refactor: expose DECOMPRESSED_DISCRIMINATOR and pubkey in decompressed accounts

[ananas-block]

Changes

1. New Constant Definition

• Added DECOMPRESSED_PDA_DISCRIMINATOR: [u8; 8] = [255u8; 8] to program-libs/compressible/src/lib.rs
• Marks placeholder compressed accounts created when PDAs are decompressed to Solana accounts
• Shared constant used across MintAction and SDK implementations

2. MintAction Changes (Compressed Token Program)

Three files updated to handle decompressed mint placeholders:

mint_output.rs:

• Modified serialize_compressed_mint() to accept validated_accounts parameter
• When mint is decompressed:
  • Allocate exactly 32 bytes for data (was 0)
  • Copy PDA pubkey to data field
  • Set discriminator to DECOMPRESSED_PDA_DISCRIMINATOR
  • Hash PDA pubkey with Sha256BE for data_hash

mint_input.rs:

• When processing input for decompressed mints:
  • Set discriminator to DECOMPRESSED_PDA_DISCRIMINATOR
  • Hash mint pubkey with Sha256BE for data_hash

zero_copy_config.rs:

• Allocate 32 bytes for decompressed mint data (was 0)

3. SDK Interface Changes

Three files updated for generic PDA compression/decompression flows:

compress_account_on_init.rs:

• Added set_decompressed_pda_output() helper with #[inline(never)] for stack optimization
• Sets PDA pubkey in data, hashes it, sets decompressed discriminator

compress_account.rs:

• Added set_decompressed_pda_input() helper with #[inline(never)] for stack optimization
• When re-compressing a decompressed PDA, fixes input to use new format

decompress_idempotent.rs:

• Added set_decompressed_pda_output() helper with #[inline(never)] for stack optimization
• Sets output for decompressed PDA placeholder with new format

4. Test Updates

• program-tests/utils/src/assert_mint_action.rs: Updated to verify discriminator and PDA pubkey in decompressed placeholder
• sdk-tests/csdk-anchor-full-derived-test/tests/basic_test.rs: Updated assertions for decompressed PDA format
• sdk-tests/csdk-anchor-full-derived-test/tests/mint/metadata_test.rs: Updated assertions for decompressed PDA format

Technical Details

• Used #[inline(never)] on helper functions to reduce stack usage and avoid SBF stack limit issues
• Sha256BE hash used for all PDA pubkey hashing to maintain consistency
• Placeholder data is always exactly 32 bytes (the PDA pubkey)
• All three SDK interface files (compress, compress_on_init, decompress) now use consistent format

Summary by CodeRabbit

• New Features

  • Decompressed accounts now use a distinct marker and store the PDA pubkey as the payload, with its hash recorded for integrity.

• Improvements

  • Serialization and account preparation paths updated to populate decompressed outputs/inputs with PDA data and computed hashes.
  • Zero-copy layout and validation updated to expect 32-byte decompressed mint data.

• Bug Fixes

  • More consistent metadata and error messages for decompressed account handling.

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[coderabbitai]

📝 Walkthrough

Walkthrough

Adds a new public constant DECOMPRESSED_PDA_DISCRIMINATOR and updates compressed-mint handling and SDK account preparation/serialization to use that discriminator with SHA-256 hashed PDA pubkeys for decompressed PDA placeholders.

Changes

Cohort / File(s)	Summary
Core Constant Definition program-libs/compressible/src/lib.rs	Adds public constant DECOMPRESSED_PDA_DISCRIMINATOR: [u8; 8] (value: [255,255,255,255,255,255,255,0]) with docs marking decompressed PDA placeholders.
Mint action — input, output, config programs/compressed-token/program/src/compressed_token/mint_action/mint_input.rs, .../mint_output.rs, .../zero_copy_config.rs	mint_input.rs: imports discriminator and uses it with SHA‑256(mint PDA) for decompressed-input hashing. mint_output.rs: serialize_compressed_mint(...) gains validated_accounts: &MintActionAccounts, when decompressed sets discriminator to DECOMPRESSED_PDA_DISCRIMINATOR, writes 32‑byte CMint PDA into data and sets data_hash = SHA‑256(pda). zero_copy_config.rs: decompressed mint output data length changed from 0 to 32.
SDK — account prepare/serialize helpers sdk-libs/sdk/src/interface/compress_account.rs, sdk-libs/sdk/src/interface/compress_account_on_init.rs, sdk-libs/sdk/src/interface/decompress_idempotent.rs	Adds private helpers (e.g., set_decompressed_pda_input / set_decompressed_pda_output) that set discriminator = DECOMPRESSED_PDA_DISCRIMINATOR, write PDA pubkey bytes to data, and compute data_hash = SHA‑256(pubkey). Integrates these into account preparation flows for compression input, init output, and idempotent decompression output.

Sequence Diagram(s)

(omitted)

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Suggested labels

ai-review

Suggested reviewers

• sergeytimoshin
• SwenSchaeferjohann

Poem

🧩 A marker set where zeros once lay,
Eight bytes beckon a clearer way,
Pubkeys hashed, discriminators shine,
Mints and SDKs now speak the same line —
Small change, tidy song, code aligned.

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: introducing a new discriminator constant and updating code to expose the PDA pubkey in decompressed account placeholders across multiple files.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 70.00%.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch jorrit/refactor-add-255u8-discriminators

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[coderabbitai]

Actionable comments posted: 2

🤖 Fix all issues with AI agents

In `@program-libs/compressible/src/lib.rs`:
- Around line 7-10: Update the doc comment for DECOMPRESSED_PDA_DISCRIMINATOR to
state that the decompressed PDA entry stores the raw PDA pubkey bytes in the
account data (not a hashed pubkey) and that the pubkey is hashed separately into
data_hash elsewhere; mention that this discriminator marks a compressed account
as a decompressed PDA placeholder so readers know where raw pubkey bytes are
stored and how the hash is derived later.

In `@sdk-libs/sdk/src/interface/compress_account_on_init.rs`:
- Around line 133-140: The code uses a temporary method call &account_info.key()
when calling set_decompressed_pda_output; update this to the established pattern
by passing a reference to the byte array from the field conversion used
elsewhere—replace the argument with &account_info.key.to_bytes() (keeping the
surrounding logic in the if !with_data branch where compressed_account,
account_info_result, with_data, and set_decompressed_pda_output are used) so the
call site matches decompress_idempotent.rs and stays consistent across the
codebase.