SIMD-0099: Feature activation status syscall

proposals/0099-feature-activation-status-syscall.md

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+---
+simd: '0099'
+title: Feature Activation Status Syscall
+authors:
+  - Hana Mumei
+category: Standard
+type: Core
+status: Draft
+created: 2024-01-03
+feature: (fill in with feature tracking issues once accepted)
+---
+
+## Summary
+
+We propose a new syscall, `is_feature_active`, which can be invoked in a
+BPF program with a feature id to check whether that feature has been activated
+on the cluster. A proof of concept is provided
+[here](https://github.com/solana-labs/solana/pull/34611).
+
+## Motivation
+
+The immediate motivation for this is to enable porting native programs to
+BPF, as part of the work on Firedancer and Runtime v2. The Stake Program is
+regularly updated using feature gates, so some mechanism for it to be aware of
+their status is necessary.
+
+Currently there are four in use by the program, most notably
+`stake_raise_minimum_delegation_to_1_sol`, which will likely not be activated
+until some unknown time in the future.
+
+## Alternatives Considered
+
+The main alternative we considered was to eliminate the use of feature gates in
+what are now native programs, and instead upgrade them via a mechanism to
+swap the bytecode implementation of the program at the appropriate time,
+whether automatically or manually. However, this would be more complex and
+likely more brittle than the proposed solution, and may also make it
+easier to railroad changes without the same level over oversight that
+feature gates provide.
+
+We may also consider a null option, i.e. no longer making consensus-breaking
+changes to the native programs at all. But this is likely not a realistic
+option as the network continues to evolve.
+
+## New Terminology
+
+N/A.
+
+## Detailed Design
+
+Implementation is fairly straightforward. A new syscall is added to
+`programs/bpf_loader` called `SyscallIsFeatureActive`. It accepts two positional
+arguments:
+
+* `var_addr`: pointer to `bool`, a memory location to write feature status
+* `feature_pubkey_addr`: pointer to `Pubkey`, the id of the feature to check
+
+The syscall begins by consuming compute. As an example, we have used
+`sysvar_base_cost` plus `size_of::<bool>()`, but this can be discussed.
+Then the syscall checks `invoke_context.feature_set.is_active()` for the
+feature id, writes the result into `var_addr`, and returns `Ok(SUCCESS)`.
+
+As-written, the syscall successfully returns `false` if the feature id is not
+found. This is identical to the behavior of the `is_active()` function and seems
+more appropriate than signalling failure.
+
+A new function is provided in `sdk/program` called `is_feature_active()`, which
+accepts the feature id as `&Pubkey` and returns `Result<bool, ProgramError>`.
+
+The stub version of the syscall completes normally with an invariant result of
+`false`, though there is no reason it couldn't be `true`.
+
+We also move all feature ids from `sdk` to `sdk/program` so that they can be
+used in a program context.
+
+## Impact
+
+A benefit of this proposal is that dapp developers will be able to query feature
+activation status in any BPF program, which may allow them to make programs that
+are more robust to new features, to preemptively code against new features and
+let the chain state handle "activation," and allow upstream more flexibility in
+designing features, knowing they can signal information downstream this way.
+
+The first obvious impact is that the Firedancer team will need to approve and
+implement this.
+
+Another impact is the way in which this SIMD interacts with the Multi-Client
+Feature Gates SIMDs detailed in
+<https://github.com/solana-foundation/solana-improvement-documents/issues/76>.
+These SIMDs will need to be coordinated with each other, but it does not appear
+at first read that they will interfere. It does mean that feature ids and
+feature state will need to remain available via `InvokeContext` and cannot
+be made resident under the `Feature111111111111111111111111111111111111`
+program, but this would likely be the case anyway since features are used
+pervasively throughout the runtime. Further investigation is needed.
+
+## Security Considerations
+
+Other than possible mistakes in implementation, it does not seem that this has
+a potential security impact. The proof-of-concept implementation is modeled
+after `Clock::get()` and does not differ substantially: it simply surfaces
+data to a BPF program with no ability for the caller to mutate state.
+
+## Backwards Compatibility
+
+Feature ids moved to `sdk/program` are reexported in their original `sdk` module
+to ensure no interface breakage downstream.