feat: prepare empty accounts

[Dodecahedr0x]

What changed

AccountAccessor::create now scans the bundled post-finalize actions for writable accounts that are absent (or only a slot-0 placeholder) and prepends a new MagicRootInstruction::Prepare per missing account to the same transaction. Prepare (magic-root-program) materializes the target as an empty system-owned account only if it still does not exist at execution time and is a no-op otherwise, making the "create the shared writables my actions need" step idempotent.

Closes #101

Impact

Post-finalize actions can initialize ER-only PDAs that did not exist when the create was composed, and concurrent creates naming the same missing account converge on whichever transaction materializes it first instead of failing each other with PostFinalize: Immutable. Adds one instruction variant to the magic-root interface; existing instructions are unchanged.

Reviewer notes

The idempotency check in account::prepare is is(AccountMode::Placeholder) && slot() == 0, i.e. "never materialized"; anything else is left untouched. missing_writable_action_accounts deduplicates and skips the created account itself. Tests: prepare_creates_missing_and_noops_on_existing and concurrent_creates_share_one_missing_writable in engine/tests/accounts.rs.

[coderabbitai]

Warning

Review limit reached

Next included review available in 6 minutes.

View limit details

Limit details: You’ve used the included review currently available.

You've used all free OSS reviews for now. Wait for the free limit to reset to keep reviewing this public repository.

Learn how review limits work.

Review configuration:

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro Plus

Run ID: 08fde231-d0ea-40e3-9c29-f546089047bb

📥 Commits

Reviewing files that changed from the base of the PR and between 965d339 and d71c8b2.

📒 Files selected for processing (8)

• engine/Cargo.toml
• engine/src/accessor.rs
• engine/tests/accounts.rs
• programs/magic-root-interface/src/lib.rs
• programs/magic-root-program/Cargo.toml
• programs/magic-root-program/README.md
• programs/magic-root-program/src/account.rs
• programs/magic-root-program/src/processor.rs

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}