Add rng support to ProcedureContext on Rust (#3865)

# Description of Changes

As the title says, required for
#3538.

# Expected complexity level and risk

1

# Testing

Will be done on the `uuid` pr.

---------

Signed-off-by: Mario Montoya <mamcx@elmalabarista.com>
Co-authored-by: Phoebe Goldman <phoebe@clockworklabs.io>

Author: mamcx

crates/bindings/src/lib.rs

@@ -1084,13 +1084,25 @@ pub struct ProcedureContext {
 
     /// Methods for performing HTTP requests.
     pub http: crate::http::HttpClient,
-    // TODO: Add rng?
+    // TODO: Change rng?
     // Complex and requires design because we may want procedure RNG to behave differently from reducer RNG,
     // as it could actually be seeded by OS randomness rather than a deterministic source.
+    #[cfg(feature = "rand08")]
+    rng: std::cell::OnceCell<StdbRng>,
 }
 
 #[cfg(feature = "unstable")]
 impl ProcedureContext {
+    fn new(sender: Identity, connection_id: Option<ConnectionId>, timestamp: Timestamp) -> Self {
+        Self {
+            sender,
+            timestamp,
+            connection_id,
+            http: http::HttpClient {},
+            #[cfg(feature = "rand08")]
+            rng: std::cell::OnceCell::new(),
+        }
+    }
     /// Read the current module's [`Identity`].
     pub fn identity(&self) -> Identity {
         // Hypothetically, we *could* read the module identity out of the system tables.

crates/bindings/src/rng.rs

@@ -1,3 +1,5 @@
+#[cfg(feature = "unstable")]
+use crate::ProcedureContext;
 use crate::{rand, ReducerContext};
 use core::cell::UnsafeCell;
 use core::marker::PhantomData;
@@ -49,6 +51,51 @@ impl ReducerContext {
     }
 }
 
+#[cfg(feature = "unstable")]
+impl ProcedureContext {
+    /// Generates a random value.
+    ///
+    /// Similar to [`rand::random()`], but using [`StdbRng`] instead.
+    ///
+    /// See also [`ProcedureContext::rng()`].
+    #[cfg(feature = "unstable")]
+    pub fn random<T>(&self) -> T
+    where
+        Standard: Distribution<T>,
+    {
+        Standard.sample(&mut self.rng())
+    }
+
+    /// Retrieve the random number generator for this procedure transaction,
+    /// seeded by the timestamp of the procedure call.
+    ///
+    /// If you only need a single random value, you can use [`ProcedureContext::random()`].
+    ///
+    /// # Examples
+    /// ```no_run
+    /// # #[cfg(target_arch = "wasm32")] mod demo {
+    /// use spacetimedb::{procedure, ProcedureContext};
+    /// use rand::Rng;
+    ///
+    /// #[spacetimedb::procedure]
+    /// fn rng_demo(ctx: &spacetimedb::ProcedureContext) {
+    ///     // Can be used in method chaining style:
+    ///     let digit = ctx.rng().gen_range(0..=9);
+    ///
+    ///     // Or, cache locally for reuse:
+    ///     let mut rng = ctx.rng();
+    ///    let floats: Vec<f32> = rng.sample_iter(rand::distributions::Standard).collect();
+    /// }
+    /// # }
+    /// ```
+    ///
+    /// For more information, see [`StdbRng`] and [`rand::Rng`].
+    #[cfg(feature = "unstable")]
+    pub fn rng(&self) -> &StdbRng {
+        self.rng.get_or_init(|| StdbRng::seed_from_ts(self.timestamp))
+    }
+}
+
 /// A reference to the random number generator for this reducer call.
 ///
 /// An instance can be obtained via [`ReducerContext::rng()`]. Import

crates/bindings/src/rt.rs

@@ -1034,12 +1034,7 @@ extern "C" fn __call_procedure__(
     let timestamp = Timestamp::from_micros_since_unix_epoch(timestamp as i64);
 
     // Assemble the `ProcedureContext`.
-    let ctx = ProcedureContext {
-        connection_id: conn_id,
-        sender,
-        timestamp,
-        http: crate::http::HttpClient {},
-    };
+    let ctx = ProcedureContext::new(sender, conn_id, timestamp);
 
     // Grab the list of procedures, which is populated by the preinit functions.
     let procedures = PROCEDURES.get().unwrap();