[schmitch]

Fixes #689

What

When the upstream connection failed while Forge staged a blob (a reset, the idle read_timeout, the download deadline), the request answered 500 internal error and the log blamed storage. It now answers like the other upstream failures.

• images/mod.rs: a new marker error, UpstreamBodyError, travels inside the io::Error, the same way push.rs tags request bodies with RequestBodyError. upstream_body_reader(response) wraps the upstream stream with it. In impl From<CasError> for Failure, a CasError::Io that carries the marker maps to upstream_unreachable: 503 the upstream registry is unreachable, logged as the warning registry proxy: upstream request failed. Every other I/O error from staging, such as a disk error, still answers 500 storage error.
• Registry::proxied_blob (images/proxy.rs) stages through the new reader. This covers blobs of at most 1 MiB, such as configs, and layers that come without a Content-Length.
• The image import (images/import.rs) stages through it too. A failed item now says the source registry is unreachable: reading <digest> failed instead of internal error.

Tests

• Unit test: images::tests::an_unreadable_upstream_body_is_the_upstreams_error sits beside an_unreadable_request_body_is_the_clients_error. A real reqwest response is cut short against its Content-Length; the test checks it maps to the unreachable 503, and that a plain I/O error still maps to 500.
• Integration test: registry.rs::an_upstream_cut_short_mid_blob_is_unreachable. MockRegistry gets a cut_blobs switch that sends half of a small blob and then drops the connection. The pull answers 503 with the unreachable message and caches nothing. Once the upstream recovers, the pull answers 200.

Run locally:

• cargo fmt --all --check passes.
• cargo clippy -p forge-server --all-targets -- -D warnings passes.
• cargo xtask changelog --check passes.
• The new unit and integration tests pass, along with the nearby proxy tests in registry.rs.

The changelog fragment is docs/changes/2.2.0/689.fixed.md.

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