config/types: support slice indexing in GetByPath

[raelga]

Summary

Teach Configuration.GetByPath to navigate slice ([]any) nodes by interpreting the next dot-segment as a non-negative integer index. Map navigation is unchanged.

After this change, a config reference like imageRegistryPolicy.extraAllowedRegistries.0 resolves to the first element of that slice, matching the existing dot-path conventions for map-valued configuration.

Why

Configuration values can be slices today, but GetByPath could only return a whole slice as an opaque value — there was no way to address an individual element. That blocks downstream tooling (notably the sdp-pipelines dunder rendering pipeline) from producing per-index placeholder references for list-valued configuration during ARO-HCP Ev2 manifest generation.

A list-valued field that uses Go + "{{ range }}" + in a Helm values.yaml currently has no working dunder representation: the build-time mapping collapses the entire slice to a single scalar placeholder, and + "text/template" + then fails with + "range can't iterate over " + (e.g. Azure/ARO-HCP#4690 hitting __imageRegistryPolicy.extraAllowedRegistries__).

This PR is the first of two changes that, together, fix that — it unblocks a sdp-pipelines change that emits per-index dunder placeholders (__path.0__, __path.1__, ...) and resolves them through GetByPath. That second PR will land separately in Azure/sdp-pipelines once this is merged and tagged.

Behaviour

Path	Before	After
items (slice node)	returns whole []any	returns whole []any (unchanged)
items.0	error	returns element 0
items.5 (len 3)	error	*IndexOutOfRangeError
items.foo	error	error: "expected non-negative integer..."
nested.key.deep (deep map walk)	unchanged	unchanged

A new IndexOutOfRangeError sentinel lets callers distinguish out-of-bounds from other failures without string matching, in the same shape as the existing MissingKeyError. The fall-through error wording changes from "expected nested map" to "expected nested map or slice" to accurately describe the new capability — one existing test that exact-matched that string was updated.

Tests

• config/types.TestGetByPathSlices (new, 13 cases): top-level slice index, last element, slice-of-map, slice-of-slice, the realistic image-registry-policy shape, and three error paths (missing key, out-of-range, non-numeric slice key).
• config.TestGetByPath (existing, 4 cases): error-string expectation updated; otherwise unchanged.
• go vet ./... clean.

Related

• Tracked in AROSLSRE-913
• Companion sdp-pipelines change to follow once this is merged.
• Mitigates a class of failures previously worked around via revert at Revert "[FSI] ValidatingAdmissionPolicy to enforce image registry allowlist" (#4690) ARO-HCP#5357.

[Copilot]

Pull request overview

Adds slice ([]any) traversal support to config/types.Configuration.GetByPath, allowing dot-path segments to index into list-valued configuration (e.g., items.0) while preserving existing map traversal behavior. This unblocks downstream tooling that needs per-index config references when rendering list-valued template placeholders.

Changes:

• Extend Configuration.GetByPath to handle slice nodes by parsing the next dot-segment as an integer index.
• Introduce IndexOutOfRangeError to distinguish out-of-bounds slice indexing from other failures.
• Add dedicated slice-navigation test coverage and update an existing error-string expectation.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
config/types/configuration.go	Implements slice indexing in GetByPath and adds IndexOutOfRangeError.
config/types/configuration_test.go	Adds a new table-driven test suite covering slice navigation and error cases.
config/types_test.go	Updates an expected error string to reflect the new “map or slice” wording.

Comments suppressed due to low confidence (1)

config/types/configuration.go:76

• The slice-index parsing uses strconv.Atoi, which accepts leading signs. That means a path segment like "-0" parses to 0 and will successfully index the slice, even though the docstring/error message says the key must be a non-negative integer literal. Consider validating the segment first (e.g., require ^[0-9]+$ and then parse), and treat negative indices consistently (either reject them with the "expected non-negative integer" error or explicitly document/support them).

		case []any:
			idx, err := strconv.Atoi(key)
			if err != nil {
				return nil, fmt.Errorf("configuration%s: expected non-negative integer to index slice, got %q", currentPath, key)
			}
			if idx < 0 || idx >= len(typed) {
				return nil, &IndexOutOfRangeError{Path: currentPath, Index: idx, Length: len(typed)}
			}

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[raelga]

Closing per architectural clarification from @stevekuznetsov (in the cc thread on this PR and #237) and @geoberle, and as documented in https://github.com/Azure/ARO-HCP/blob/main/docs/configuration.md#limitations ("Avoid using arrays in configuration. Instead, represent arrays as a list of comma separated values"), per-region array configuration is not architecturally supported.

The intended consumer of slice-indexed GetByPath lookups (the dunder placeholder generator walking __foo.0__, __foo.1__, ...) only exists to support arrays in per-region configuration, which the policy disallows.

Closing in favor of #237, which addresses the surviving scalar (boolean / integer / number) dunder-validation case and stays inside the documented configuration limitations. Tracking via AROSLSRE-900.