set the close-on-exec flag on all file descriptors

Added the "fd_util" library, which attempts to use the new thread-safe
Linux system calls pipe2(), accept4() and the options O_CLOEXEC,
SOCK_CLOEXEC.  Without these, it falls back to FD_CLOEXEC, which is
not thread safe.

This is particularly important for the "pipe" output plugin (and
others, such as JACK/PulseAudio), because we were heavily leaking file
descriptors to child processes.

src/fd_util.c

@@ -0,0 +1,171 @@
+/*
+ * Copyright (C) 2003-2009 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#include "fd_util.h"
+#include "config.h"
+
+#if !defined(_GNU_SOURCE) && (defined(HAVE_PIPE2) || defined(HAVE_ACCEPT4))
+#define _GNU_SOURCE
+#endif
+
+#include <assert.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <errno.h>
+
+#ifdef WIN32
+#include <winsock2.h>
+#else
+#include <sys/socket.h>
+#endif
+
+#ifndef WIN32
+
+static int
+fd_mask_flags(int fd, int and_mask, int xor_mask)
+{
+	int ret;
+
+	assert(fd >= 0);
+
+	ret = fcntl(fd, F_GETFD, 0);
+	if (ret < 0)
+		return ret;
+
+	return fcntl(fd, F_SETFD, (ret & and_mask) ^ xor_mask);
+}
+
+#endif /* !WIN32 */
+
+int
+fd_set_cloexec(int fd, bool enable)
+{
+#ifndef WIN32
+	return fd_mask_flags(fd, ~FD_CLOEXEC, enable ? FD_CLOEXEC : 0);
+#else
+	(void)fd;
+	(void)enable;
+#endif
+}
+
+int
+open_cloexec(const char *path_fs, int flags)
+{
+	int fd;
+
+#ifdef O_CLOEXEC
+	flags |= O_CLOEXEC;
+#endif
+
+#ifdef O_NOCTTY
+	flags |= O_NOCTTY;
+#endif
+
+	fd = open(path_fs, flags, 0666);
+	fd_set_cloexec(fd, true);
+
+	return fd;
+}
+
+int
+creat_cloexec(const char *path_fs, int mode)
+{
+	int flags = O_CREAT|O_WRONLY|O_TRUNC;
+	int fd;
+
+#ifdef O_CLOEXEC
+	flags |= O_CLOEXEC;
+#endif
+
+#ifdef O_NOCTTY
+	flags |= O_NOCTTY;
+#endif
+
+	fd = open(path_fs, flags, mode);
+	fd_set_cloexec(fd, true);
+
+	return fd;
+}
+
+int
+pipe_cloexec(int fd[2])
+{
+#ifdef WIN32
+	return _pipe(event_pipe, 512, _O_BINARY);
+#else
+	int ret;
+
+#ifdef HAVE_PIPE2
+	ret = pipe2(fd, O_CLOEXEC);
+	if (ret >= 0 || errno != ENOSYS)
+		return ret;
+#endif
+
+	ret = pipe(fd);
+	if (ret >= 0) {
+		fd_set_cloexec(fd[0], true);
+		fd_set_cloexec(fd[1], true);
+	}
+
+	return ret;
+#endif
+}
+
+int
+socket_cloexec(int domain, int type, int protocol)
+{
+	int fd;
+
+#ifdef SOCK_CLOEXEC
+	fd = socket(domain, type | SOCK_CLOEXEC, protocol);
+	if (fd >= 0 || errno != EINVAL)
+		return fd;
+#endif
+
+	fd = socket(domain, type, protocol);
+	if (fd >= 0)
+		fd_set_cloexec(fd, true);
+
+	return fd;
+}
+
+int
+accept_cloexec(int fd, struct sockaddr *address, size_t *address_length_r)
+{
+	int ret;
+	socklen_t address_length = *address_length_r;
+
+#ifdef HAVE_ACCEPT4
+	ret = accept4(fd, address, &address_length, SOCK_CLOEXEC);
+	if (ret >= 0 || errno != ENOSYS) {
+		if (ret >= 0)
+			*address_length_r = address_length;
+
+		return ret;
+	}
+#endif
+
+	ret = accept(fd, address, &address_length);
+	if (ret >= 0) {
+		fd_set_cloexec(ret, true);
+		*address_length_r = address_length;
+	}
+
+	return ret;
+}