Add support for "optimize tcp buffers" option per profile.

[notsure2]

• It tunes down the local sslocal listener socket buffers which is the right thing to do in 99.999% of cases of mobile app.
• It additionally also tunes down the remote socket buffers of sslocal when a plugin is used. This is needed to ensure there's no bufferbloat between sslocal and the plugin itself.
• Ideally the plugin on its side should also choke its local listening socket buffers and leave only its remote side socket buffers unrestricted.
  • An additional follow up fix is needed which is to use a version of shadowsocks-rust which implements the buffer controls on the listening socket side. I have opened a PR there: https://github.com/shadowsocks/shadowsocks-rust/pull/1381
  • Given that ss-android is using an older release of ss-rust. The PR will probably need to be backported to 1.5.x branch, or ss-android needs to upgrade to latest ss-rust after my PR is merged and a release is made.

• This fixes the following issue which happens when using shadowsocks with or without a plugin:

• Large file uploads immediately jump to 80-90% and hang because the unrestricted local socket of ss, copies most or the file inside it giving a premature acknowledgement to the app, then the app hangs while the actual proxy uploads the file bit by bit (this issue is highly apparent on slow upload internet condition). Most apps will consider this hang as a timeout and fail to upload.

• This issue affects for example Whatsapp Google Drive backup which fails to upload while shadowsocks is working when the backup size becomes big enough. It also causes failures to google drive sync.

• I added translation for all languages by carefully finessing google bard and translating back and forth multiple times until both sides give the same result, feel free to improve them.

[Mygod]

Hi please see here for the translation contribution: https://discourse.shadowsocks.org/t/poeditor-translation-main-thread/30

Is there a reason to still use the default buffer size? Why not enforce this size?

[notsure2]

@Mygod Thank you for looking at this PR.

The small buffer size should be only incoming socket if plugin is not used, and in case of plugin used both incoming and outgoing socket to plugin should have their buffers stripped down.

I'm not sure what to do about translation ? I should remove from PR and add in that POEditor website ?

[Mygod]

Only include the English texts and add them later.

Thanks for the PR but I think if the issues are real then shadowsocks-rust should probably provide a better default instead. @zonyitoo Thoughts?

[notsure2]

I kind of disagree it will be a breaking change of behavior to all users who are not aware the effect of this change, that's why I don't even default this setting to on. The goal of this change is fix upload buffer bloat by removing buffer between app and ss and between ss and cloak in upload direction.

[notsure2]

@Mygod done

[Mygod]

So is this a Cloak specific issue? Other plugins don't seem to have this problem.

[notsure2]

So is this a Cloak specific issue? Other plugins don't seem to have this problem.

Negative, this affects all plugins actually. It's most visible when using a connection with a slow upload. It looks like this, try to upload a large file with ss or ss + plugin without optimize buffers, on a slow upload internet.

What you will see, upload will jump to a high % immediately, then slowly actually upload to the background: Cause is that the browser copied to ss buffer very quickly and ss accepted it because default buffering is huge, but actually the underlying upload can't handle it, so the upload just hangs a for a long time with no progress until it catches up, then continues at the real speed to the end.

Many upload controls will cancel the upload with "Timeout" error if no bytes move for a long time (which happens in this case due to the buffer bloat at the local ss).

App ----------------> ss ------------------> internet
        (balloon here)         

In ss alone the balloon must be small or it will inflate with data much faster than ss can empty it to the internet giving a false impression of upload speed to the app potentially leading to timeouts as the app thinks the connection stalled.

App ----------------> ss ------------------> plugin --------------------> internet
        (balloon here)         (balloon here)

Same problem here, the idea is all buffers from the app to the internet except the last one must be set very small so that the app gets a correct TCP backpressure and correct "feeling" of upload speed and works properly.

PS: This problem doesn't affect download, only upload

[Mygod]

It seems that the default buffer size is 16383*20 bytes unless I'm mistaken? This probably wouldn't cause the issue you described unless I'm mistaken.

[notsure2]

That's 20 times what I set :-) I set to just 16384. Also default is whatever is core_wmem i think in kernel, for localhost it's very big.