#4088: CredentialSelector extension point in PD matcher

[stevenvegt]

Summary

• Introduces CredentialSelector function type in vcr/pe for pluggable credential selection
• Extracts FirstMatchSelector as the default (preserves existing behavior)
• Refactors matchConstraints to collect all matching VCs per input descriptor, then call the selector
• Adds MatchWithSelector to PresentationDefinition — Match delegates to it with FirstMatchSelector
• Adds SetCredentialSelector to PresentationSubmissionBuilder — Build uses it when set
• Adds ErrMultipleCredentials error for selectors that enforce strict matching
• Documents the CredentialSelector contract (nil vs error return semantics)
• All existing tests pass unchanged — pure refactor with additive API

Closes #4088
Part of #4067

Usage example

// Define a custom selector that picks the credential with a specific patient ID
mySelector := func(descriptor pe.InputDescriptor, candidates []vc.VerifiableCredential) (*vc.VerifiableCredential, error) {
    if len(candidates) == 0 {
        return nil, pe.ErrNoCredentials
    }
    if len(candidates) > 1 {
        return nil, pe.ErrMultipleCredentials
    }
    return &candidates[0], nil
}

// Use it with the builder
builder := presentationDefinition.PresentationSubmissionBuilder()
builder.AddWallet(holderDID, walletCredentials)
builder.SetCredentialSelector(mySelector)

submission, signInstruction, err := builder.Build("ldp_vp")

Without SetCredentialSelector, the builder uses FirstMatchSelector which picks the first matching credential per input descriptor — identical to the previous behavior.

Test plan

• Custom selector picks non-first credential
• Default behavior preserved (first match without selector)
• Selector receives all matching candidates
• Selector error propagates through Build
• All existing PD matching tests pass
• All existing PresentationSubmissionBuilder tests pass

[qltysh]

0 new issues

Tool	Category	Rule	Count

[qltysh]

Coverage Impact

⬇️ Merging this pull request will decrease total coverage on feature/4067-credential-selection-dcql by 0.01%.

Modified Files with Diff Coverage (2)

Rating	File	% Diff	Uncovered Line #s
	vcr/pe/presentation_definition.go	100.0%
	vcr/pe/presentation_submission.go	100.0%
	Total	100.0%

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[stevenvegt]

Review finding: ErrNoCredentials soft-fail for submission requirements

During review, we identified that a custom CredentialSelector returning ErrNoCredentials would hard-fail in matchConstraints, preventing submission requirements (e.g., pick rules with min: 0) from evaluating. This breaks backward compatibility: previously, when no VC matched a descriptor, it resulted in a nil candidate that submission requirements could handle.

Example

A PD with pick rule, min: 0:

• Match(emptyVCs) → succeeds (FirstMatchSelector returns (nil, nil), submission requirement accepts zero fulfilled descriptors)
• MatchWithSelector(emptyVCs, strictSelector) → hard-failed because ErrNoCredentials aborted matchConstraints before submission requirements could evaluate

Fix

matchConstraints now catches ErrNoCredentials from the selector and converts it to a nil selection (soft fail). This allows submission requirements to decide whether zero fulfilled descriptors is acceptable. ErrMultipleCredentials and other errors remain hard failures.

Changes

• matchConstraints: soft-fail on ErrNoCredentials
• Updated CredentialSelector contract docs to clarify soft vs hard failure semantics
• New test TestPresentationDefinition_MatchWithSelector_SubmissionRequirements verifying Match and MatchWithSelector behave identically with min: 0