Low: unix: new qb_sys_unlink_or_truncate{,_at} helpers

These are intended for subsequent qb_rb_{force_,}close refactorization
and utilization of this new truncate as a fallback after unlink failure
as detailed in the commit to follow.

For newer POSIX revision compliant systems, there's "at" variant using
openat/unlinkat functions so that paths do not have to be traversed
in full anew when not needed (as both unlink and truncate operate on
the same path).

configure.ac

@@ -217,7 +217,8 @@ AC_CHECK_FUNCS([alarm clock_gettime ftruncate gettimeofday \
                 pthread_mutexattr_setpshared \
                 pthread_condattr_setpshared \
 		sem_timedwait semtimedop \
-		getpeerucred getpeereid])
+		getpeerucred getpeereid \
+		openat unlinkat])
 
 AM_CONDITIONAL(HAVE_SEM_TIMEDWAIT,
 	       [test "x$ac_cv_func_sem_timedwait" = xyes])

lib/unix.c

@@ -255,6 +255,55 @@ qb_sys_fd_nonblock_cloexec_set(int32_t fd)
 	return res;
 }
 
+int32_t
+qb_sys_unlink_or_truncate(const char *path, int32_t truncate_fallback)
+{
+	int32_t res = 0;
+
+	if (unlink(path) == -1) {
+		res = errno;
+		qb_util_perror(LOG_DEBUG,
+			       "Unlinking file: %s",
+			       path);
+		if (res != ENOENT && truncate_fallback) {
+			res = errno = 0;
+			if (truncate(path, 0) == -1) {
+				res = errno;
+				qb_util_perror(LOG_DEBUG,
+					       "Truncating file: %s", path);
+			}
+		}
+	}
+	return -res;
+}
+
+#if defined(HAVE_OPENAT) && defined(HAVE_UNLINKAT)
+int32_t
+qb_sys_unlink_or_truncate_at(int32_t dirfd, const char *path,
+			     int32_t truncate_fallback)
+{
+	int32_t fd, res = 0;
+
+	if (unlinkat(dirfd, path, 0) == -1) {
+		res = errno;
+		qb_util_perror(LOG_DEBUG,
+			       "Unlinking file at dir: %s", path);
+		if (res != ENOENT && truncate_fallback) {
+			res = errno = 0;
+			if ((fd = openat(dirfd, path, O_WRONLY|O_TRUNC)) == -1) {
+				res = errno;
+				qb_util_perror(LOG_DEBUG,
+					       "Truncating file at dir: %s",
+					       path);
+			} else {
+				close(fd);
+			}
+		}
+	}
+	return -res;
+}
+#endif
+
 void
 qb_sigpipe_ctl(enum qb_sigpipe_ctl ctl)
 {

lib/util_int.h

@@ -83,6 +83,25 @@ int32_t qb_sys_circular_mmap(int32_t fd, void **buf, size_t bytes);
  */
 int32_t qb_sys_fd_nonblock_cloexec_set(int32_t fd);
 
+/**
+ * Try to unlink file, and possibly truncate it as a fallback.
+ * @param path the file to be unlinked or truncated.
+ * @param truncate_fallback whether to truncate the file when unlink fails.
+ * @return 0 (success) or -errno
+ */
+int32_t qb_sys_unlink_or_truncate(const char *path, int32_t truncate_fallback);
+
+#if defined(HAVE_OPENAT) && defined(HAVE_UNLINKAT)
+/**
+ * Try to unlinkat file, and possibly truncate it as a fallback ("at" variant).
+ * @param path the file to be unlinked or truncated.
+ * @param truncate_fallback whether to truncate the file when unlink fails.
+ * @return 0 (success) or -errno
+ */
+int32_t qb_sys_unlink_or_truncate_at(int32_t dirfd, const char *path,
+				     int32_t truncate_fallback);
+#endif
+
 enum qb_sigpipe_ctl {
        QB_SIGPIPE_IGNORE,
        QB_SIGPIPE_DEFAULT,