rt: Turn forwards from librt to libc into compat symbols [BZ #24194]

As the  result of commit 6e6249d0b461b952d0f544792372663feb6d792a
("BZ#14743: Move clock_* symbols from librt to libc."), in glibc 2.17,
clock_gettime, clock_getres, clock_settime, clock_getcpuclockid,
clock_nanosleep were added to libc, and the file rt/clock-compat.c
was added with forwarders to the actual implementations in libc.
These forwarders were wrapped in

#if SHLIB_COMPAT (librt, GLIBC_2_2, GLIBC_2_17)

so that they are not present for newer architectures (such as
powerpc64le) with a 2.17 or later ABI baseline.  But the forwarders
were not marked as compatibility symbols.  As a result, on older
architectures, historic configure checks such as

AC_CHECK_LIB(rt, clock_gettime)

still cause linking against librt, even though this is completely
unnecessary.  It also creates a needless porting hazard because
architectures behave differently when it comes to symbol availability.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index 29f4ac31e1..20ff76ab62 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+2019-02-07  Florian Weimer  <fweimer@redhat.com>
+
+	* rt/clock-compat.c (COMPAT_REDIRECT): Turn librt forwarders into
+	compatibility symbols.
+
 2019-02-07  Stefan Liebler  <stli@linux.ibm.com>
 
 	[BZ #24180]