From aa533828e6164731006dab92665fa92b7b058d6f Mon Sep 17 00:00:00 2001
From: Andres Freund <andres@anarazel.de>
Date: Mon, 21 May 2018 15:43:30 -0700
Subject: [PATCH v2 2/6] Add functions to send/receive data & FD over a unix
 domain socket.

This'll be used by a followup patch changing how the fsync request
queue works, to make it safe on linux.

TODO: This probably should live elsewhere.

Author: Andres Freund
Reviewed-By:
Discussion: https://postgr.es/m/
Backpatch:
---
 src/backend/storage/file/fd.c | 102 ++++++++++++++++++++++++++++++++++
 src/include/storage/fd.h      |   4 ++
 2 files changed, 106 insertions(+)

diff --git a/src/backend/storage/file/fd.c b/src/backend/storage/file/fd.c
index 441f18dcf56..65e46483a44 100644
--- a/src/backend/storage/file/fd.c
+++ b/src/backend/storage/file/fd.c
@@ -3572,3 +3572,105 @@ MakePGDirectory(const char *directoryName)
 {
 	return mkdir(directoryName, pg_dir_create_mode);
 }
+
+/*
+ * Send data over a unix domain socket, optionally (when fd != -1) including a
+ * file descriptor.
+ */
+ssize_t
+pg_uds_send_with_fd(int sock, void *buf, ssize_t buflen, int fd)
+{
+	ssize_t     size;
+	struct msghdr   msg = {0};
+	struct iovec    iov;
+	/* cmsg header, union for correct alignment */
+	union
+	{
+		struct cmsghdr  cmsghdr;
+		char        control[CMSG_SPACE(sizeof (int))];
+	} cmsgu;
+	struct cmsghdr  *cmsg;
+
+	iov.iov_base = buf;
+	iov.iov_len = buflen;
+
+	msg.msg_name = NULL;
+	msg.msg_namelen = 0;
+	msg.msg_iov = &iov;
+	msg.msg_iovlen = 1;
+
+	if (fd >= 0)
+	{
+		msg.msg_control = cmsgu.control;
+		msg.msg_controllen = sizeof(cmsgu.control);
+
+		cmsg = CMSG_FIRSTHDR(&msg);
+		cmsg->cmsg_len = CMSG_LEN(sizeof (int));
+		cmsg->cmsg_level = SOL_SOCKET;
+		cmsg->cmsg_type = SCM_RIGHTS;
+
+		*((int *) CMSG_DATA(cmsg)) = fd;
+	}
+
+	size = sendmsg(sock, &msg, 0);
+
+	/* errors are returned directly */
+	return size;
+}
+
+/*
+ * Receive data from a unix domain socket. If a file is sent over the socket,
+ * store it in *fd.
+ */
+ssize_t
+pg_uds_recv_with_fd(int sock, void *buf, ssize_t bufsize, int *fd)
+{
+	ssize_t     size;
+	struct msghdr   msg;
+	struct iovec    iov;
+	/* cmsg header, union for correct alignment */
+	union
+	{
+		struct cmsghdr  cmsghdr;
+		char        control[CMSG_SPACE(sizeof (int))];
+	} cmsgu;
+	struct cmsghdr  *cmsg;
+
+	Assert(fd != NULL);
+
+	iov.iov_base = buf;
+	iov.iov_len = bufsize;
+
+	msg.msg_name = NULL;
+	msg.msg_namelen = 0;
+	msg.msg_iov = &iov;
+	msg.msg_iovlen = 1;
+	msg.msg_control = cmsgu.control;
+	msg.msg_controllen = sizeof(cmsgu.control);
+
+	size = recvmsg (sock, &msg, 0);
+
+	if (size < 0)
+	{
+		*fd = -1;
+		return size;
+	}
+
+	cmsg = CMSG_FIRSTHDR(&msg);
+	if (cmsg && cmsg->cmsg_len == CMSG_LEN(sizeof(int)))
+	{
+		if (cmsg->cmsg_level != SOL_SOCKET)
+			elog(FATAL, "unexpected cmsg_level");
+
+		if (cmsg->cmsg_type != SCM_RIGHTS)
+			elog(FATAL, "unexpected cmsg_type");
+
+		*fd = *((int *) CMSG_DATA(cmsg));
+
+		/* FIXME: check / handle additional cmsg structures */
+	}
+	else
+		*fd = -1;
+
+	return size;
+}
diff --git a/src/include/storage/fd.h b/src/include/storage/fd.h
index 8e7c9728f4b..5e016d69a5a 100644
--- a/src/include/storage/fd.h
+++ b/src/include/storage/fd.h
@@ -143,4 +143,8 @@ extern void SyncDataDirectory(void);
 #define PG_TEMP_FILES_DIR "pgsql_tmp"
 #define PG_TEMP_FILE_PREFIX "pgsql_tmp"
 
+/* XXX; This should probably go elsewhere */
+ssize_t pg_uds_send_with_fd(int sock, void *buf, ssize_t buflen, int fd);
+ssize_t pg_uds_recv_with_fd(int sock, void *buf, ssize_t bufsize, int *fd);
+
 #endif							/* FD_H */
-- 
2.17.0.rc1.dirty