Support R2Bucket#list() startAfter option

Note this is returning slightly different, but still correct, results
to the real implementation. This is due a restriction of Miniflare's
storage abstraction, see the comment in `list()` for more details.
We're planning to replace this very soon though, so this shouldn't be
a problem for long.

Author: mrbbot

packages/tre/src/plugins/r2/gateway.ts

@@ -139,18 +139,26 @@ export class R2Gateway {
   async list(listOptions: R2ListOptions = {}): Promise<R2Objects> {
     validate.limit(listOptions.limit);
 
-    const { prefix = "", include = [], cursor = "" } = listOptions;
+    const { prefix = "", include = [], cursor = "", startAfter } = listOptions;
     let { delimiter, limit = MAX_LIST_KEYS } = listOptions;
     if (delimiter === "") delimiter = undefined;
 
-    // if include contains inputs, we reduce the limit to max 100
+    // If include contains inputs, we reduce the limit to max 100
     if (include.length > 0) limit = Math.min(limit, 100);
 
+    // If startAfter is set, we should increment the limit here, as `startAfter`
+    // is exclusive, but we're using inclusive `start` to implement it.
+    // Ideally we want to return `limit` number of keys here. However, doing
+    // this would be incompatible with the returned opaque `cursor`. Luckily,
+    // the R2 list contract allows us to return less than `limit` keys, as
+    // long as we set `truncated: true`. We'll fix this behaviour when we switch
+    // to the new storage system.
     const res = await this.storage.list<R2ObjectMetadata>({
       prefix,
       limit,
       cursor,
       delimiter,
+      start: startAfter,
     });
     const delimitedPrefixes = new Set(res.delimitedPrefixes ?? []);
 
@@ -169,6 +177,11 @@ export class R2Gateway {
         return new R2Object(metadata);
       });
 
+    if (startAfter !== undefined && objects[0]?.key === startAfter) {
+      // If the first key matched `startAfter`, remove it as this is exclusive
+      objects.splice(0, 1);
+    }
+
     const cursorLength = res.cursor.length > 0;
     return {
       objects,