[fadedreamz]

Backport of upstream curl/curl@ef6b17cf12 (curl#23412). It's on upstream master after 8.23.0 and will first ship in 8.24.0.

Problem

curl 8.22.0 (upstream 2f1e94e) skips the connection alive check if it ran less than a second ago. Returning a connection to the pool counts as a check. So an idle HTTP/1.1 connection the server has closed is handed out as healthy within that second:

1. The request goes out on the dead connection and gets nothing back.
2. curl retries on a fresh connection (Curl_retry_request) and must rewind the upload.
3. For a streaming upload whose read callback already returned CURL_READFUNC_PAUSE and has no seek support, the rewind fails with CURLE_SEND_FAIL_REWIND ("seek callback returned error 2").

In Unity this breaks HttpHandlerBehaviors.SendAsync_{Get,Put}WithLateWrittenBody_ServerReceivesFullBody on iOS. They pass with curl 8.20 (Yamato 77970446) and fail with 8.22 (Yamato 77880251).

Change

Poll for pending input on every reuse of an idle, non-multiplexed connection, as 8.20 did. The once-a-second limit stays for multiplexed connections and protocols with their own connection_is_dead.

Adapted to 8.22.0: upstream's cpool_conn_can_be_closed(conn) is spelled out as !CONN_INUSE(conn) && !conn->bits.connect_only. Upstream test 1701 is included.

Verification

• Not built or run locally.
• To verify: build through the libcurl submodule bump, then re-run the iOS PlayMode CoreTests job on unity/unity PR #130795.

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