--- a/src/lib/istream-sized.c	Fri Dec 23 12:26:17 2016 -0500
+++ b/src/lib/istream-sized.c	Fri Dec 23 12:51:41 2016 -0500
@@ -99,9 +99,10 @@
 	} else if (stream->istream.stream_errno == ENOENT) {
 		/* lost the file */
 	} else {
+		/* EOF before we reached the wanted size */
 		error = sstream->error_callback(&data, sstream->error_context);
 		io_stream_set_error(&stream->iostream, "%s", error);
-		stream->istream.stream_errno = EINVAL;
+		stream->istream.stream_errno = EPIPE;
 	}
 
 	ret = pos > stream->pos ? (ssize_t)(pos - stream->pos) :

--- a/src/lib/istream-sized.h	Fri Dec 23 12:26:17 2016 -0500
+++ b/src/lib/istream-sized.h	Fri Dec 23 12:51:41 2016 -0500
@@ -16,8 +16,9 @@
 istream_sized_callback_t(const struct istream_sized_error_data *data,
 			 void *context);
 
-/* Assume that input is exactly the given size. If it's smaller, log an error
-   and fail with EINVAL error. If it's larger, log an error but don't fail. */
+/* Assume that input stream is exactly the given size. If the stream is too
+   small, fail with stream_errno=EPIPE. If stream is too large, fail with
+   stream_errno=EINVAL. */
 struct istream *i_stream_create_sized(struct istream *input, uoff_t size);
 struct istream *i_stream_create_sized_range(struct istream *input,
 					    uoff_t offset, uoff_t size);

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/lib/test-istream-sized.c	Fri Dec 23 12:51:41 2016 -0500
@@ -0,0 +1,66 @@
+/* Copyright (c) 2016 Dovecot authors, see the included COPYING file */
+
+#include "test-lib.h"
+#include "str.h"
+#include "istream-private.h"
+#include "istream-sized.h"
+
+static const struct {
+	const char *input;
+	uoff_t size;
+	int stream_errno;
+} tests[] = {
+	{ "", 0, 0 },
+	{ "", 1, EPIPE },
+	{ "a", 1, 0 },
+	{ "ab", 1, EINVAL },
+	{ "ab", 0, EINVAL },
+	{ "ab", (uoff_t)-1, EPIPE },
+};
+
+static void
+run_test(const char *sized_input, uoff_t sized_size, int stream_errno)
+{
+	unsigned int sized_input_len = strlen(sized_input);
+	struct istream *input_data, *input;
+	const unsigned char *data;
+	size_t i, size;
+	int ret = 0;
+
+	input_data = test_istream_create_data(sized_input, sized_input_len);
+	test_istream_set_allow_eof(input_data, FALSE);
+	input = i_stream_create_sized(input_data, sized_size);
+
+	for (i = 1; i < sized_input_len; i++) {
+		test_istream_set_size(input_data, i);
+		while ((ret = i_stream_read(input)) > 0) ;
+		if (ret == -1 && stream_errno != 0)
+			break;
+		test_assert(ret == 0);
+	}
+	if (ret == 0) {
+		test_istream_set_allow_eof(input_data, TRUE);
+		test_istream_set_size(input_data, i);
+		while ((ret = i_stream_read(input)) > 0) ;
+	}
+	test_assert(ret == -1);
+	test_assert(input->stream_errno == stream_errno);
+
+	data = i_stream_get_data(input, &size);
+	test_assert(size == I_MIN(sized_input_len, sized_size));
+	if (size > 0)
+		test_assert(memcmp(data, sized_input, size) == 0);
+	i_stream_unref(&input);
+	i_stream_unref(&input_data);
+}
+
+void test_istream_sized(void)
+{
+	unsigned int i;
+
+	for (i = 0; i < N_ELEMENTS(tests); i++) {
+		test_begin(t_strdup_printf("istream sized %u", i+1));
+		run_test(tests[i].input, tests[i].size, tests[i].stream_errno);
+		test_end();
+	}
+}