core: Allow building on very old systems with kernels < 3.15

Open file descriptor locks were introduced in Linux 3.15, so we
cannot use that type of locking on systems with older kernels
(e.g. CentOS 7, which of course is no longer supported).

Since this problem only occurs on kernels no longer supported
by keepalived this commit simply removes the file locking, rather
than implementing a more comprehensive solution.

It is expected that at some point, in order to simplify the code,
support for kernels no longer supported by any of the main
distros will be removed from keepalived.

Signed-off-by: Quentin Armitage <[email protected]>

configure.ac

@@ -1695,7 +1695,7 @@ AC_CHECK_LIB(magic, magic_open,
 AM_CONDITIONAL([MAGIC], [test $MAGIC -eq 1])
 unset LIBS
 
-dnl -- Check for the following variables introduced at various times into Linux
+dnl -- Check for the following definitions introduced at various times into Linux
 dnl --RTAX_QUICKACK		dnl -- Linux 3.11
 dnl --FRA_SUPPRESS_PREFIXLEN	dnl -- Linux 3.12
 dnl --FRA_SUPPRESS_IFGROUP	dnl -- Linux 3.12
@@ -1736,6 +1736,19 @@ for flag in IFA_FLAGS; do
   fi
 done
 
+dnl -- Linux 3.15
+AC_CHECK_DECLS([F_OFD_SETLK], [], [],
+  [[
+    #include <unistd.h>
+    #include <fcntl.h>
+  ]])
+for flag in F_OFD_SETLK; do
+  AS_VAR_COPY([decl_var], [ac_cv_have_decl_$flag])
+  if test ${decl_var} = yes; then
+    add_system_opt[${flag}]
+  fi
+done
+
 dnl - Introduced in Linux 5.18
 AC_CHECK_DECLS([IFA_PROTO], [], [], [[#include <linux/if_addr.h>]])
 for flag in IFA_PROTO; do

keepalived/core/pidfile.c

@@ -158,7 +158,9 @@ create_pidfile(pidfile_t *pidf)
 	struct stat st, fd_st;
 	int error;
 	int ret;
+#if HAVE_DECL_F_OFD_SETLK == 1
 	struct flock fl = { .l_type = F_WRLCK, .l_whence = SEEK_SET, .l_start = 0, .l_len = 0 };
+#endif
 
 	for (;;) {
 		/* We want to create the file with permissions rw-r--r-- */
@@ -177,6 +179,7 @@ create_pidfile(pidfile_t *pidf)
 			return true;
 		}
 
+#if HAVE_DECL_F_OFD_SETLK == 1
 		fl.l_pid = 0;
 		while ((ret = fcntl(pidf->fd, F_OFD_SETLK, &fl)) && errno == EINTR);
 		if (ret) {
@@ -187,6 +190,7 @@ create_pidfile(pidfile_t *pidf)
 
 			break;
 		}
+#endif
 
 		/* Make sure the file has not been removed/moved */
 		if (stat(pidf->path, &st)) {