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()`.

src/java.base/windows/native/libnio/ch/Net.c line 225:

> 223:      * SOMAXCONN_HINT will adjust the value N to be within the range (200, 65535).
> 224:      */
> 225:     if (backlog >= 200) {

Should this is > 200 rather than >= 200?

This is a Windows specific file so the comment can be trimmed down to a one sentence to say that SOMAXCONN_HINT is used when requested backlog is larger than SOMAXCONN.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25819#discussion_r2149261450