luaext/Pexec: optimize setting CLOEXEC

In case maximum number of open files limit is set too high,
Pexec() spends way too much time trying to set FD_CLOEXEC
for all those file descriptors, resulting in severe increase
of time it takes to execute rpm or dnf. For example, this
happens while running under Docker because:

> $ docker run fedora ulimit -n
> 1048576

One obvious fix is when open_max is too big, use procfs to get
the actual list of opened file descriptors and iterate over those.
My quick-n-dirty benchmark shows the /proc approach is about 10x
faster than iterating through a list of 1024 fds.

Note that the old method is still used in case /proc is not available.

This should fix:
- moby/moby#23137
- https://bugzilla.redhat.com/show_bug.cgi?id=1537564

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

luaext/lposix.c

@@ -330,26 +330,58 @@ static int Pmkfifo(lua_State *L)		/** mkfifo(path) */
 }
 
 
+static void set_cloexec(int fd)
+{
+	int flags = fcntl(fd, F_GETFD);
+
+	if (flags == -1 || (flags & FD_CLOEXEC))
+		return;
+
+	fcntl(fd, F_SETFD, flags | FD_CLOEXEC);
+}
+
+static void set_cloexec_all(void)
+{
+	const int min_fd = 3; // don't touch stdin/out/err
+	int fd, open_max;
+
+	open_max = sysconf(_SC_OPEN_MAX);
+	if (open_max == -1) {
+	    open_max = 1024;
+	}
+
+	// iterate over fds obtained from /proc
+	DIR *dir = opendir("/proc/self/fd");
+	if (dir == NULL) {
+		goto fallback;
+	}
+
+	struct dirent *entry;
+	while ((entry = readdir(dir)) != NULL) {
+		fd = atoi(entry->d_name);
+		if (fd >= min_fd)
+			set_cloexec(fd);
+	}
+	closedir(dir);
+
+	return;
+
+fallback:
+	// iterate over all possible fds
+	for (fd = min_fd; fd < open_max; fd++)
+		set_cloexec(fd);
+}
+
 static int Pexec(lua_State *L)			/** exec(path,[args]) */
 {
 	const char *path = luaL_checkstring(L, 1);
 	int i,n=lua_gettop(L);
 	char **argv;
-	int flag, fdno, open_max;
 
 	if (!have_forked)
 	    return luaL_error(L, "exec not permitted in this context");
 
-	open_max = sysconf(_SC_OPEN_MAX);
-	if (open_max == -1) {
-	    open_max = 1024;
-	}
-	for (fdno = 3; fdno < open_max; fdno++) {
-	    flag = fcntl(fdno, F_GETFD);
-	    if (flag == -1 || (flag & FD_CLOEXEC))
-		continue;
-	    fcntl(fdno, F_SETFD, FD_CLOEXEC);
-	}
+	set_cloexec_all();
 
 	argv = malloc((n+1)*sizeof(char*));
 	if (argv==NULL) return luaL_error(L,"not enough memory");