Add opt-in persistent connection pooling to PostsData

[maximus1108]

Summary

• Adds opt-in persistent_connections class attribute to PostsData module
• When enabled, raw_ssl_request uses a per-class Net::HTTP::Persistent pool instead of creating a fresh Net::HTTP per request
• Eliminates TCP+TLS handshake overhead (~345ms at 115ms RTT) on repeat requests to the same host
• Zero behavior change for consumers that don't opt in (default: false)

Motivation

Carrier service rate requests in Shopify Core create a new TCP+TLS connection for every single HTTP request via ActiveUtils::PostsData → Connection → Net::HTTP.new. For high-latency third-party hosts (e.g., carriers in Argentina at ~115ms RTT), the handshake alone costs ~345ms — 63% of the total request time.

By pooling connections and reusing them for subsequent requests to the same host, we can eliminate this overhead entirely on warm hits, reducing p95 latency from ~600ms to ~250ms for these carriers.

See the full context document for detailed analysis of the problem, host distribution (55K hosts, power-law — top 50 handle 84% of volume), worker threading model (29 threads/pod), and pool sizing rationale.

Design

• Opt-in per class: self.persistent_connections = true after include ActiveUtils::PostsData
• Per-class pool isolation: Each including class gets its own Net::HTTP::Persistent instance
• Per-request timeout override: pool.read_timeout is set before each request call, supporting dynamic timeouts
• Error compatibility: Network errors caught and re-raised as ActiveUtils::ConnectionError / ResponseError
• Return type unchanged: raw_ssl_request still returns Net::HTTP::HTTPResponse

Configuration

class ShippingRateProvider
  include ActiveUtils::PostsData

  self.persistent_connections = true
  self.pool_size = 100          # 29 threads + warm connection headroom
  self.pool_idle_timeout = 60   # seconds
  self.pool_keep_alive = 60     # seconds
  self.pool_max_requests = 100  # recycle after N requests
end

Test plan

• All existing posts_data_test.rb tests pass unchanged (verified: 8/8)
• All existing connection_test.rb tests pass unchanged (verified: 32/32)
• All existing network_connection_retries_test.rb tests pass unchanged (verified: 23/23)
• 13 new tests covering: default off, pool reuse, per-request timeout, error mapping (timeout/reset/refused → ConnectionError, non-2xx → ResponseError), GET/POST, headers, pool clearing, config defaults
• Consumer opt-in in Core to be done as a follow-up PR (ShippingRateProvider behind feature flag)

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