fix(core): fix underflow bug with desired slot count

Author: NathanFlurry

Cargo.lock

@@ -29,11 +29,11 @@ impl ServerlessDesiredSlotsKey {
 
 impl FormalKey for ServerlessDesiredSlotsKey {
 	/// Count.
-	type Value = u32;
+	type Value = i64;
 
 	fn deserialize(&self, raw: &[u8]) -> Result<Self::Value> {
 		// NOTE: Atomic ops use little endian
-		Ok(u32::from_le_bytes(raw.try_into()?))
+		Ok(i64::from_le_bytes(raw.try_into()?))
 	}
 
 	fn serialize(&self, value: Self::Value) -> Result<Vec<u8>> {

out/openapi.json

@@ -6,3 +6,30 @@ macro_rules! div_up {
 		($a + ($b - 1)) / $b
 	};
 }
+
+/// Performs ceiling division for i64 values.
+///
+/// Returns the smallest integer greater than or equal to `a / b`.
+///
+/// # Examples
+/// ```
+/// assert_eq!(div_ceil_i64(10, 3), 4);  // 10/3 = 3.33.. -> 4
+/// assert_eq!(div_ceil_i64(9, 3), 3);   // 9/3 = 3 -> 3
+/// assert_eq!(div_ceil_i64(-10, 3), -3); // -10/3 = -3.33.. -> -3
+/// ```
+///
+/// # Panics
+/// Panics if `b` is zero.
+pub fn div_ceil_i64(a: i64, b: i64) -> i64 {
+	if b == 0 {
+		panic!("attempt to divide by zero");
+	}
+
+	if a == 0 || (a > 0 && b > 0) || (a < 0 && b < 0) {
+		// Standard ceiling division when signs match or a is zero
+		(a + b - 1) / b
+	} else {
+		// When signs differ, regular division gives the ceiling
+		a / b
+	}
+}

packages/common/types/src/keys/pegboard/ns.rs

@@ -15,6 +15,7 @@ reqwest.workspace = true
 rivet-config.workspace = true
 rivet-runner-protocol.workspace = true
 rivet-types.workspace = true
+rivet-util.workspace = true
 tracing.workspace = true
 universaldb.workspace = true
 vbare.workspace = true

packages/common/util/core/src/math.rs

@@ -142,10 +142,25 @@ async fn tick(
 		// Remove finished and draining connections from list
 		curr.retain(|conn| !conn.handle.is_finished() && !conn.draining.load(Ordering::SeqCst));
 
-		let desired_count = (desired_slots.div_ceil(*slots_per_runner).max(*min_runners)
-			+ *runners_margin)
-			.min(*max_runners)
-			.try_into()?;
+		// Log warning and reset to 0 if negative
+		let adjusted_desired_slots = if *desired_slots < 0 {
+			tracing::warn!(
+				?ns_id,
+				?runner_name,
+				desired_slots = ?desired_slots,
+				"Negative desired_slots detected, resetting to 0"
+			);
+			0
+		} else {
+			*desired_slots
+		};
+
+		let desired_count =
+			(rivet_util::math::div_ceil_i64(adjusted_desired_slots, *slots_per_runner as i64)
+				.max(*min_runners as i64)
+				+ *runners_margin as i64)
+				.min(*max_runners as i64)
+				.try_into()?;
 
 		// Calculate diff
 		let drain_count = curr.len().saturating_sub(desired_count);

packages/core/pegboard-serverless/Cargo.toml

@@ -95,7 +95,7 @@ pub async fn namespace_runner_config_upsert(ctx: &OperationCtx, input: &Input) -
 							input.namespace_id,
 							input.name.clone(),
 						),
-						&0u32.to_le_bytes(),
+						&0i64.to_le_bytes(),
 						MutationType::Add,
 					);
 				}

packages/core/pegboard-serverless/src/lib.rs

@@ -240,7 +240,7 @@ pub(crate) async fn clear_slot(
 				namespace_id,
 				runner_name_selector.to_string(),
 			),
-			&(-1i32).to_le_bytes(),
+			&(-1i64).to_le_bytes(),
 			MutationType::Add,
 		);
 	}

packages/services/namespace/src/ops/runner_config/upsert.rs

@@ -129,7 +129,7 @@ async fn allocate_actor(
 						namespace_id,
 						input.runner_name_selector.clone(),
 					),
-					&1u32.to_le_bytes(),
+					&1i64.to_le_bytes(),
 					MutationType::Add,
 				);
 			}
@@ -367,7 +367,7 @@ pub async fn deallocate(ctx: &ActivityCtx, input: &DeallocateInput) -> Result<()
 						namespace_id,
 						runner_name_selector.clone(),
 					),
-					&(-1i32).to_le_bytes(),
+					&(-1i64).to_le_bytes(),
 					MutationType::Add,
 				);
 			}