--- a/src/lib/Makefile.am	Mon Jun 05 19:05:58 2017 +0300
+++ b/src/lib/Makefile.am	Sat Apr 30 12:50:53 2016 +0200
@@ -116,6 +116,7 @@
 	ostream-hash.c \
 	ostream-null.c \
 	ostream-rawlog.c \
+	ostream-unix.c \
 	pkcs5.c \
 	primes.c \
 	printf-format-fix.c \
@@ -255,6 +256,7 @@
 	ostream-private.h \
 	ostream-null.h \
 	ostream-rawlog.h \
+	ostream-unix.h \
 	pkcs5.h \
 	primes.h \
 	printf-format-fix.h \

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/lib/ostream-unix.c	Sat Apr 30 12:50:53 2016 +0200
@@ -0,0 +1,90 @@
+/* Copyright (c) 2015-2016 Dovecot authors, see the included COPYING file */
+
+#include "lib.h"
+#include "fdpass.h"
+#include "ostream-file-private.h"
+#include "ostream-unix.h"
+
+struct unix_ostream {
+	struct file_ostream fstream;
+	int write_fd;
+};
+
+static void
+o_stream_unix_close(struct iostream_private *stream, bool close_parent)
+{
+	struct unix_ostream *ustream = (struct unix_ostream *)stream;
+
+	if (ustream->write_fd != -1)
+		i_close_fd(&ustream->write_fd);
+	o_stream_file_close(stream, close_parent);
+}
+
+static ssize_t o_stream_unix_writev(struct file_ostream *fstream,
+				   const struct const_iovec *iov,
+				   unsigned int iov_count)
+{
+	struct unix_ostream *ustream = (struct unix_ostream *)fstream;
+	size_t sent;
+	ssize_t ret;
+
+	if (ustream->write_fd == -1) {
+		/* no fd */
+		return o_stream_file_writev(fstream, iov, iov_count);
+	}
+
+	/* send first iovec along with fd */
+	if (iov_count == 0)
+		return 0;
+	i_assert(iov[0].iov_len > 0);
+	ret = fd_send(fstream->fd, ustream->write_fd,
+		iov[0].iov_base, iov[0].iov_len);
+	if (ret < 0)
+		return ret;
+
+	/* update stream */
+	sent = ret;
+	fstream->real_offset += sent;
+
+	ustream->write_fd = -1;
+
+	if (sent < iov[0].iov_len || iov_count == 1) {
+		/* caller will call us again to write the rest */
+		return sent;
+	}
+
+	/* send remaining iovecs */
+	ret = o_stream_file_writev(fstream, &iov[1], iov_count-1);
+	if (ret < 0)
+		return  (errno == EAGAIN || errno == EINTR ? (ssize_t)sent : ret);
+	sent += ret;
+	return sent;
+}
+
+struct ostream *o_stream_create_unix(int fd, size_t max_buffer_size)
+{
+	struct unix_ostream *ustream;
+	struct ostream *output;
+
+	i_assert(fd != -1);
+
+	ustream = i_new(struct unix_ostream, 1);
+	ustream->write_fd = -1;
+	output = o_stream_create_file_common(&ustream->fstream, fd,
+					    max_buffer_size, FALSE);
+	output->real_stream->iostream.close = o_stream_unix_close;
+	ustream->fstream.writev = o_stream_unix_writev;
+
+	return output;
+}
+
+bool o_stream_unix_write_fd(struct ostream *output, int fd)
+{
+	struct unix_ostream *ustream =
+		(struct unix_ostream *)output->real_stream;
+
+	if (ustream->write_fd >= 0)
+		return FALSE;
+	ustream->write_fd = fd;
+	return TRUE;
+}

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/lib/ostream-unix.h	Sat Apr 30 12:50:53 2016 +0200
@@ -0,0 +1,10 @@
+#ifndef OSTREAM_UNIX_H
+#define OSTREAM_UNIX_H
+
+struct ostream *o_stream_create_unix(int fd, size_t max_buffer_size);
+/* Write fd to UNIX socket along with the next outgoing data block.
+   Returns TRUE if fd is accepted, and FALSE if a previous fd still
+   needs to be sent. */
+bool o_stream_unix_write_fd(struct ostream *output, int fd);
+
+#endif