`rewrite` / `verify` hard-fail with no rules file — should pass through unfiltered

[miroslavpojer]

Describe the bug

When a project has neither jmf.globalRules nor a jmf-rules.txt file, the Maven rewrite
and verify goals abort the build:

Failed to execute goal io.github.moranaapps:jacoco-method-filter-maven-plugin:2.2.0:rewrite
  (jmf-rewrite) on project X: Configuration problems detected:
  - Rules configuration missing at: .../jmf-rules.txt
    Solution: execute 'mvn io.github.moranaapps:jacoco-method-filter-maven-plugin:2.2.0:init-rules'

The CLI enforces the same rule — CoverageRewriterCli.checkConfig:

} else if (cfg.globalRules.isEmpty && cfg.localRules.isEmpty) {
  failure("At least one of --global-rules or --local-rules must be specified")
}

Expected: no rules configured ⇒ every class passes through unchanged ⇒ a normal, unfiltered
JaCoCo report. Method filtering is an opt-in refinement; its absence should not be a build error.

Why this matters

It blocks wiring the plugin into a shared / parent POM for a group of projects. The moment the
coverage profile runs, every project that has not yet authored a jmf-rules.txt fails its build.
The only escapes are -Djmf.skip=true (turns the whole thing off, per project) or forcing every
project to commit a rules file before it is ready. That defeats the point of a common, centrally
managed coverage setup.

This is a regression AND an internal inconsistency

1. CHANGELOG [1.0.0] (2024-08-24) lists "Quiet-exit mode for builds without a rules file" as
   a shipped feature. The 2.0.0 rework (--global-rules / --local-rules, the checkConfig
   "at least one" requirement, the new Maven mojos) dropped it.

2. The sbt plugin already does the right thing. JacocoFilterPlugin.jmfRewrite / jmfVerify:

   val hasRulesConfig = globalRules.isDefined || localRules.exists(_.exists) || rulesFile.exists
   if (!hasRulesConfig) { log.warn(s"[jmf] rules file missing: ...; skipping."); classesIn }

   It warns and proceeds on the original classes. Only the Maven plugin + CLI hard-fail.

3. The core engine already supports it. Rules.loadAll(None, None) returns Seq.empty;
   CoverageRewriter.run with an empty rule set walks every class, marks 0 methods, and writes a
   complete pass-through copy to --out:
   [info] Processed N class file(s), marked 0 method(s).

So the "no rules" path is blocked only by two artificial guards (CLI checkConfig +
RewriteMojo/VerifyMojo.checkInputs). The desired behaviour needs no core changes.

Docs currently document it as an error

docs/rules-reference.md → CLI Reference:

| --global-rules <path\|url> | At least one of the two | Global rules file path or URL |
| --local-rules <path> | At least one of the two | Local rules file path |

Steps to reproduce

Minimal Maven project, jacoco-method-filter-maven-plugin:2.2.0 rewrite execution bound in a
profile, no jmf-rules.txt:

mvn -Pcoverage clean verify

→ BUILD FAILURE at jmf:rewrite, exit 1. Reproduced on Maven 3.9.16 / Temurin JDK 1.8.0_504 /
Windows 11 / JMF 2.2.0 (a parent+consumer repro is available on request).

Expected state

• rewrite with no rules → BUILD SUCCESS; target/classes-filtered holds an unchanged copy of
  every class; a later report produces a normal JaCoCo report.
• verify with no rules → BUILD SUCCESS, "0 methods matched".
• CLI --in … --out … with neither --global-rules nor --local-rules → exit 0, pass-through.
• Maven and sbt behave identically for the no-rules case.

Proposed fix

1. CLI — CoverageRewriterCli.checkConfig
Remove the cfg.globalRules.isEmpty && cfg.localRules.isEmpty → failure(...) branch. With zero
sources, run() already produces a pass-through copy and logs Loaded 0 rule(s).

2. CLI — missing --local-rules file
Rules.loadFromPath throws NoSuchFileException for a non-existent path. Preferred: when a
--local-rules path does not exist, emit
[warn] local rules file not found: <path> — proceeding with 0 local rules and treat it as empty.
(Otherwise wrappers must be careful never to pass the flag for an absent file.)

3. Maven — RewriteMojo / VerifyMojo

• Drop the hasRulesConfig hard-fail in checkInputs(). When nothing resolves, log
  [INFO] No JMF rules configured (no jmf.globalRules, no <basedir>/jmf-rules.txt) — classes pass through unfiltered.
  and continue.
• Only pass --local-rules to the CLI when the file actually exists. (localRules is never null
  — it has a default of ${project.basedir}/jmf-rules.txt — so today --local-rules <nonexistent>
  is always sent.)
• Net effect: rewrite still populates target/classes-filtered (pass-through), so the resources
  overlay and report downstream are unaffected.

4. Opt-in strictness (optional but recommended)
Add jmf.requireRules (Maven) / --require-rules (CLI), default false, that restores today's
"fail when no rules" behaviour for teams that want it enforced in CI.

5. Docs

• docs/rules-reference.md CLI table: --global-rules / --local-rules → Optional. With neither,
  all classes pass through unfiltered (no-op).
• maven-plugin/README.md: document the pass-through behaviour and jmf.requireRules.
• CHANGELOG.md: note restoration of the v1.0.0 "quiet-exit mode for builds without a rules file".

Acceptance criteria

1. mvn …:rewrite with no jmf.globalRules and no jmf-rules.txt → BUILD SUCCESS;
   target/classes-filtered contains an unchanged copy of every .class; a subsequent report
   yields a normal JaCoCo report.
2. mvn …:verify with no rules → BUILD SUCCESS, 0 matched methods.
3. CLI --in … --out … with no rules sources → exit 0, pass-through copy.
4. Today's behaviour is still reachable via jmf.requireRules=true / --require-rules.
5. sbt and Maven behave identically for the no-rules case.
6. Integration tests cover the no-rules path for CLI, Maven, and sbt.
7. Docs and CHANGELOG updated.

Impact / Severity

High — blocks adoption via a shared parent POM, which is the intended model for org-wide
standardisation.

Attachments / Evidence

• CoverageRewriterCli.scala checkConfig (the CLI guard)
• RewriteMojo.java / VerifyMojo.java checkInputs() (the Maven guard)
• JacocoFilterPlugin.scala jmfRewrite / jmfVerify (the sbt path that already warns-and-skips)
• CoverageRewriter.scala run() + Rules.loadAll (core already yields a pass-through on empty rules)
• CHANGELOG.md [1.0.0] — "Quiet-exit mode for builds without a rules file"

Desktop

• OS: Windows 11
• Maven: 3.9.16
• JDK: Eclipse Temurin 1.8.0_504
• JMF: 2.2.0 (maven plugin + core)

Related / References

• Downstream rollout: feat: Adding the JaCoCo Method Filter to the project AbsaOSS/commons#147, Adopt Java Method Filtration JaCoCo Extension AbsaOSS/commons#144