Follow up on feedback from private-attribution#1332

ipa-core/src/helpers/gateway/send.rs

@@ -255,16 +255,27 @@ impl SendChannelConfig {
         total_records: TotalRecords,
         record_size: usize,
     ) -> Self {
+        // this computes the greatest positive power of 2 that is
+        // less than or equal to target.
+        fn non_zero_prev_power_of_two(target: usize) -> usize {
+            let bits = usize::BITS - target.leading_zeros();
+
+            1 << (std::cmp::max(1, bits) - 1)
+        }
+
         assert!(record_size > 0, "Message size cannot be 0");
 
         let total_capacity = gateway_config.active.get() * record_size;
         // define read size as a multiplier of record size. The multiplier must be
         // a power of two to align perfectly with total capacity. We don't want to exceed
-        // the target read size, so we pick a power of two <= read_size.
-        let read_size_multiplier =
-            // this computes the highest power of 2 less than or equal to read_size / record_size.
-            // Note, that if record_size is greater than read_size, we round it to 1
-            1 << (std::cmp::max(1, usize::BITS - (gateway_config.read_size.get() / record_size).leading_zeros()) - 1);
+        // the target read size, so multiplier * record_size <= read_size. We want to get
+        // as close as possible to read_size.
+        let read_size_multiplier = {
+            let target = gateway_config.read_size.get() / record_size;
+            // If record_size is greater than read_size, we set the multiplier to 1
+            // as read size cannot be 0.
+            non_zero_prev_power_of_two(target)
+        };
 
         let this = Self {
             total_capacity: total_capacity.try_into().unwrap(),
@@ -279,7 +290,11 @@ impl SendChannelConfig {
             total_records,
         };
 
+        // If capacity can't fit all active work items, the protocol deadlocks on
+        // inserts above the total capacity.
         assert!(this.total_capacity.get() >= record_size * gateway_config.active.get());
+        // if capacity is not aligned with read size, we can get a deadlock
+        // described in ipa/1300
         assert_eq!(0, this.total_capacity.get() % this.read_size.get());
 
         this