Investigate behavior of interrupted execution of long-to-execute repository rules

[irengrig]

Copying the link and text of original [description:]

(bazel-contrib/rules_nodejs#802 (comment))

Canceling a build at its loading phase will also trigger this.

The first build:

Loading:
Loading: 0 packages loaded
Loading: 0 packages loaded
Loading: 0 packages loaded
# Received cancellation signal, interrupting

The second build:

Loading:
Loading: 0 packages loaded
ERROR: error loading package '': in /var/lib/buildkite-agent/builds/buildkite-agents-preemptible-nzjp-2/<redacted>/javascript/npm.bzl: Every .bzl file must have a corresponding package, but '@npm//:install_bazel_dependencies.bzl' does not have one. Please create a BUILD file in the same or any parent directory. Note that this BUILD file does not need to do anything except exist.
ERROR: error loading package '': in /var/lib/buildkite-agent/builds/buildkite-agents-preemptible-nzjp-2/<redacted>/javascript/npm.bzl: Every .bzl file must have a corresponding package, but '@npm//:install_bazel_dependencies.bzl' does not have one. Please create a BUILD file in the same or any parent directory. Note that this BUILD file does not need to do anything except exist.
INFO: Elapsed time: 0.200s
INFO: 0 processes.
FAILED: Build did NOT complete successfully (0 packages loaded)
INFO: Build Event Protocol files produced successfully.
FAILED: Build did NOT complete successfully (0 packages loaded)

My speculation is that repository_ctx doesn't have a way to tell if a repository has been fully loaded or not, so it will use a corrupted external repository without loading it again.

[irengrig]

/cc @laurentlb

[uri-canva]

Is there any update on this? In the meantime we've modified some of our repository rules to output a sentinel file when they successfully complete, and we check for that file's existence in our bazel wrapper.