Deduplicate OAuth2/OIDC link nonce creation

[Copilot]

CreateLinkNonce had two near-identical implementations in OAuth2Handler and OIDCHandler, which created avoidable maintenance risk around nonce generation, hashing, persistence, and response handling. This change consolidates that flow into one shared path while preserving each handler’s existing nonce-generation entry point and TTL.

• Shared link-nonce flow

  • Added createLinkNonce in handler/oauth2_common.go for the common account-linking nonce lifecycle:
    • store availability guard
    • authenticated user lookup from request context
    • nonce generation
    • high-entropy token hashing
    • nonce persistence with expiration
    • JSON response payload

• Handler delegation

  • Replaced duplicated CreateLinkNonce bodies in:
    • OAuth2Handler
    • OIDCHandler
  • Both handlers now delegate to the shared helper.

• Behavior preserved

  • The helper accepts a nonce generator, so each handler keeps its original generation path:
    • OAuth2 continues to use auth.GenerateRandomBase64(32)
    • OIDC continues to use generateOIDCState
  • Existing handler-specific TTLs remain unchanged.

func (h *OIDCHandler) CreateLinkNonce(w http.ResponseWriter, r *http.Request) {
	createLinkNonce(w, r, h.LinkNonces, oidcStateCookieTTL, generateOIDCState)
}

func (h *OAuth2Handler) CreateLinkNonce(w http.ResponseWriter, r *http.Request) {
	createLinkNonce(w, r, h.LinkNonces, oauth2StateCookieTTL, func() (string, error) {
		return auth.GenerateRandomBase64(32)
	})
}

Greptile Summary

This PR eliminates duplicated CreateLinkNonce implementations in OAuth2Handler and OIDCHandler by extracting a shared createLinkNonce helper in oauth2_common.go. Both handlers delegate to it while retaining their original nonce generators and TTLs.

• handler/oauth2_common.go: New createLinkNonce function handles the full lifecycle — store nil-guard, user-ID extraction, nonce generation via injected func, high-entropy hashing, persistence, and JSON response.
• handler/oauth2.go / handler/oidc.go: Each CreateLinkNonce method is replaced with a single delegation call, with no change to observable behavior.

Confidence Score: 5/5

Safe to merge — the refactor is a straightforward delegation with no behavioral changes to nonce generation, hashing, persistence, or response handling.

Both handlers preserve their original nonce generators and TTLs; the shared helper replicates the exact same control flow that existed in both originals. No logic has been altered.

No files require special attention.

Important Files Changed

Filename	Overview
handler/oauth2_common.go	New shared createLinkNonce helper correctly consolidates the store-guard, user-ID extraction, nonce generation, hashing, persistence, and JSON response; minor inconsistency in structured log attributes.
handler/oauth2.go	Delegates CreateLinkNonce to the shared helper with the original auth.GenerateRandomBase64(32) generator and oauth2StateCookieTTL; behavior preserved.
handler/oidc.go	Delegates CreateLinkNonce to the shared helper with generateOIDCState and oidcStateCookieTTL; behavior preserved.

Sequence Diagram

sequenceDiagram
    participant Client
    participant OAuth2Handler/OIDCHandler
    participant createLinkNonce
    participant Store as auth.OIDCLinkNonceStore

    Client->>OAuth2Handler/OIDCHandler: POST /link-nonce
    OAuth2Handler/OIDCHandler->>createLinkNonce: delegate(w, r, store, ttl, generateNonce)
    createLinkNonce->>createLinkNonce: store nil-guard
    createLinkNonce->>createLinkNonce: auth.UserIDFromContext(r.Context())
    createLinkNonce->>createLinkNonce: generateNonce()
    createLinkNonce->>createLinkNonce: auth.HashHighEntropyToken(nonce)
    createLinkNonce->>Store: CreateLinkNonce(ctx, userID, hash, expiry)
    Store-->>createLinkNonce: ok / err
    createLinkNonce-->>Client: "200 {nonce} or error"

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Prompt To Fix All With AI

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 1
handler/oauth2_common.go:85
The `user_id` structured attribute is included in the generate-nonce error log (line 78) but is dropped from the store-nonce error log, even though `userID` is already in scope. Without it, a store failure in the persistence layer loses the user identity that would be needed for incident triage.

```suggestion
		slog.ErrorContext(r.Context(), "failed to store link nonce", slog.Any("error", err), slog.String("user_id", userID))
```

_{Reviews (2): Last reviewed commit: "Update handler/oauth2_common.go" | Re-trigger Greptile}

[Copilot]

Pull request overview

This PR reduces maintenance risk in the account-linking flow by deduplicating the OAuth2 and OIDC implementations of link-nonce creation into a shared helper while keeping each handler’s existing nonce generation approach and TTL.

Changes:

• Added a shared createLinkNonce helper in handler/oauth2_common.go to centralize nonce generation, hashing, persistence, and JSON response writing.
• Updated OIDCHandler.CreateLinkNonce and OAuth2Handler.CreateLinkNonce to delegate to the shared helper.

Show a summary per file

File	Description
handler/oidc.go	Replaces the inline OIDC link-nonce creation logic with a call to the shared helper.
handler/oauth2.go	Replaces the inline OAuth2 link-nonce creation logic with a call to the shared helper, preserving its nonce generation method.
handler/oauth2_common.go	Introduces the shared createLinkNonce helper that encapsulates the link-nonce lifecycle.

Copilot's findings

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• Files reviewed: 3/3 changed files
• Comments generated: 1