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libexpr: rethrow failed realisation error - #16682
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A cached string's context paths each required one `isValidPath` call from `getStringWithContext()`. Each path cost a daemon round-trip. The function switched to `addTempRoot` for each path. Those calls deduplicate per connection in `RemoteStore`. A single `queryValidPaths` call can check all paths.
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Attributes that import the same failing derivation ask the store to build that derivation again. Many attributes import the same broken derivation. These importers cause one attempt each. The output lock of the store serialized concurrent evaluator attempts. Build with `buildPathsWithResults`. Save a copy of the build error for each derivation that failed alone. Reasons include failure of the builder, rejection of inputs or outputs, hash mismatch and log limit. Same for a timeout. A second attempt can wait for the full timeout again. Rethrow that copy rather than rebuild. Rebuild transient failures such as network errors from a substituter, dependency failure and loss of the daemon connection. The `:reload` command in the REPL can call `resetFileCache()`. Stored failures disappear after `resetFileCache()`.
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Motivation
I run a small CI system. Many attributes of a flake import the same derivation. Attributes that import a failing derivation cause the store to build that derivation again. Importers each trigger one attempt for a broken derivation. Concurrent evaluator attempts serialize on the lock for a store output.
Context
The evaluator can now keep a copy of the exception for each
EvalStateand rethrow that copy for the rest of its lifetime. The evaluator can capture the copy before the exception propagates. Handlers add traces to a caught exception. Tests check that a replayed failure and the first failure have the same number of traces.The changes sit atop #16677. The test store and its
DummyStoreImplheader come from that PR.Add 👍 to pull requests you find important.
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