Periodic NAT Type Measurement Support (Fix #40075)

This pull request adds periodic NAT Type measurement support to snowflake. The default behaviour is unmodified. To enable this periodic NAT Type measurement set nat-type-measurement-interval-second to a non-zero value.

Things to look at:

• Copyright. This pull request includes ported infrastructure codes from V2Ray for managed periodic tasks. If reusing MIT code is not allowed, this code needs to be rewritten.
• No exhaustive test. The network environment cannot be easily modified. Lookup Table is not tested.
• Flag name too long. There should be a shorter name for nat-type-measurement-interval-second that contains the same or more information.
• Change of default behaviour. If necessary, flag nat-type-measurement-interval-second can have a default value of 86400 to recheck every day by default.

You already mentioned it in the MR description, I think it makes sense to set something else than 0 already here as default and your proposed 86400 sounds good.

The default probe interval is set to 86400 in !62 (1b79962c).

changed this line in version 7 of the diff

nat-type-measurement-interval-second is a bit long, can we come up with something sorter? nat-measure-seconds? check-nat-seconds? ...

If we don't need to be very specific, maybe we can also use nat-retest-seconds / nat-retest-seconds / nat-reassignment-seconds?

That sounds better than my proposals. I like nat-retest-seconds, but I'm happy with any.

How about using time.ParseDuration, instead of always using a unit of seconds? The argument could be something like 24h, 3h, 3600s, etc.

Personally, I think time.ParseDuration is a great choice. The only issue used to be the users cannot guess the format of input based on the flag name. Now that the user cannot reliably guess the meaning of the flag after the flag is renamed, and have to consult the document anyway, this change has no adverse effects at all.

I will update the code to reflect this change.

changed this line in version 7 of the diff

This recommendation have been adopted in 9bdb87ea efdb850d .

I don't know if it's better, but there is also flag.Duration which would avoid a separate time.ParseDuration step. (But the error message for a syntax error may not be customizable in this case.)

I have adopted this recommendation in c49f72eb. I don't think there will be a need to customize it in the near future while this change makes the code more idiomatic.

Ups, I clicked on send the review while I was still looking at it. Is looking good, but I'll finish my review.

Now yes, I finished reviewing it. Amazing, thanks for your first contribution

I left a bunch of comments, I tend to search for simpler/sorter code when possible. But I'm happy to talk about it, I don't have strong opinions on any of my comments.

mentioned in commit shelikhoo/snowflake@67506ed0

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• 67506ed0 - Extract function getCurrentNATType()

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mentioned in commit shelikhoo/snowflake@102d410b

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• 102d410b - Extract function getCurrentNATType()

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mentioned in commit shelikhoo/snowflake@4159f44a

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• 4159f44a - Extract function getCurrentNATType()

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mentioned in commit shelikhoo/snowflake@2547883c

mentioned in commit shelikhoo/snowflake@59af9927

mentioned in commit shelikhoo/snowflake@1b79962c

added 9 commits

• 4159f44a...ead5a960 - 2 commits from branch tpo/anti-censorship/pluggable-transports:main
• 04bc471a - Support recurring NAT Type measurement
• 4c8a1661 - Port V2Ray periodic task standard library to snowflake
• ac97ce71 - Add NAT Type measurement command line flag
• a6a53ff8 - Add NAT Type test periodic task
• 2547883c - Extract function getCurrentNATType()
• 59af9927 - Refactor state transfer logic to simplify it
• 1b79962c - Rename flag to nat-retest-seconds and retest daily by default

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mentioned in commit shelikhoo/snowflake@d4fdb35e

added 1 commit

• d4fdb35e - Add in source indicator of file origin

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