unified-storage: save previous_resource_version as microsecond timestamp in compat mode (#116179)

* unified-storage: save `previous_resource_version` as microsecond timestamp in compat mode

* debug test

* fix test

* fix typo

* Fix misleading var name.

`ms` is typically the abbreviation for millisecond, not microsecond.

pkg/storage/unified/sql/rvmanager/rv_manager.go

@@ -307,7 +307,7 @@ func (m *ResourceVersionManager) execBatch(ctx context.Context, group, resource
 		// Allocate the RVs
 		for i, guid := range guids {
 			guidToRV[guid] = rv
-			guidToSnowflakeRV[guid] = SnowflakeFromRv(rv)
+			guidToSnowflakeRV[guid] = SnowflakeFromRV(rv)
 			rvs[i] = rv
 			rv++
 		}
@@ -364,12 +364,20 @@ func (m *ResourceVersionManager) execBatch(ctx context.Context, group, resource
 	}
 }
 
-// takes a unix microsecond rv and transforms into a snowflake format. The timestamp is converted from microsecond to
+// takes a unix microsecond RV and transforms into a snowflake format. The timestamp is converted from microsecond to
 // millisecond (the integer division) and the remainder is saved in the stepbits section. machine id is always 0
-func SnowflakeFromRv(rv int64) int64 {
+func SnowflakeFromRV(rv int64) int64 {
 	return (((rv / 1000) - snowflake.Epoch) << (snowflake.NodeBits + snowflake.StepBits)) + (rv % 1000)
 }
 
+// It is generally not possible to convert from a snowflakeID to a microsecond RV due to the loss in precision
+// (snowflake ID stores timestamp in milliseconds). However, this implementation stores the microsecond fraction
+// in the step bits (see SnowflakeFromRV), allowing us to compute the microsecond timestamp.
+func RVFromSnowflake(snowflakeID int64) int64 {
+	microSecFraction := snowflakeID & ((1 << snowflake.StepBits) - 1)
+	return ((snowflakeID>>(snowflake.NodeBits+snowflake.StepBits))+snowflake.Epoch)*1000 + microSecFraction
+}
+
 // helper utility to compare two RVs. The first RV must be in snowflake format. Will convert rv2 to snowflake and retry
 // if comparison fails
 func IsRvEqual(rv1, rv2 int64) bool {
@@ -377,7 +385,7 @@ func IsRvEqual(rv1, rv2 int64) bool {
 		return true
 	}
 
-	return rv1 == SnowflakeFromRv(rv2)
+	return rv1 == SnowflakeFromRV(rv2)
 }
 
 // Lock locks the resource version for the given key

pkg/storage/unified/sql/rvmanager/rv_manager_test.go

@@ -63,3 +63,13 @@ func TestResourceVersionManager(t *testing.T) {
 		require.Equal(t, rv, int64(200))
 	})
 }
+
+func TestSnowflakeFromRVRoundtrips(t *testing.T) {
+	// 2026-01-12 19:33:58.806211 +0000 UTC
+	offset := int64(1768246438806211) // in microseconds
+
+	for n := range int64(100) {
+		ts := offset + n
+		require.Equal(t, ts, RVFromSnowflake(SnowflakeFromRV(ts)))
+	}
+}