Included automated poc code and FINDINGS.md

Author: rozza

.gitignore

@@ -52,6 +52,7 @@ local.properties
 *.sh
 !.evergreen/*.sh
 !scripts/*.sh
+!testing/java-6279-poc/*.sh
 
 # security-sensitive files
 *.gpg

testing/java-6279-poc/FINDINGS.md

@@ -0,0 +1,12 @@
+// Gradle init script used by run.sh to discover module runtime classpaths without touching any
+// build file. Applied with -I; adds no behaviour to a normal build.
+allprojects {
+    tasks.register("printRuntimeCp") {
+        doLast {
+            def cp = project.configurations.findByName("runtimeClasspath")
+            if (cp != null) {
+                println "CP:" + cp.files.join(File.pathSeparator)
+            }
+        }
+    }
+}

testing/java-6279-poc/classpath.gradle

@@ -0,0 +1,110 @@
+#!/usr/bin/env bash
+# Runs the JAVA-6279 proofs of concept.
+#
+# Usage: ./testing/java-6279-poc/run.sh [primer|driver|executor|all]
+#
+#   primer    synthetic classes in a throwaway child class loader -- reproduces the finding that a live thread
+#             started from a class's static initializer pins that class loader. No driver code.
+#   driver    the driver loaded into a child class loader -- shows that BufferPoolPruner keeps that loader, and
+#             therefore every driver class and all its static state, strongly reachable after the client is closed.
+#   executor  no driver code and no class loaders: checks that a self-terminating, self-resurrecting pruner is
+#             implementable on ScheduledThreadPoolExecutor without ever shutting it down. Run under every JDK found
+#             on the machine, because it leans on unspecified implementation behaviour and the driver's baseline is
+#             Java 8. Set JAVA_HOMES to a colon separated list to override JDK discovery.
+#
+# No MongoDB server is needed: the driver scenarios never issue an operation.
+# Nothing under any module's src/main is modified.
+set -euo pipefail
+
+REPO_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/../.." && pwd)"
+POC_DIR="$REPO_ROOT/testing/java-6279-poc"
+WHAT="${1:-all}"
+cd "$REPO_ROOT"
+
+OUT="$(mktemp -d)"
+trap 'rm -rf "$OUT"' EXIT
+STATUS=0
+
+# ---------------------------------------------------------------------------------------------------------------------
+# executor mechanism: standalone, Java 8 clean, no driver jars needed
+# ---------------------------------------------------------------------------------------------------------------------
+if [ "$WHAT" = "executor" ] || [ "$WHAT" = "all" ]; then
+  echo "== compiling the executor mechanism checks at --release 8 =="
+  mkdir -p "$OUT/mechanism"
+  javac -nowarn --release 8 -d "$OUT/mechanism" "$POC_DIR/src/java6279/ExecutorMechanism.java"
+
+  # Discover JDKs. JAVA_HOMES wins; otherwise ask java_home on macOS; always include whatever java is on the PATH.
+  JDKS=""
+  if [ -n "${JAVA_HOMES:-}" ]; then
+    JDKS="$(echo "$JAVA_HOMES" | tr ':' '\n')"
+  elif [ -x /usr/libexec/java_home ]; then
+    JDKS="$(/usr/libexec/java_home -V 2>&1 | sed -n 's|.* \(/.*/Contents/Home\)$|\1|p' || true)"
+  fi
+  JDKS="$(printf '%s\n%s\n' "$JDKS" "$(dirname "$(dirname "$(command -v java)")")" | sed '/^$/d' | sort -u)"
+
+  for JDK in $JDKS; do
+    [ -x "$JDK/bin/java" ] || continue
+    echo
+    echo "############ $("$JDK/bin/java" -version 2>&1 | head -1) ############"
+    "$JDK/bin/java" ${POC_JAVA_ARGS:-} -cp "$OUT/mechanism" java6279.ExecutorMechanism || STATUS=1
+  done
+fi
+
+# ---------------------------------------------------------------------------------------------------------------------
+# class loader retention: needs the driver jars
+# ---------------------------------------------------------------------------------------------------------------------
+if [ "$WHAT" = "primer" ] || [ "$WHAT" = "driver" ] || [ "$WHAT" = "all" ]; then
+  need_jar() {
+    local pattern="$1"
+    local found
+    found="$(ls $pattern 2>/dev/null | grep -v -- '-sources' | grep -v -- '-javadoc' | head -1 || true)"
+    if [ -z "$found" ]; then
+      echo "missing jar: $pattern" >&2
+      exit 1
+    fi
+    echo "$found"
+  }
+
+  echo
+  if [ -n "${JAVA6279_DRIVER_REPO:-}" ]; then
+    # Point the harness at another checkout -- e.g. the backpressure branch in a git worktree -- to see what that
+    # tree's threads do. The scenarios are branch agnostic; only the jars under test change.
+    echo "== using driver jars from $JAVA6279_DRIVER_REPO =="
+    DRIVER_REPO="$JAVA6279_DRIVER_REPO"
+  else
+    echo "== building driver jars =="
+    ./gradlew -q -PskipCryptVerify=true :bson:jar :driver-core:jar :driver-sync:jar
+    DRIVER_REPO="$REPO_ROOT"
+  fi
+
+  # The classpath handed to the *child* loader. It must be self-contained down to the platform class loader, so it
+  # carries the third party jars as well, taken from driver-sync's own runtime classpath so versions match the build.
+  DRIVER_CP="$(need_jar "$DRIVER_REPO/bson/build/libs/bson-*.jar")"
+  DRIVER_CP="$DRIVER_CP:$(need_jar "$DRIVER_REPO/driver-core/build/libs/mongodb-driver-core-*.jar")"
+  DRIVER_CP="$DRIVER_CP:$(need_jar "$DRIVER_REPO/driver-sync/build/libs/mongodb-driver-sync-*.jar")"
+
+  DEPS="$(./gradlew -q -I "$POC_DIR/classpath.gradle" :driver-sync:printRuntimeCp \
+          | sed -n 's/^CP://p' | tr ':' '\n' | sort -u | grep -v -- '-SNAPSHOT.jar$' | paste -sd: -)"
+  if [ -z "$DEPS" ]; then
+    echo "could not resolve third party jars from Gradle" >&2
+    exit 1
+  fi
+  DRIVER_CP="$DRIVER_CP:$DEPS"
+
+  echo "== compiling the class loader proof of concept =="
+  find "$POC_DIR/src" -name '*.java' > "$OUT/sources.txt"
+  javac -nowarn -d "$OUT/classes" @"$OUT/sources.txt"
+
+  # java6279.Poc goes on the application classpath; java6279.primer.* is also there, but the primer class loader
+  # defines it from $OUT/classes itself rather than delegating, which is what gives each scenario a fresh loader.
+  echo "== running =="
+  SCENARIOS="$WHAT"
+  [ "$WHAT" = "all" ] && SCENARIOS="all"
+  java -cp "$OUT/classes" \
+    -Djava6279.classesDir="$OUT/classes" \
+    -Djava6279.driverCp="$DRIVER_CP" \
+    ${POC_JAVA_ARGS:-} \
+    java6279.Poc "$SCENARIOS" || STATUS=1
+fi
+
+exit $STATUS