* Add `open_timeout` as an overall timeout option for `Socket.tcp`
[Background]
Currently, `TCPSocket.new` and `Socket.tcp` accept two kind of timeout options:
- `resolv_timeout`, which controls the timeout for DNS resolution
- `connect_timeout`, which controls the timeout for the connection attempt
With the introduction of Happy Eyeballs Version 2 (as per [RFC 8305](https://datatracker.ietf.org/doc/html/rfc8305)) in[ Feature #20108](https://bugs.ruby-lang.org/issues/20108) and [Feature #20782](https://bugs.ruby-lang.org/issues/20782), both address resolution and connection attempts are now parallelized.
As a result, the sum of `resolv_timeout` and `connect_timeout` no longer represents the total timeout duration. This is because, in HEv2, name resolution and connection attempts are performed concurrently, causing the two timeouts to overlap.
Example:
When `resolv_timeout: 200ms` and `connect_timeout: 100ms` are set:
1. An IPv6 address is resolved after the method starts immediately (IPv4 is still being resolved).
2. A connection attempt is initiated to the IPv6 address
3. After 100ms, `connect_timeout` is exceeded. However, since `resolv_timeout` still has 100ms left, the IPv4 resolution continues.
4. After 200ms from the start, the method raises a `resolv_timeout` error.
In this case, the total elapsed time before a timeout is 200ms, not the expected 300ms (100ms + 200ms).
Furthermore, in HEv2, connection attempts are also parallelized.
It starts a new connection attempts every 250ms for resolved addresses. This makes the definition of `connect_timeout` even more ambiguous—specifically, it becomes unclear from which point the timeout is counted.
Additionally, these methods initiate new connection attempts every 250ms (Connection Attempt Delay) for each candidate address, thereby parallelizing connection attempts. However, this behavior makes it unclear from which point in time the connect_timeout is actually measured.
Currently, a `connect_timeout` is raised only after the last connection attempt exceeds the timeout.
Example:
When `connect_timeout: 100ms` is set and 3 address candidates:
1. Start a connection attempt to the address `a`
2. 250ms after step 1, start a new connection attempt to the address `b`
3. 500ms after step 1, start a new connection attempt to the address `c`
4. 1000ms after step 3 (1000ms after starting the connection to `c`, 1250ms after starting the connection to `b,` and 1500ms after starting the connection to `a`) `connect_timeout` is raised
This behavior aims to favor successful connections by allowing more time for each attempt, but it results in a timeout model that is difficult to reason about.
These methods have supported `resolv_timeout` and `connect_timeout` options even before the introduction of HEv2. However, in many use cases, it would be more convenient if a timeout occurred after a specified duration from the start of the method. Similar functions in other languages (such as PHP, Python, and Go) typically allow specifying only an overall timeout.
[Proposal]
I propose adding an `open_timeout` option to `Socket.tcp` in this PR, which triggers a timeout after a specified duration has elapsed from the start of the method.
The name `open_timeout` aligns with the existing accessor used in `Net::HTTP`.
If `open_timeout` is specified together with `resolv_timeout` and `connect_timeout`, I propose that only `open_timeout` be used and the others be ignored. While it is possible to support combinations of `open_timeout`, `resolv_timeout`, and `connect_timeout`, doing so would require defining which timeout takes precedence in which situations. In this case, I believe it is more valuable to keep the behavior simple and easy to understand, rather than supporting more complex use cases.
If this proposal is accepted, I also plan to extend `open_timeout` support to `TCPSocket.new`.
While the long-term future of `resolv_timeout` and `connect_timeout` may warrant further discussion, I believe the immediate priority is to offer a straightforward way to specify an overall timeout.
[Outcome]
If `open_timeout` is also supported by `TCPSocket.new`, users would be able to manage total connection timeouts directly in `Net::HTTP#connect` without relying on `Timeout.timeout`.
https://github.com/ruby/ruby/blob/aa0f689bf45352c4a592e7f1a044912c40435266/lib/net/http.rb#L1657
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* Raise an exception if it is specified together with other timeout options
> If open_timeout is specified together with resolv_timeout and connect_timeout, I propose that only open_timeout be used and the others be ignored.
Since this approach may be unclear to users, I’ve decided to explicitly raise an `ArgumentError` if these options are specified together.
* Add doc
* Fix: open_timeout error should be raised even if there are still addresses that have not been tried