RFR: 8330940: Impossible to create a socket backlog greater than 200 on Windows 8+

[Alan Bateman]

On Mon, 16 Jun 2025 06:54:56 GMT, Jaikiran Pai <jpai at openjdk.org> wrote:

> Can I please get a review of this change which proposes to enhance the implementation of `ServerSocket` and `ServerSocketChannel` to allow for `backlog` values to be greater than 200 on Windows? This addresses https://bugs.openjdk.org/browse/JDK-8330940.
> 
> As noted in that enhancement request, right now on Windows, if the backlog is specified to be more than 200, then Windows caps it to a platform internal `SOMAXCONN`. As noted in the documentation here https://learn.microsoft.com/en-us/windows/win32/api/winsock2/nf-winsock2-listen applications can increase that limit by using the `SOMAXCONN_HINT` macro. That macro then adjusts the value to be between 200 and 65535, thus allowing for a higher backlog of connections.
> 
> The commit in this PR uses this macro when the specified backlog is 200 or more. A new jtreg test has been introduced to verify this change. This test and other existing tests in tier1, tier2 and tier3 continue to pass.
> 
> A similar restriction on the backlog value applies in Linux too https://github.com/torvalds/linux/blob/master/Documentation/networking/ip-sysctl.rst#tcp-variables. But from what I can see, unlike Windows, it cannot be adjusted when calling `listen()`.

test/jdk/java/net/ServerSocket/LargeBacklogTest.java line 43:

> 41:  * @run junit LargeBacklogTest
> 42:  */
> 43: class LargeBacklogTest {

How reliable is this test? If it is reliable then we can add tests for SocketChannelChannel::socket and AsynchronousServerSocket too.

test/jdk/java/net/ServerSocket/LargeBacklogTest.java line 72:

> 70:     private static void testBackloggedConnects(final int backlog, final int serverPort) {
> 71:         int numSuccessfulConnects = 0;
> 72:         System.out.println("attempting " + backlog + " connections to port " + serverPort);

I don't know if this is a left over trace message or not. If intended then you might have to change it to use System.err so that it inlines with the test (at least I think JUnit uses System.err, TestNG uses System.out).

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PR Review Comment: https://git.openjdk.org/jdk/pull/25819#discussion_r2149266221
PR Review Comment: https://git.openjdk.org/jdk/pull/25819#discussion_r2149267956