fix: update default willCommit semantics to make race conditions slig…

…htly safer

packages/json-api/src/-private/cache.ts

@@ -670,6 +670,40 @@ export default class JSONAPICache implements Cache {
    */
   willCommit(identifier: StableRecordIdentifier): void {
     const cached = this.__peek(identifier, false);
+
+    /*
+      if we have multiple saves in flight at once then
+      we have information loss no matter what. This
+      attempts to lose the least information.
+
+      If we were to clear inflightAttrs, previous requests
+      would not be able to use it during their didCommit.
+
+      If we upsert inflightattrs, previous requests incorrectly
+      see more recent inflight changes as part of their own and
+      will incorrectly mark the new state as the correct remote state.
+
+      We choose this latter behavior to avoid accidentally removing
+      earlier changes.
+
+      If apps do not want this behavior they can either
+
+      - chain save requests serially vs allowing concurrent saves
+      - move to using a request handler that caches the inflight state
+        on a per-request basis
+      - change their save requests to only send a "PATCH" instead of a "PUT"
+        so that only latest changes are involved in each request, and then also
+        ensure that the API or their handler reflects only those changes back
+        for upsert into the cache.
+    */
+    if (cached.inflightAttrs) {
+      if (!cached.localAttrs) {
+        return;
+      }
+      Object.assign(cached.inflightAttrs, cached.localAttrs);
+      cached.localAttrs = null;
+      return;
+    }
     cached.inflightAttrs = cached.localAttrs;
     cached.localAttrs = null;
   }