Add support for TCP send buffer auto tuning.

Send buffer size in TCP indicates the amount of bytes available for the sender
to transmit. This change will allow TCP to update the send buffer size when
- TCP enters established state.
- ACK is received.

The auto tuning is disabled when the send buffer size is set with the
SO_SNDBUF option.

PiperOrigin-RevId: 390312274

1 files changed, 60 insertions, 0 deletions

diff --git a/test/syscalls/linux/tcp_socket.cc b/test/syscalls/linux/tcp_socket.cc
index cb77986c2..3fbbf1423 100644
--- a/test/syscalls/linux/tcp_socket.cc
+++ b/test/syscalls/linux/tcp_socket.cc
@@ -2088,6 +2088,66 @@ TEST_P(SimpleTcpSocketTest, ConnectUnspecifiedAddress) {
   }
 }
 
+// Tests that send will return EWOULDBLOCK initially with large buffer and will
+// succeed after the send buffer size is increased.
+TEST_P(TcpSocketTest, SendUnblocksOnSendBufferIncrease) {
+  // Set the FD to O_NONBLOCK.
+  int opts;
+  ASSERT_THAT(opts = fcntl(first_fd, F_GETFL), SyscallSucceeds());
+  opts |= O_NONBLOCK;
+  ASSERT_THAT(fcntl(first_fd, F_SETFL, opts), SyscallSucceeds());
+
+  // Get maximum buffer size by trying to set it to a large value.
+  constexpr int kSndBufSz = 0xffffffff;
+  ASSERT_THAT(setsockopt(first_fd, SOL_SOCKET, SO_SNDBUF, &kSndBufSz,
+                         sizeof(kSndBufSz)),
+              SyscallSucceeds());
+
+  int max_buffer_sz = 0;
+  socklen_t max_len = sizeof(max_buffer_sz);
+  ASSERT_THAT(
+      getsockopt(first_fd, SOL_SOCKET, SO_SNDBUF, &max_buffer_sz, &max_len),
+      SyscallSucceeds());
+
+  int buffer_sz = max_buffer_sz >> 2;
+  EXPECT_THAT(setsockopt(first_fd, SOL_SOCKET, SO_SNDBUF, &buffer_sz,
+                         sizeof(buffer_sz)),
+              SyscallSucceedsWithValue(0));
+
+  // Create a large buffer that will be used for sending.
+  std::vector<char> buffer(max_buffer_sz);
+
+  // Write until we receive an error.
+  while (RetryEINTR(send)(first_fd, buffer.data(), buffer.size(), 0) != -1) {
+    // Sleep to give linux a chance to move data from the send buffer to the
+    // receive buffer.
+    usleep(10000);  // 10ms.
+  }
+
+  // The last error should have been EWOULDBLOCK.
+  ASSERT_EQ(errno, EWOULDBLOCK);
+
+  ScopedThread send_thread([this]() {
+    int flags = 0;
+    ASSERT_THAT(flags = fcntl(first_fd, F_GETFL), SyscallSucceeds());
+    EXPECT_THAT(fcntl(first_fd, F_SETFL, flags & ~O_NONBLOCK),
+                SyscallSucceeds());
+
+    // Expect the send() to succeed.
+    char buffer;
+    ASSERT_THAT(RetryEINTR(send)(first_fd, &buffer, sizeof(buffer), 0),
+                SyscallSucceeds());
+  });
+
+  // Set SO_SNDBUF to maximum buffer size allowed.
+  buffer_sz = max_buffer_sz >> 1;
+  EXPECT_THAT(setsockopt(first_fd, SOL_SOCKET, SO_SNDBUF, &buffer_sz,
+                         sizeof(buffer_sz)),
+              SyscallSucceedsWithValue(0));
+
+  send_thread.Join();
+}
+
 INSTANTIATE_TEST_SUITE_P(AllInetTests, SimpleTcpSocketTest,
                          ::testing::Values(AF_INET, AF_INET6));