities cause more favorable scheduling.
 Only the super-user may lower priorities.
@@ -60,8 +60,9 @@ Children inherit the priority of their parent processes via
 .Sh RETURN VALUES
 Upon successful completion,
 .Fn nice
-returns the new nice value minus
-.Dv NZERO .
+returns 0, and
+.Va errno
+is unchanged.
 Otherwise, \-1 is returned, the process' nice value is not changed, and
 .Va errno
 is set to indicate the error.
@@ -84,7 +85,11 @@ argument is negative and the caller does not have appropriate privileges.
 The
 .Fn nice
 function conforms to
-.St -xpg4.2 .
+.St -p1003.1-2008
+except for the return value.
+This implementation returns 0 upon successful completion but 
+the standard requires returning the new nice value,
+which could be \-1.
 .Sh HISTORY
 A
 .Fn nice
diff --git a/lib/libc/gen/nice.c b/lib/libc/gen/nice.c
index 58cde98..ba9524b 100644
--- a/lib/libc/gen/nice.c
+++ b/lib/libc/gen/nice.c
@@ -45,16 +45,18 @@ __FBSDID("$FreeBSD$");
 int
 nice(int incr)
 {
-	int prio;
+	int saverrno, prio;
 
+	saverrno = errno;
 	errno = 0;
 	prio = getpriority(PRIO_PROCESS, 0);
-	if (prio == -1 && errno)
-		return -1;
+	if (prio == -1 && errno != 0)
+		return (-1);
 	if (setpriority(PRIO_PROCESS, 0, prio + incr) == -1) {
 		if (errno == EACCES)
 			errno = EPERM;
-		return -1;
+		return (-1);
 	}
-	return getpriority(PRIO_PROCESS, 0);
+	errno = saverrno;
+	return (0);
 }