feat: Introduce new random_seed_array method and update docs (#692)

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Changelog
 
+## [unreleased]
+
+### Features
+- Added `env::random_seed_array` to return a fixed length array of the `random_seed` and optimizes the existing function. [PR 692](https://github.com/near/near-sdk-rs/pull/692)
+
 ## `4.0.0-pre.5` [12-23-2021]
 - fix(standards): Fix NFT impl macros to not import `HashMap` and `near_sdk::json_types::U128`. [PR 571](https://github.com/near/near-sdk-rs/pull/571).
 - Add drain iterator for `near_sdk::store::UnorderedMap`. [PR 613](https://github.com/near/near-sdk-rs/pull/613).

near-sdk/src/environment/env.rs

@@ -232,9 +232,25 @@ pub fn used_gas() -> Gas {
 // ############
 // # Math API #
 // ############
-/// Get random seed from the register.
+
+/// Returns the random seed from the current block. This 32 byte hash is based on the VRF value from
+/// the block. This value is not modified in any way each time this function is called within the
+/// same method/block.
 pub fn random_seed() -> Vec<u8> {
-    method_into_register!(random_seed)
+    random_seed_array().to_vec()
+}
+
+/// Returns the random seed from the current block. This 32 byte hash is based on the VRF value from
+/// the block. This value is not modified in any way each time this function is called within the
+/// same method/block.
+pub fn random_seed_array() -> [u8; 32] {
+    //* SAFETY: random_seed syscall will always generate 32 bytes inside of the atomic op register
+    //*         so the read will have a sufficient buffer of 32, and can transmute from uninit
+    //*         because all bytes are filled. This assumes a valid random_seed implementation.
+    unsafe {
+        sys::random_seed(ATOMIC_OP_REGISTER);
+        read_register_fixed_32(ATOMIC_OP_REGISTER)
+    }
 }
 
 /// Hashes the random sequence of bytes using sha256.
@@ -845,6 +861,16 @@ mod tests {
         );
     }
 
+    #[cfg(not(target_arch = "wasm32"))]
+    #[test]
+    fn random_seed_smoke_test() {
+        crate::testing_env!(crate::test_utils::VMContextBuilder::new()
+            .random_seed([8; 32])
+            .build());
+
+        assert_eq!(super::random_seed(), [8; 32]);
+    }
+
     #[cfg(not(target_arch = "wasm32"))]
     #[cfg(feature = "unstable")]
     #[test]

near-sdk/src/test_utils/context.rs

@@ -112,8 +112,8 @@ impl VMContextBuilder {
         self
     }
 
-    pub fn random_seed(&mut self, seed: Vec<u8>) -> &mut Self {
-        self.context.random_seed = seed;
+    pub fn random_seed(&mut self, seed: [u8; 32]) -> &mut Self {
+        self.context.random_seed = seed.to_vec();
         self
     }