fix: Retry individual calls to the /missing_objects endpoint

Split out the individual API call into its own function so retry
attempts continue from the position of the loop in the calling function,
rather than looping all over again.

flat-manager-client

@@ -220,22 +220,28 @@ def chunks(l, n):
     retry=TENACITY_RETRY_EXCEPTIONS,
     reraise=True,
 )
+async def _missing_chunk(session, build_url, chunk, headers):
+    wanted_json = json.dumps({"wanted": chunk}).encode("utf-8")
+    data = gzip.compress(wanted_json)
+
+    resp = await session.get(build_url + "/missing_objects", data=data, headers=headers)
+    async with resp:
+        if resp.status != 200:
+            raise ApiError(resp, await resp.text())
+
+        return await resp.json()
+
+
 async def missing_objects(session, build_url, token, wanted):
     missing=[]
     for chunk in chunks(wanted, 2000):
-        wanted_json=json.dumps({'wanted': chunk}).encode('utf-8')
-        data=gzip.compress(wanted_json)
         headers = {
             'Authorization': 'Bearer ' + token,
             'Content-Encoding': 'gzip',
             'Content-Type': 'application/json'
         }
-        resp = await session.get(build_url + "/missing_objects", data=data, headers=headers)
-        async with resp:
-            if resp.status != 200:
-                raise ApiError(resp, await resp.text())
-            data = await resp.json()
-            missing.extend(data["missing"])
+        data = await _missing_chunk(session, build_url, chunk, headers)
+        missing.extend(data["missing"])
     return missing
 
 @retry(