Map proxy errors to specific status codes instead of always 502

[alexspeller]

Problem

httputil.ReverseProxy's ErrorHandler is invoked for several distinct failure modes, but Thruster collapses all of them into a 502 (logged at info level as "Unable to proxy request").

The most damaging case is client disconnection. When a client closes its connection before the response is ready — an aborted fetch, a Turbo Frame swapping its src, a navigation away, an upstream load balancer giving up — the proxy is called with a context.Canceled error. The client is already gone, so the 502 is written to a closed socket and discarded, but the access log still records status=502. Ordinary client cancellations therefore show up as server errors and pollute 5xx dashboards.

Change

Distinguish the error cases, mirroring the handling in kamal-proxy's handleProxyError:

Condition	Before	After
Client disconnect (context.Canceled)	502	499 Client Closed Request (nginx convention), logged at debug
Upstream timeout (net.Error with Timeout())	502	504 Gateway Timeout
Malformed chunked encoding from client	502	400 Bad Request
Connection refused / EOF / other	502	502 (unchanged)
Request entity too large	413	413 (unchanged)

The client-disconnect case still sets a status code (rather than skipping the write) so the request is recorded in the access log — just as a 499 rather than a 502, keeping it out of the 5xx bucket. Only genuine upstream failures now land there.

Tests

Adds proxy_handler_test.go covering each branch, including an end-to-end test that performs a real mid-flight client cancellation through the proxy and asserts it is recorded as 499 rather than 502.

[claude]

Claude Code Review

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[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds more nuanced reverse-proxy error handling so client cancellations are recorded as 499 (instead of 5xx), and certain upstream/transport errors map to more appropriate HTTP status codes.

Changes:

• Introduces StatusClientClosedRequest (499) and treats context.Canceled as a client-closed request without logging it as a proxy error.
• Classifies timeout errors as 504 Gateway Timeout and chunked-encoding parse failures as 400 Bad Request.
• Adds unit + end-to-end tests covering the new error classification behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
internal/proxy_handler.go	Adds 499 status constant and expands ProxyErrorHandler to classify cancellations/timeouts/chunked-encoding errors.
internal/proxy_handler_test.go	Adds tests validating status codes and logging behavior, including an end-to-end disconnect case.

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