[PATCH net v3 2/3] vsock/test: fix MSG_PEEK handling in recv_buf()

[Luigi Leonardi <leonardi@redhat.com>]

`recv_buf` does not handle the MSG_PEEK flag correctly: it keeps calling
`recv` until all requested bytes are available or an error occurs.

The problem is how it calculates the amount of bytes read: MSG_PEEK
doesn't consume any bytes, will re-read the same bytes from the buffer
head, so, summing the return value every time is wrong.

Moreover, MSG_PEEK doesn't consume the bytes in the buffer, so if the
requested amount is more than the bytes available, the loop will never
terminate, because `recv` will never return EOF. For this reason we need
to compare the amount of read bytes with the number of bytes expected.

Add a check, and if the MSG_PEEK flag is present, update the counter of
read bytes differently, and break if we read the expected amount.

This allows us to simplify the `test_stream_credit_update_test`, by
reusing `recv_buf`, like some other tests already do.

This also fixes callers that pass MSG_PEEK to recv_buf().

Suggested-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Luigi Leonardi <leonardi@redhat.com>
---
 tools/testing/vsock/util.c       | 15 +++++++++++++++
 tools/testing/vsock/vsock_test.c | 13 +------------
 2 files changed, 16 insertions(+), 12 deletions(-)

diff --git a/tools/testing/vsock/util.c b/tools/testing/vsock/util.c
index 1fe1338c79cd..2c9ee3210090 100644
--- a/tools/testing/vsock/util.c
+++ b/tools/testing/vsock/util.c
@@ -381,7 +381,13 @@ void send_buf(int fd, const void *buf, size_t len, int flags,
 	}
 }
 
+#define RECV_PEEK_RETRY_USEC 10
+
 /* Receive bytes in a buffer and check the return value.
+ *
+ * MSG_PEEK note: MSG_PEEK doesn't consume bytes from the buffer, so partial
+ * reads cannot be summed. Instead, the function retries until recv() returns
+ * exactly expected_ret bytes in a single call.
  *
  * expected_ret:
  *  <0 Negative errno (for testing errors)
@@ -403,6 +409,15 @@ void recv_buf(int fd, void *buf, size_t len, int flags, ssize_t expected_ret)
 		if (ret <= 0)
 			break;
 
+		if (flags & MSG_PEEK) {
+			if (ret == expected_ret) {
+				nread = ret;
+				break;
+			}
+			timeout_usleep(RECV_PEEK_RETRY_USEC);
+			continue;
+		}
+
 		nread += ret;
 	} while (nread < len);
 	timeout_end();
diff --git a/tools/testing/vsock/vsock_test.c b/tools/testing/vsock/vsock_test.c
index 5bd20ccd9335..bdb0754965df 100644
--- a/tools/testing/vsock/vsock_test.c
+++ b/tools/testing/vsock/vsock_test.c
@@ -1500,18 +1500,7 @@ static void test_stream_credit_update_test(const struct test_opts *opts,
 	}
 
 	/* Wait until there will be 128KB of data in rx queue. */
-	while (1) {
-		ssize_t res;
-
-		res = recv(fd, buf, buf_size, MSG_PEEK);
-		if (res == buf_size)
-			break;
-
-		if (res <= 0) {
-			fprintf(stderr, "unexpected 'recv()' return: %zi\n", res);
-			exit(EXIT_FAILURE);
-		}
-	}
+	recv_buf(fd, buf, buf_size, MSG_PEEK, buf_size);
 
 	/* There is 128KB of data in the socket's rx queue, dequeue first
 	 * 64KB, credit update is sent if 'low_rx_bytes_test' == true.

-- 
2.53.0