feat: add configurable timeout for remote GETs to prevent build hangs

[bjoern-weidlich-anchorage]

Problem

When a TCP connection goes half-dead during a remote GET (e.g. over cloud interconnects or unreliable network paths), handleGet blocks indefinitely. The S3 response body is streamed directly into local disk storage via io.Copy, which blocks forever waiting for data on a dead connection. This hangs the Go build process since handleGet is synchronous.

The circuit breaker doesn't help because it only counts completed errors — a hanging request never returns an error.

Solution

Add CACHEPROG_REMOTE_GET_TIMEOUT (and --remote-get-timeout flag) that wraps the remote GET path in handleGet with a context timeout. When the timeout fires:

• The context is cancelled, aborting the S3 read
• The response is reported as a cache miss (not an error), so Go compiles from source instead of failing the build
• This applies to both the S3 fetch and the subsequent local write (which streams from the S3 body)

Testing

• Added unit test verifying timeout returns a miss within the expected time window
• Manual testing with a black hole TCP server (accepts connections, never responds):
  • Without timeout: build hangs indefinitely
  • With CACHEPROG_REMOTE_GET_TIMEOUT=5s: timeout fires, miss reported, circuit breaker trips, build completes successfully

Usage

CACHEPROG_REMOTE_GET_TIMEOUT=2m

Setting to 0 (default) disables the timeout, preserving existing behavior.

[bjoern-weidlich-anchorage]

@xakep666 what do you think? Without this I'm currently not able to use it.

[xakep666]

Looks good. Maybe put timeouts should be added too. Timeout errors also should trigger circuit breaker.