Coverage gate: JDK 21 floors for modules that cover more on JDK 21

The Jetty launcher covers line 85.8%, branch 86.7% on JDK 21, where its
virtual-thread pool runs, but 76.4% and 70.0% on JDK 17; one floor for both
let the JDK 21 build lose 16 points of branches unnoticed. A module may now
declare jacoco.jdk21.minimum.line and .branch, which the coverage-jdk21
profile (JDK 21+) makes the gate check; without them JDK 21 checks the
module's floors as before. The launcher gets 0.83 line, 0.84 branch on JDK 21.

Author: Mark Pollack

README.md

@@ -512,7 +512,9 @@ that authenticates in front. Browser requests whose `Origin` is not a loopback o
 `./mvnw verify` writes a JaCoCo coverage report for each module to
 `<module>/target/site/jacoco/index.html`, and fails if a module's line or branch coverage drops
 below the floor declared in its `pom.xml` (measured coverage less 2 points). Each module counts
-only its own classes, covered by its own tests. The classes that choose between virtual threads
+only its own classes, covered by its own tests. A module whose JDK 21 build covers more (its
+virtual-thread code runs only there) also declares JDK 21 floors, which a JDK 21 build checks
+instead. The classes that choose between virtual threads
 (JDK 21 and later) and platform threads (JDK 17) must have every branch covered on whichever JDK
 builds, so CI's JDK 17 and JDK 21 builds each prove both choices. When running a subset of tests
 under `verify` (`-Dtest=...`), add `-Djacoco.skip` to skip the gate.

acp-streamable-http-jetty/pom.xml

@@ -18,11 +18,13 @@
 
     <properties>
         <!-- Coverage floors: the launcher alone (the servlet host it runs is measured in
-             acp-http-servlet). Measured line 85.8%, branch 86.7% on JDK 21, and line 76%, branch
-             70% on JDK 17, where the virtual-thread pool paths cannot run; the JDK 17 figures less
-             2 points (parent pom, JaCoCo) -->
+             acp-http-servlet). Measured line 76.4%, branch 70.0% on JDK 17, where the
+             virtual-thread pool paths cannot run, and line 85.8%, branch 86.7% on JDK 21; each
+             JDK's figures less 2 points (parent pom, JaCoCo) -->
         <jacoco.minimum.line>0.74</jacoco.minimum.line>
         <jacoco.minimum.branch>0.68</jacoco.minimum.branch>
+        <jacoco.jdk21.minimum.line>0.83</jacoco.jdk21.minimum.line>
+        <jacoco.jdk21.minimum.branch>0.84</jacoco.jdk21.minimum.branch>
         <!-- JaCoCo's prepare-agent sets argLine; empty when JaCoCo is skipped. -->
         <argLine/>
     </properties>

pom.xml

@@ -119,6 +119,13 @@
         <!-- Coverage floors; each module overrides them with its own (see the JaCoCo plugin) -->
         <jacoco.minimum.line>1.00</jacoco.minimum.line>
         <jacoco.minimum.branch>1.00</jacoco.minimum.branch>
+        <!-- A module whose JDK 21 build runs code JDK 17 cannot (virtual threads) declares
+             higher floors for it; otherwise JDK 21 checks the same floors. -->
+        <jacoco.jdk21.minimum.line>${jacoco.minimum.line}</jacoco.jdk21.minimum.line>
+        <jacoco.jdk21.minimum.branch>${jacoco.minimum.branch}</jacoco.jdk21.minimum.branch>
+        <!-- The floors the gate checks on this JDK (the coverage-jdk21 profile switches them) -->
+        <jacoco.gate.line>${jacoco.minimum.line}</jacoco.gate.line>
+        <jacoco.gate.branch>${jacoco.minimum.branch}</jacoco.gate.branch>
         <owasp.version>13.0.0</owasp.version>
         <cyclonedx-maven-plugin.version>2.9.3</cyclonedx-maven-plugin.version>
         <maven-pmd-plugin.version>3.28.0</maven-pmd-plugin.version>
@@ -531,7 +538,12 @@
                  (integration-testing/) is outside the Maven build and is not counted.
 
                  The gate (check, in verify) fails when a module's LINE or BRANCH covered ratio
-                 falls below its floor. Every module declares jacoco.minimum.line and
+                 falls below its floor. CI builds on JDK 17 and on JDK 21, and a module may cover
+                 more on JDK 21, where its virtual-thread code runs: such a module also declares
+                 jacoco.jdk21.minimum.line and jacoco.jdk21.minimum.branch, which the JDK 21
+                 build checks instead (the coverage-jdk21 profile); without them JDK 21 checks
+                 the same floors. Measure a floor on both JDKs: the JDK 17 one from the JDK 17
+                 build, the JDK 21 one from the JDK 21 build, and a shared floor from the lower. Every module declares jacoco.minimum.line and
                  jacoco.minimum.branch; the default below is full coverage, so a new module
                  fails until it declares measured floors. A floor is the coverage measured from a clean verify less a margin of
                  2 points, rounded down to a whole percent: enough for the run-to-run variation
@@ -586,12 +598,12 @@
                                         <limit>
                                             <counter>LINE</counter>
                                             <value>COVEREDRATIO</value>
-                                            <minimum>${jacoco.minimum.line}</minimum>
+                                            <minimum>${jacoco.gate.line}</minimum>
                                         </limit>
                                         <limit>
                                             <counter>BRANCH</counter>
                                             <value>COVEREDRATIO</value>
-                                            <minimum>${jacoco.minimum.branch}</minimum>
+                                            <minimum>${jacoco.gate.branch}</minimum>
                                         </limit>
                                     </limits>
                                 </rule>
@@ -622,6 +634,17 @@
     </build>
 
     <profiles>
+        <!-- The coverage gate's JDK 21 floors (see the JaCoCo plugin). -->
+        <profile>
+            <id>coverage-jdk21</id>
+            <activation>
+                <jdk>[21,)</jdk>
+            </activation>
+            <properties>
+                <jacoco.gate.line>${jacoco.jdk21.minimum.line}</jacoco.gate.line>
+                <jacoco.gate.branch>${jacoco.jdk21.minimum.branch}</jacoco.gate.branch>
+            </properties>
+        </profile>
         <!-- Error Prone: Error Prone's own bug checks and NullAway, at ERROR, on main sources.
              Error Prone 2.50 runs only on JDK 21+, while the SDK targets Java 17 and its
              release build runs on JDK 17, so the profile activates by JDK: every build on