SIMD-0163: Lift the CPI caller restriction (#163)

* First draft

* Review feedback.

proposals/0163-lift-cpi-caller-restriction.md

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+---
+simd: '0163'
+title: Lift the CPI caller restriction
+authors:
+  - Alexander Meißner
+category: Standard
+type: Core
+status: Draft
+created: 2024-07-16
+feature: TBD
+---
+
+## Summary
+
+Remove the check which forces CPI callers to have the program account of the
+callee available to them as an instruction account.
+
+## Motivation
+
+The restriction is purely historical and not necessary for the protocol or its
+implementations.
+
+Removing it would improve composability and reduce transaction building
+complexity because program accounts would no longer be passed all the way from
+transaction level instructions to the inner most CPI instructions.
+
+Additionally, not passing these program accounts is a significant reduction in
+data copied during serialization as program accounts of loader v1, v2 and v4
+contain the entire 10 MB ELF. This massively reduces the CU cost for nested CPI
+and disincentivizes programs from accessing program accounts, which could later
+allow us to remove program loading from transaction loading.
+
+### Example transaction
+
+- Accounts:
+  - Fee payer
+  - Caller Program
+  - Callee Program
+  - Token
+- Transaction-level instruction:
+  - Program Account: Caller Program
+  - Instruction Accounts:
+    - **Callee Program** (this could be removed)
+    - Token
+  - CPI:
+    - Program Account: Callee Program
+    - Instruction Accounts:
+      - Token
+
+Currently, in order to be used as a program account for a nested instruction
+the callee program must be passed as an instruction account to all outer
+instructions recursively.
+
+## New Terminology
+
+None.
+
+## Detailed Design
+
+In CPI a feature gate must switch the search for the program id of the callee
+from the instruction accounts of the caller to the account list of the
+transaction.
+
+Invoking a program in CPI which was un/re/deployed in the same transaction is
+prevented by the "delay visibility" feature and thus unproblematic.
+
+## Alternatives Considered
+
+None.
+
+## Impact
+
+See motivation.
+
+Dapp developers who wish to benefit from the lifting of the restriction shall:
+
+- Hard-code the callee address in the CPI call, in case they want a static
+dispatch
+- Use the instruction **data**, not instruction **accounts**, to receive the
+callee address as a parameter, in case they want a dynamic dispatch
+
+Transaction building should append the required program accounts, which are not
+passed as instruction accounts, at the end of the transaction accounts list.
+How the dapps describe which callee prorgams they require to be present in the
+transaction is explicitly left unspecified.
+
+## Security Considerations
+
+None.
+
+## Backwards Compatibility
+
+Programs remain unaffected the way they are, unless they want to profit from
+this change. In that case they can do one of the following:
+
+Existing programs, which have hard-coded the callee statically and only need it
+as any instruction account to satisfy the constraint imposed by the runtime,
+can be fed a placeholder like `NativeLoader1111111111111111111111111111111`, in
+order not to shift the indices of the other instruction accounts.
+
+All other existing programs, which dynamically call whatever is passed in a
+specific instruction account will have to be updated and redeployed to benefit
+from the lifting of the restriction.