Ensure disk cache root exists

[tylerwilliams]

The disk cache root directory was previously being manually created in two places before creating the disk cache client. This CL instead ensures that the disk cache root directory is created when createDiskCache() is called, and simplifies the other callsites.

This fixes an issue where the disk cache directory might not exist if using --digest_function=BLAKE3.

[brentleyjones]

@bazel-io flag

[BalestraPatrick]

I tested this change (after manually cherry-picking it into release-.6.40 and fixing conflicts) and it fixed the issue we were seeing. Thank you!

[iancha1992]

@bazel-io fork 6.4.0

[iancha1992]

@tylerwilliams @brentleyjones @coeuvre @BalestraPatrick

When cherry-picking this to release-6.4.0, there were some conflicts.

1. src/main/java/com/google/devtools/build/lib/remote/RemoteCacheClientFactory.java

createDiskCache returns return new DiskCacheClient(cacheDir, verifyDownloads, checkActionResult, digestUtil) in the release-6.4.0 branch. But it's return new DiskCacheClient(cacheDir, verifyDownloads, digestUtil) in the master branch.

RemoteCacheClient httpCache = createHttp(options, cred, authAndTlsOptions, digestUtil); is in the release-6.4.0 branch. but it is RemoteCacheClient httpCache = createHttp(options, cred, authAndTlsOptions, digestUtil, retrier); in the master branch.

2. src/main/java/com/google/devtools/build/lib/remote/disk/DiskCacheClient.java
   In the release-6.4.0 branch currently,

public DiskCacheClient(
      Path root, boolean verifyDownloads, boolean checkActionResult, DigestUtil digestUtil) 

In the master branch currently,

public DiskCacheClient(Path root, boolean verifyDownloads, DigestUtil digestUtil)

It does seem small conflicts. But if there's any commit you want me to push in order to resolve these conflicts, please let me know so we can cherry-pick this for our release-6.4.0. Thanks.

cc: @bazelbuild/triage

[brentleyjones]

I'll submit a PR.

[brentleyjones]

#19225

[iancha1992]

The changes in this PR have been included in Bazel 6.4.0 RC1. Please test out the release candidate and report any issues as soon as possible. If you're using Bazelisk, you can point to the latest RC by setting USE_BAZEL_VERSION=last_rc.
Thanks!