Description of the problem / feature request:

Running a build with the --bes_upload_mode=fully_async doesn't have the expected effect. The build events are still uploaded synchronously after the command is run. This seems to happen when remote cache is used.

Bugs: what's the simplest, easiest way to reproduce this bug?

git clone git@github.com:bazelbuild/rules_java.git && cd rules_java

And then run at least twice so the Bazel server is running and the cache is populated:

env USE_BAZEL_VERSION=5.0.0 bazelisk build --remote_cache=grpcs://remote.buildbuddy.io --bes_backend=grpcs://remote.buildbuddy.io //java:srcs --bes_upload_mode=fully_async

After the build is complete it still takes a bit of time to do something, which I believe is uploading the BEP events. Changing USE_BAZEL_VERSION to 4.2.0 makes it snappier. This upload time is more noticeable in short builds that run more targets.

What operating system are you running Bazel on?

macOS 12.0.1

What's the output of bazel info release?

release 5.0.0

Have you found anything relevant by searching the web?

This seems similar to #14576.