--- a/src/lib-test/Makefile.am	Fri Sep 30 13:10:13 2016 +0300
+++ b/src/lib-test/Makefile.am	Fri Sep 30 13:42:45 2016 +0300
@@ -6,7 +6,8 @@
 
 libtest_la_SOURCES = \
 	test-common.c \
-	test-istream.c
+	test-istream.c \
+	test-ostream.c
 
 headers = \
 	test-common.h

--- a/src/lib-test/test-common.h	Fri Sep 30 13:10:13 2016 +0300
+++ b/src/lib-test/test-common.h	Fri Sep 30 13:42:45 2016 +0300
@@ -7,6 +7,14 @@
 void test_istream_set_allow_eof(struct istream *input, bool allow);
 void test_istream_set_max_buffer_size(struct istream *input, size_t size);
 
+struct ostream *test_ostream_create(buffer_t *output);
+struct ostream *test_ostream_create_nonblocking(buffer_t *output,
+						size_t max_internal_buffer_size);
+/* When output->used reaches max_size, start buffering output internally.
+   When internal buffer reaches max_internal_buffer_size, start returning 0 for
+   o_stream_send*(). */
+void test_ostream_set_max_output_size(struct ostream *output, size_t max_size);
+
 void test_begin(const char *name);
 #define test_assert(code) STMT_START { \
 	if (!(code)) test_assert_failed(#code, __FILE__, __LINE__); \

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/lib-test/test-ostream.c	Fri Sep 30 13:42:45 2016 +0300
@@ -0,0 +1,193 @@
+/* Copyright (c) 2016 Dovecot authors, see the included COPYING file */
+
+#include "lib.h"
+#include "buffer.h"
+#include "ostream-private.h"
+#include "test-common.h"
+
+struct test_ostream {
+	struct ostream_private ostream;
+	buffer_t *internal_buf;
+	buffer_t *output_buf;
+	size_t max_output_size;
+	struct timeout *to;
+	bool flush_pending;
+};
+
+static void o_stream_test_destroy(struct iostream_private *stream)
+{
+	struct test_ostream *tstream = (struct test_ostream *)stream;
+
+	if (tstream->to != NULL)
+		timeout_remove(&tstream->to);
+	if (tstream->internal_buf != NULL)
+		i_free(tstream->internal_buf);
+}
+
+static int o_stream_test_flush(struct ostream_private *stream)
+{
+	struct test_ostream *tstream = (struct test_ostream *)stream;
+
+	if (tstream->internal_buf == NULL || tstream->internal_buf->used == 0)
+		return 1;
+	if (tstream->output_buf->used >= tstream->max_output_size)
+		return 0;
+
+	size_t left = tstream->max_output_size - tstream->output_buf->used;
+	size_t n = I_MIN(left, tstream->internal_buf->used);
+	buffer_append(tstream->output_buf, tstream->internal_buf->data, n);
+	buffer_delete(tstream->internal_buf, 0, n);
+	return tstream->internal_buf->used == 0 ? 1 : 0;
+}
+
+static ssize_t
+o_stream_test_sendv(struct ostream_private *stream,
+		    const struct const_iovec *iov, unsigned int iov_count)
+{
+	struct test_ostream *tstream = (struct test_ostream *)stream;
+	struct const_iovec cur_iov = { NULL, 0 };
+	size_t left, n;
+	ssize_t ret = 0;
+	unsigned int i;
+
+	/* first we need to try to flush the internal buffer */
+	if ((ret = o_stream_test_flush(stream)) <= 0)
+		return ret;
+
+	/* append to output_buf until max_output_size is reached */
+	ret = 0;
+	for (i = 0; i < iov_count; i++) {
+		left = tstream->max_output_size < tstream->output_buf->used ? 0 :
+			tstream->max_output_size - tstream->output_buf->used;
+		n = I_MIN(left, iov[i].iov_len);
+		buffer_append(tstream->output_buf, iov[i].iov_base, n);
+		stream->ostream.offset += n;
+		ret += n;
+		if (n != iov[i].iov_len) {
+			cur_iov.iov_base = CONST_PTR_OFFSET(iov[i].iov_base, n);
+			cur_iov.iov_len = iov[i].iov_len - n;
+			break;
+		}
+	}
+	/* if we've internal_buf, append to it until max_buffer_size is
+	   reached */
+	if (i == iov_count || tstream->internal_buf == NULL)
+		return ret;
+	do {
+		left = tstream->ostream.max_buffer_size -
+			tstream->internal_buf->used;
+		n = I_MIN(left, cur_iov.iov_len);
+		buffer_append(tstream->internal_buf, cur_iov.iov_base, n);
+		stream->ostream.offset += n;
+		ret += n;
+		if (n != cur_iov.iov_len)
+			break;
+		cur_iov = iov[++i];
+	} while (i < iov_count);
+
+	tstream->flush_pending = TRUE;
+	return ret;
+}
+
+static void test_ostream_send_more(struct test_ostream *tstream)
+{
+	struct ostream *ostream = &tstream->ostream.ostream;
+	int ret;
+
+	o_stream_ref(ostream);
+	tstream->flush_pending = FALSE;
+	if (tstream->ostream.callback != NULL)
+		ret = tstream->ostream.callback(tstream->ostream.context);
+	else
+		ret = o_stream_test_flush(&tstream->ostream);
+	if (ret == 0 || (tstream->internal_buf != NULL &&
+			 tstream->internal_buf->used > 0))
+		tstream->flush_pending = TRUE;
+	if (!tstream->flush_pending ||
+	    tstream->output_buf->used >= tstream->max_output_size)
+		timeout_remove(&tstream->to);
+	o_stream_unref(&ostream);
+}
+
+static void test_ostream_set_send_more_timeout(struct test_ostream *tstream)
+{
+	if (tstream->to == NULL && tstream->flush_pending &&
+	    tstream->output_buf->used < tstream->max_output_size)
+		tstream->to = timeout_add_short(0, test_ostream_send_more, tstream);
+}
+
+static void
+o_stream_test_flush_pending(struct ostream_private *stream, bool set)
+{
+	struct test_ostream *tstream = (struct test_ostream *)stream;
+
+	if (tstream->internal_buf != NULL && tstream->internal_buf->used > 0) {
+		/* we have internal data, won't reset flush_pending */
+		i_assert(tstream->flush_pending);
+	} else {
+		tstream->flush_pending = set;
+	}
+	if (set)
+		test_ostream_set_send_more_timeout(tstream);
+}
+
+static size_t
+o_stream_test_get_used_size(const struct ostream_private *stream)
+{
+	struct test_ostream *tstream = (struct test_ostream *)stream;
+
+	return tstream->internal_buf == NULL ? 0 :
+		tstream->internal_buf->used;
+}
+
+struct ostream *test_ostream_create(buffer_t *output)
+{
+	struct test_ostream *tstream;
+	struct ostream *ostream;
+
+	tstream = i_new(struct test_ostream, 1);
+	tstream->ostream.max_buffer_size = (size_t)-1;
+	tstream->ostream.iostream.destroy = o_stream_test_destroy;
+	tstream->ostream.sendv = o_stream_test_sendv;
+	tstream->ostream.flush = o_stream_test_flush;
+	tstream->ostream.flush_pending = o_stream_test_flush_pending;
+	tstream->ostream.get_used_size = o_stream_test_get_used_size;
+	tstream->ostream.ostream.blocking = TRUE;
+
+	tstream->output_buf = output;
+	tstream->max_output_size = (size_t)-1;
+	ostream = o_stream_create(&tstream->ostream, NULL, -1);
+	o_stream_set_name(ostream, "(test-ostream)");
+	return ostream;
+}
+
+struct ostream *test_ostream_create_nonblocking(buffer_t *output,
+						size_t max_internal_buffer_size)
+{
+	struct test_ostream *tstream;
+
+	tstream = (struct test_ostream *)test_ostream_create(output)->real_stream;
+	tstream->internal_buf = buffer_create_dynamic(default_pool, 128);
+	tstream->ostream.ostream.blocking = FALSE;
+	tstream->ostream.max_buffer_size = max_internal_buffer_size;
+	return &tstream->ostream.ostream;
+}
+
+static struct test_ostream *test_ostream_find(struct ostream *output)
+{
+	struct ostream *out;
+
+	for (out = output; out != NULL; out = out->real_stream->parent) {
+		if (out->real_stream->sendv == o_stream_test_sendv)
+			return (struct test_ostream *)out->real_stream;
+	}
+	i_panic("%s isn't test-ostream", o_stream_get_name(output));
+}
+
+void test_ostream_set_max_output_size(struct ostream *output, size_t max_size)
+{
+	struct test_ostream *tstream = test_ostream_find(output);
+
+	tstream->max_output_size = max_size;
+	test_ostream_set_send_more_timeout(tstream);
+}