Cache config file layer discovery on startup (#319)

[leynos]

Summary

Closes #319

Eliminates the duplicate config-file I/O on startup: resolve_merged_diag_json and merge_with_config each independently discovered and loaded all file layers, so every config file was opened, read, and deserialised twice per run.

Stacked on #365 (merge-pipeline debug logging) because both touch merge.rs/discovery.rs/main.rs; this PR retargets to main automatically once #365 merges.

Changes

• src/cli/discovery.rs: new ConfigFileLayers (discovery outcome loaded once; MergeLayer is Clone, so the merge clones cached layers instead of re-reading disk). push_file_layers now consumes the cache; collect_diag_file_layers is gone.
• src/cli/diag.rs / src/cli/merge.rs: new resolve_merged_diag_json_with_layers and merge_with_config_layers; the original functions remain as thin self-loading wrappers, keeping the existing API for tests.
• src/main.rs: loads the layers once and shares them between the pre-pass and the merge.
• build.rs: verify_public_api_symbols entries for the new symbols (project convention for the build-script copy of the CLI modules).

Testing

• New config_layers_are_loaded_once_and_shared test: loads the layers, deletes the config file, then verifies both the diag pre-pass and the merge still honour the file's contents — proving neither re-reads the disk.

Validation

• make check-fmt / make lint / make test — pass (37 suites)

🤖 Generated with Claude Code

Summary by Sourcery

Cache configuration file layer discovery so diagnostic pre-pass and full merge share loaded config layers instead of re-reading from disk on startup.

Enhancements:

• Introduce a ConfigFileLayers type to encapsulate discovered configuration layers for reuse across CLI operations.
• Add layer-aware variants of diagnostic resolution and configuration merge functions while preserving the existing public API.
• Wire main execution path to load config layers once and pass them to both diagnostic mode resolution and configuration merging.
• Extend build-time public API symbol verification to cover the new config-layer types and functions.

Tests:

• Add an integration test ensuring configuration layers are loaded once and reused even if the underlying config file is deleted after loading.

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[sourcery-ai]

Reviewer's Guide

Introduces a cached configuration file layer abstraction that is loaded once on startup and shared between the diagnostic pre-pass and full config merge, eliminating duplicate config-file I/O while preserving the existing public API surface.

Sequence diagram for cached config file layer reuse on startup

sequenceDiagram
    participant Run as run_with_args
    participant Layers as ConfigFileLayers
    participant Diag as resolve_merged_diag_json_with_layers
    participant MergeEntry as merge_cli_or_exit
    participant Merge as merge_with_config_layers
    participant Composer as MergeComposer

    Run->>Layers: ConfigFileLayers::load(parsed_cli)
    activate Layers
    Layers-->>Run: ConfigFileLayers
    deactivate Layers

    Run->>Diag: resolve_merged_diag_json_with_layers(parsed_cli, matches, &config_layers)
    activate Diag
    Diag->>Layers: ConfigFileLayers::as_result()
    Diag-->>Run: bool (DiagMode input)
    deactivate Diag

    Run->>MergeEntry: merge_cli_or_exit(parsed_cli, matches, mode, &config_layers)
    activate MergeEntry
    MergeEntry->>Merge: merge_with_config_layers(parsed_cli, matches, &config_layers)
    activate Merge
    Merge->>Composer: MergeComposer::with_capacity(4)
    Merge->>Merge: push_defaults_layer(composer, errors)
    Merge->>Merge: push_file_layers(&config_layers, composer, errors)
    Merge->>Composer: push_environment_layer(composer, errors)
    Merge->>Composer: push_cli_layer(cli, matches, composer, errors)
    Merge-->>MergeEntry: OrthoResult<Cli>
    deactivate Merge
    MergeEntry-->>Run: Result<Cli, ExitCode>
    deactivate MergeEntry

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File-Level Changes

Change	Details	Files
Add ConfigFileLayers abstraction to discover and cache config file layers once per invocation, and refactor discovery to work over cached layers.	Introduce ConfigFileLayers wrapper around OrthoResult<Vec<MergeLayer<'static>>> with load and as_result helpers Refactor push_file_layers to consume pre-loaded ConfigFileLayers instead of recomputing layers and to clone error Arcs when pushing to the error vector Change push_discovered_layers to accept a slice of MergeLayer and clone each layer before pushing into the MergeComposer, avoiding ownership moves from the cached layers Remove the now-redundant collect_diag_file_layers helper which separately loaded layers for diagnostics	src/cli/discovery.rs
Refactor diagnostic JSON resolution to operate on pre-loaded config layers while keeping a wrapper with the old signature for API compatibility.	Keep resolve_merged_diag_json as a thin wrapper that internally calls the new resolve_merged_diag_json_with_layers using freshly loaded ConfigFileLayers Add resolve_merged_diag_json_with_layers that takes ConfigFileLayers and iterates over cached layers to determine diag_json, falling back to defaults when discovery fails Change diag_json_from_file_layers to take ConfigFileLayers and return a bool directly, using as_result and cloning layers before into_value to preserve cached state	src/cli/diag.rs
Refactor config merge to reuse cached file layers and expose a new API that accepts them explicitly.	Keep merge_with_config as a thin wrapper that loads ConfigFileLayers and delegates to merge_with_config_layers Add merge_with_config_layers that accepts pre-loaded ConfigFileLayers and uses push_file_layers(file_layers, ...) instead of recomputing file layers from Cli Preserve existing merge sequencing (defaults, file, env, CLI) while changing only how file layers are obtained	src/cli/merge.rs
Wire the cached config layers through the main entrypoint so both diagnostic pre-pass and main merge share the same discovery result.	Load ConfigFileLayers once in run_with_args immediately after parsing CLI arguments Use resolve_merged_diag_json_with_layers with the shared ConfigFileLayers when computing DiagMode Pass the shared ConfigFileLayers into merge_cli_or_exit and call merge_with_config_layers there instead of merge_with_config	src/main.rs
Expose new symbols and validate them in the build script, and add a regression test proving layers are not reloaded from disk.	Re-export ConfigFileLayers and the *_with_layers variants from cli::mod so they are available to crate consumers Extend verify_public_api_symbols in build.rs with size and function-signature checks for ConfigFileLayers, ConfigFileLayers::load, resolve_merged_diag_json_with_layers, and merge_with_config_layers Add config_layers_are_loaded_once_and_shared test that loads config layers, deletes the backing file, and then asserts both diag resolution and merge still observe the original config values	src/cli/mod.rs build.rs tests/cli_tests/config_discovery.rs

Possibly linked issues

• Cache config file layer discovery to avoid double I/O on startup #319: The PR introduces ConfigFileLayers and shared usage, eliminating the double config-file discovery and I/O described in the issue.

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