fix: mysql: Replaces the user_id%10 in batch_id with a global counter.

Since the batch_uploads primary key already contains user_id, it does
nothing to increase time resolution. The counter may still see
conflicts with other processes, but the timing issue is mostly a
concern in E2E tests.

syncstorage-mysql/src/batch.rs

@@ -1,5 +1,8 @@
 use base64::Engine;
-use std::collections::HashSet;
+use std::{
+    collections::HashSet,
+    sync::atomic::{AtomicU32, Ordering},
+};
 
 use diesel::{
     self,
@@ -21,6 +24,8 @@ use super::{
 
 const MAXTTL: i32 = 2_100_000_000;
 
+static COUNTER: AtomicU32 = AtomicU32::new(0);
+
 pub fn create(db: &MysqlDb, params: params::CreateBatch) -> DbResult<results::CreateBatch> {
     let user_id = params.user_id.legacy_id as i64;
     let collection_id = db.get_collection_id(&params.collection)?;
@@ -36,7 +41,7 @@ pub fn create(db: &MysqlDb, params: params::CreateBatch) -> DbResult<results::Cr
     // while still letting us treat these ids as millisecond timestamps.  It's
     // yuck, but it works and it keeps the weirdness contained to this single
     // line of code.
-    let batch_id = db.timestamp().as_i64() + (user_id % 10);
+    let batch_id = db.timestamp().as_i64() + COUNTER.fetch_add(1, Ordering::Relaxed) as i64 % 10;
     insert_into(batch_uploads::table)
         .values((
             batch_uploads::batch_id.eq(&batch_id),