java: google-cloud-java patch version bump breaks librarian version assumption

[zhumin8]

(To scope this: this affect README.md generation when a partial release is done. When last release is a patch release, librarian generated README.md has wrong versions.
When regular releases that bumps minor, no such generate issue.)
In readme generation, we assumed released version can be worked out from current version:

librarian/internal/librarian/java/postprocess.go

Lines 389 to 396 in f6099e8

	// deriveLastReleasedVersion derives the last released version from a snapshot version
	// (e.g., x.y.z-SNAPSHOT or x.y.z-beta-SNAPSHOT) by decrementing the patch or minor version.
	// If the version has a prerelease tag (like "beta") before "SNAPSHOT", that tag is preserved
	// in the derived version (e.g., x.y.z-beta-SNAPSHOT becomes x.y.(z-1)-beta).
	//
	// It returns an error if both minor and patch versions are zero, as it's
	// ambiguous what the last released version was in that case.
	func deriveLastReleasedVersion(v string) (string, error) {

This assumption is based on google-cloud-java uses release-please "versioning": "always-bump-minor",.

https://github.com/googleapis/google-cloud-java/blob/dcc2a68b7cfc36ffcfa7e34e3ce91253a4cca074/release-please-config.json#L4

This assumption does not hold true in recent manual partial releases https://github.com/googleapis/google-cloud-java/releases/tag/v1.86.2

As result, seeing generation diffs in recent check for googleapis/google-cloud-java#13319

Run git diff --exit-code
diff --git a/java-bigtable/README.md b/java-bigtable/README.md
index 07882cc1fc0..c8a2b142ba7 100644
--- a/java-bigtable/README.md
+++ b/java-bigtable/README.md
@@ -56,13 +56,13 @@ implementation 'com.google.cloud:google-cloud-bigtable'
 If you are using Gradle without BOM, add this to your dependencies:
 
 ```Groovy
-implementation 'com.google.cloud:google-cloud-bigtable:2.78.1'
+implementation 'com.google.cloud:google-cloud-bigtable:2.78.0'
 ```
 
 If you are using SBT, add this to your dependencies:
 
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-bigtable" % "2.78.1"
+libraryDependencies += "com.google.cloud" % "google-cloud-bigtable" % "2.78.0"
 ```
 
 ## Authentication
@@ -452,7 +452,7 @@ Java is a registered trademark of Oracle and/or its affiliates.
 [javadocs]: https://cloud.google.com/java/docs/reference/google-cloud-bigtable/latest/history
 [stability-image]: https://img.shields.io/badge/stability-stable-green
 [maven-version-image]: https://img.shields.io/maven-central/v/com.google.cloud/google-cloud-bigtable.svg
-[maven-version-link]: https://central.sonatype.com/artifact/com.google.cloud/google-cloud-bigtable/2.78.1
+[maven-version-link]: https://central.sonatype.com/artifact/com.google.cloud/google-cloud-bigtable/2.78.0
 [authentication]: https://github.com/googleapis/google-cloud-java#authentication
 [auth-scopes]: https://developers.google.com/identity/protocols/oauth2/scopes
 [predefined-iam-roles]: https://cloud.google.com/iam/docs/understanding-roles#predefined_roles
Error: Process completed with exit code 1.

[zhumin8]

In this case, we should not derive released version, instead keep a release_version config under java section.

  - name: bigtable
    version: 2.79.0-SNAPSHOT
     ... ...
    java:
      released_version: 2.78.1

For this to work properly with releases, release-please needs to also update this library.java.release_version config similar to library.version.

Do you see any concerns with this approach from release-please side since you worked on #4976? @JoeWang1127

[JoeWang1127]

In the partial release, the version.txt is updated in step 6.

Do you think we should fix the script to update the version in librarian.yaml: https://github.com/googleapis/google-cloud-java/blob/05df6bdbd330c679365d20df31f283ca0e9bcf6b/.github/release/partial_release.py

[zhumin8]

I don't think partial release step 6 (bump to snapshot) is the problem here. This latest bump PR seems as expected to me: googleapis/google-cloud-java#13213

The change is that if I introduce an explicit config for released_version, I believe we need to do change in release please so that it can update it similar to version?

[zhumin8]

Oh, I see your point. So googleapis/release-please#2750 only updates librarian.yaml as part of the java monorepo strategy. You are saying for partial releases, we need it to update librarian.yaml as well?

[zhumin8]

For partial release:

• in step 1. "Update the versions of the library in versions.txt" is done manually, add text to also update librarian.yaml versions.
• in step 6, update partial_release.py to update the version in librarian.yaml or add a step to manually update librarian.yaml version.

[JoeWang1127]

Oh, I see your point. So googleapis/release-please#2750 only updates librarian.yaml as part of the java monorepo strategy. You are saying for partial releases, we need it to update librarian.yaml as well?

Yes, I think putting the logic together is helpful for us to understand what it does in during partial release and for future refactor (IMO, we should support partial release in librarian).

Also, adding release_version makes sense as we can't derive this version after a partial release.

[zhumin8]

SG.
Created #6248 to track work for partial release guide/script.
#6249 to track update needed in release please.

[zhumin8]

Re: adding release_version. I am making this required field and explicitly list for every library to start with.

[JoeWang1127]

Why required since we can derive released version for most of the libaries?

[zhumin8]

I wanted to do this incremental, and my next step is exactly to make it derivable and think through how it works release-please.

I am thinking use Fill to derive and Tidy to remove extras.
For release-please logic,

• when there is explicit release_version, update it.
• when there is no explicit release_version, write one.

So after release-please updates it at release time, librarian.yaml has explicit release_version for all libraries.
Then on the next librarian tidy it removed derivable ones.