SIMD-0162: Remove Accounts is_executable Flag Checks (#162)

* First draft

* Review feedback.

proposals/0162-remove-accounts-executable-flag-checks.md

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+---
+simd: '0162'
+title: Remove Accounts `is_executable` Flag Checks
+authors:
+  - Alexander Meißner
+category: Standard
+type: Core
+status: Draft
+created: 2024-07-16
+feature: TBD
+extends: 0093
+---
+
+## Summary
+
+Remove all checks of accounts `is_executable` flag which can throw errors in
+the runtime.
+
+## Motivation
+
+Currently, a program account which satisfies the following conditions can be
+invoked / executed:
+
+- Has the `is_executable` flag set
+- Is owned by a loader
+- Contains a successfully verified ELF, contains the address of a program data
+account which does or is a built-in program
+
+The second condition can be used as a performance optimization to filter out
+accounts which definitely do not contain programs and the third condition alone
+would be sufficient to guarantee correct execution. The first condition is
+however is not only useless, it is downright deceptive and entangled with the
+confusing proxy account workaround in loader-v3.
+
+Originally, the `is_executable` flag meant that a program was deployed and
+finalized, which effectively renders its program account read-only forever,
+as the `is_executable` can not be cleared / reset.
+
+With the introduction of the upgradeable loader (loader-v3), instead of
+removing the first condition back then, a workaround was created: The program
+account became a proxy account which has the `is_executable` flag set and then
+points to the actual program data account which does not have the
+`is_executable` flag set. Meaning the `is_executable` flag now neither does
+reliably indicate that an account contains a program (as the program data
+account does not have the flag set), nor does it indicate that a program is
+finalized (as the program account has it set while the program remains
+upgradeable).
+
+Removing the first condition not only removes some redundant error checks from
+the runtime and confusion for future maintainers, but more importantly enables
+us to deploy loader-v4 which is upgradeable like loader-v3 but without the need
+for proxy accounts.
+
+## New Terminology
+
+None.
+
+## Detailed Design
+
+This proposal aims to unblock loader-v4 with minimal impact on the ecosystem by
+only removing checks of the `is_executable` flag. A complete removal of the
+flag can be addressed in a subsequent proposal. Thus, the following must remain
+unaffected for now:
+
+- Setting of the `is_executable` flag during program deployment in loader-v3
+- Calculation of the account hashes
+- Minimization of snapshots
+- Serialization of instruction accounts `is_executable` flag for dapps
+- CPI ignores changes made by the caller to instruction accounts which have
+the `is_executable` flag set
+
+These checks of the `is_executable` flag must be removed:
+
+- `ExecutableLamportChange` during execution (transaction succeeds instead)
+
+These checks of the `is_executable` flag do not influence whether transactions
+fail or succeed because they are all covered by other checks. Thus, only the
+thrown error codes will change, which does not affect consensus. Nevertheless,
+they should still be removed because all implementations should aim to produce
+the same error codes:
+
+- during transaction loading:
+  - `InvalidProgramForExecution` (fallthrough to `UnsupportedProgramId` during
+  execution)
+  - `AccountNotExecutable` (fallthrough to `UnsupportedProgramId` during
+  execution)
+  - Meaning the transaction loading checks are effectively deferred until
+execution, which gives users more complete transaction logs.
+
+- in the `Upgrade` instruction of loader-v3:
+  - `AccountNotExecutable`
+  (fallthrough to `IncorrectProgramId` or `InvalidAccountData`, depending on
+  the owner)
+
+- during execution:
+  - `AccountNotExecutable` (fallthrough to `UnsupportedProgramId` during
+  execution)
+  - `IncorrectProgramId` (fallthrough to `UnsupportedProgramId` during
+  execution)
+  - `ExecutableDataModified`
+  (fallthrough to `ExternalAccountDataModified` during execution)
+  - `ExecutableModified`
+  (is unreachable, as it is overshadowed by owner and writability checks)
+  - `ModifiedProgramId`
+  (is unreachable, as it is overshadowed by owner and writability checks)
+
+Similarly the following checks during execution, unrelated to the
+`is_executable` flag, but related to whether an account contains a program or
+not, should be changed to throw `UnsupportedProgramId` instead:
+
+- `InvalidAccountData` for programs which are closed, in visibility delay,
+failed verification or not owned by a loader
+- `IncorrectProgramId` for unrecognized built-in programs
+
+All in all, the following error messages related to invocation of program
+accounts will be coalesced into `UnsupportedProgramId`, but some of them will
+remain in use in other circumstances unrelated to program execution:
+
+- `InvalidProgramForExecution`
+- `AccountNotExecutable`
+- `InvalidAccountData`
+- `IncorrectProgramId`
+- `UnsupportedProgramId`
+
+## Alternatives Considered
+
+None.
+
+## Impact
+
+Error codes of these conditions, which are rarely triggered, will change.
+
+The only consensus relevant change is that it will become possible (for
+everybody) to donate funds to program accounts. That however is expected not to
+break any existing programs.
+
+## Security Considerations
+
+None.