Honor server Keep-Alive timeout hints in the HTTP/1 connection pool

[quinnj]

Fixes #1155.

Problem

Servers such as Apache, nginx and HAProxy advertise their idle keep-alive window on responses as Keep-Alive: timeout=<seconds> (Apache's default is 5 seconds). The HTTP/1 pool only knew its own idle_timeout_ns (default 90 seconds), so a pooled connection could be handed out again well after the server had already closed it at its own deadline. An idempotent request then burned a retry (_retryable_reused_conn_error), and a non-idempotent one failed outright.

Change

• Conn gains a keepalive_timeout_ns field. After a reusable response is read, _note_keepalive_hint! parses the response's Keep-Alive header and records the advertised window less a one-second safety margin. The margin exists because the server's clock starts when it finishes sending the response, before the client has finished reading it (this mirrors Node's undici keepAliveTimeoutThreshold).
• _evict_expired_idle_locked! now expires an idle connection when it exceeds either the pool's idle_timeout_ns or the connection's own server-advertised bound, whichever is tighter.
• A hint with no headroom (timeout=1 or timeout=0) marks the connection non-reusable, so the pool never keeps a connection the server is about to drop.
• Parsing is tolerant of parameter order, whitespace, case (Timeout = 3), quoted values, and multiple header lines. Malformed or signed values are ignored (treated as no hint). Absurd values saturate instead of overflowing.
• max=<n> is intentionally ignored: a server that exhausts it answers with Connection: close, which the pool already honors.
• Transport docstring and CHANGELOG updated.

Tests

Added to test/http_client_transport_tests.jl:

• Keep-Alive timeout hint parsing (#1155): 17 parser cases.
• HTTP client transport evicts an idle conn past its server keep-alive window (#1155): pool idle timeout disabled, only the per-connection bound evicts the stale peer and the fresh one is reused.
• HTTP client transport honors the server Keep-Alive timeout hint end to end (#1155): raw TCP server replies with timeout=5, max=100 (one connection serves both requests, pooled bound is 4s), max=100 (reused, no bound), and timeout=1 (not pooled, second request dials a new connection).

Locally test/http_client_transport_tests.jl passes in full (36 testsets, 0 failures) on Julia 1.12.6 with 4 threads.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.45%. Comparing base (d6f049a) to head (c8f91d0).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1358      +/-   ##
==========================================
+ Coverage   89.42%   89.45%   +0.03%     
==========================================
  Files          31       31              
  Lines       12594    12627      +33     
==========================================
+ Hits        11262    11296      +34     
+ Misses       1332     1331       -1     

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