Test that SSL_write(0) does not write a record

It behaves just fine, but I thought it didn't and we have no test that
asserts this. Add a test.

Change-Id: I0e061870a6a4ac56191559ed0e324e9b92a5f9d2
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/102267
Reviewed-by: Xiangfei Ding <xfding@google.com>
Commit-Queue: Xiangfei Ding <xfding@google.com>
Auto-Submit: David Benjamin <davidben@google.com>
Presubmit-BoringSSL-Verified: boringssl-scoped@luci-project-accounts.iam.gserviceaccount.com <boringssl-scoped@luci-project-accounts.iam.gserviceaccount.com>

ssl/ssl_test.cc

diff --git a/ssl/ssl_test.cc b/ssl/ssl_test.cc
index 4a6c99c..b5e9dbb 100644
--- a/ssl/ssl_test.cc
+++ b/ssl/ssl_test.cc
@@ -6613,6 +6613,22 @@
   EXPECT_EQ(1, SSL_has_pending(client_.get()));
 }
 
+// Test that `SSL_write(0)` does not write a zero-length record.
+TEST_P(SSLVersionTest, WriteZeroLength) {
+  // Disable session tickets. In TLS 1.3, the NewSessionTicket is deferred to
+  // the first write and flushed on an `SSL_write` call. This confuses the test.
+  SSL_CTX_set_options(client_ctx_.get(), SSL_OP_NO_TICKET);
+  SSL_CTX_set_options(server_ctx_.get(), SSL_OP_NO_TICKET);
+
+  ASSERT_TRUE(Connect());
+  uint64_t old_bytes_written = BIO_number_written(SSL_get_wbio(client_.get()));
+  ASSERT_EQ(0, SSL_write(client_.get(), nullptr, 0));
+  uint64_t new_bytes_written = BIO_number_written(SSL_get_wbio(client_.get()));
+
+  // No new bytes should be written. We shouldn't write a zero-length record.
+  EXPECT_EQ(old_bytes_written, new_bytes_written);
+}
+
 // Test that post-handshake tickets consumed by `SSL_shutdown` are ignored.
 TEST(SSLTest, ShutdownIgnoresTickets) {
   bssl::UniquePtr<SSL_CTX> ctx(CreateContextWithTestCertificate(TLS_method()));