[PATCH liburing v2 01/12] test/helpers: Add `t_bind_ephemeral_port()` function

[Ammar Faizi <ammarfaizi2@gnuweeb.org>]

From: Ammar Faizi <ammarfaizi2@gnuweeb.org>

This is a prep patch to fix an intermittent issue with the port number.

We have many places where we need to bind() a socket to any unused port
number. To achieve that, the current approach does one of the following
mechanisms:

  1) Randomly brute force the port number until the bind() syscall
     succeeds.

  2) Use a static port at compile time (randomly chosen too).

This is not reliable and it results in an intermittent issue (test
fails when the selected port is in use).

Setting @addr->sin_port to zero on a bind() syscall lets the kernel
choose a port number that is not in use. The caller then can know the
port number to be bound by invoking a getsockname() syscall after
bind() succeeds.

Wrap this procedure in a new function called t_bind_ephemeral_port().
The selected port will be returned into @addr->sin_port, the caller
can use it later to connect() or whatever they need.

Link: https://lore.kernel.org/r/918facd1-78ba-2de7-693a-5f8c65ea2fcd@gnuweeb.org
Cc: Dylan Yudaken <dylany@fb.com>
Cc: Facebook Kernel Team <kernel-team@fb.com>
Cc: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: Alviro Iskandar Setiawan <alviro.iskandar@gnuweeb.org>
Tested-by: Alviro Iskandar Setiawan <alviro.iskandar@gnuweeb.org>
Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
---
 test/helpers.c | 18 ++++++++++++++++++
 test/helpers.h |  7 +++++++
 2 files changed, 25 insertions(+)

diff --git a/test/helpers.c b/test/helpers.c
index 0146533..4d5c402 100644
--- a/test/helpers.c
+++ b/test/helpers.c
@@ -24,14 +24,32 @@ void *t_malloc(size_t size)
 {
 	void *ret;
 	ret = malloc(size);
 	assert(ret);
 	return ret;
 }
 
+/*
+ * Helper for binding socket to an ephemeral port.
+ * The port number to be bound is returned in @addr->sin_port.
+ */
+int t_bind_ephemeral_port(int fd, struct sockaddr_in *addr)
+{
+	socklen_t addrlen;
+
+	addr->sin_port = 0;
+	if (bind(fd, (struct sockaddr *)addr, sizeof(*addr)))
+		return -errno;
+
+	addrlen = sizeof(*addr);
+	assert(!getsockname(fd, (struct sockaddr *)addr, &addrlen));
+	assert(addr->sin_port != 0);
+	return 0;
+}
+
 /*
  * Helper for allocating size bytes aligned on a boundary.
  */
 void t_posix_memalign(void **memptr, size_t alignment, size_t size)
 {
 	int ret;
 	ret = posix_memalign(memptr, alignment, size);
diff --git a/test/helpers.h b/test/helpers.h
index 6d5726c..9ad9947 100644
--- a/test/helpers.h
+++ b/test/helpers.h
@@ -18,14 +18,21 @@ enum t_setup_ret {
 
 enum t_test_result {
 	T_EXIT_PASS   = 0,
 	T_EXIT_FAIL   = 1,
 	T_EXIT_SKIP   = 77,
 };
 
+/*
+ * Helper for binding socket to an ephemeral port.
+ * The port number to be bound is returned in @addr->sin_port.
+ */
+int t_bind_ephemeral_port(int fd, struct sockaddr_in *addr);
+
+
 /*
  * Helper for allocating memory in tests.
  */
 void *t_malloc(size_t size);
 
 
 /*
-- 
Ammar Faizi