Require Java 21 exactly and pin the unit to it

NeoForge 21.1.x targets Java 21. Fedora 43 ships an early-access JDK 26 as the
default java, which satisfied the ">= 21" check and then failed inside
ModLauncher — the server started, logged "JVM identified as ... 26-ea+32", and
died with no obvious cause.

mc-service-setup.sh now locates a real Java 21 under /usr/lib/jvm, refuses to
continue if there is none, and writes its absolute path into ExecStart so the
service does not follow whatever /usr/bin/java points at later.

install.sh checks for Java 21 specifically rather than for any java, so the
prerequisite step actually installs it on a host that already has a newer JDK.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

install.sh

@@ -63,7 +63,6 @@ step "Checking prerequisites"
 
 MISSING=()
 need() { command -v "$1" >/dev/null || MISSING+=("$2"); }
-need java   java-21-openjdk-headless
 need curl   curl
 need go     golang
 need python3 python3
@@ -71,6 +70,11 @@ need zfs    zfs
 need findmnt util-linux
 need mkfifo coreutils
 
+# Any java is not enough: NeoForge 21.1.x needs 21 exactly.
+if ! ls /usr/lib/jvm/java-21-openjdk*/bin/java >/dev/null 2>&1; then
+    MISSING+=(java-21-openjdk-headless)
+fi
+
 if [ "${#MISSING[@]}" -gt 0 ]; then
     info "missing: ${MISSING[*]}"
     if (( CHECK_ONLY )); then
@@ -84,8 +88,12 @@ else
     info "all present"
 fi
 
-jver="$(java -version 2>&1 | head -1 | grep -oE '"[0-9]+' | tr -d '"' || echo 0)"
-[ "${jver:-0}" -ge 21 ] || warn "java is $jver; NeoForge for 1.21.x wants 21"
+if ls /usr/lib/jvm/java-21-openjdk*/bin/java >/dev/null 2>&1; then
+    info "java 21 present: $(ls -d /usr/lib/jvm/java-21-openjdk*/bin/java | head -1)"
+else
+    die "Java 21 is still missing after the install step.
+     sudo dnf install java-21-openjdk-headless"
+fi
 
 if ! mountpoint -q "$SHARE"; then
     die "$SHARE is not a mountpoint — the ZFS dataset is not mounted.

mc-service-setup.sh

@@ -102,10 +102,38 @@ mountpoint -q "$SHARE" || die "$SHARE is not a mountpoint — the ZFS dataset is
      Check 'zpool status' and 'zfs mount -a' first; installing a server onto the
      unmounted path would fill the root filesystem instead of the pool."
 
-command -v java >/dev/null || die "java not found — install it first: dnf install java-21-openjdk-headless"
+# NeoForge 21.1.x targets Java 21 exactly. A newer JDK on PATH — Fedora ships
+# early-access builds — passes a ">= 21" test and then fails inside ModLauncher,
+# so find a real 21 and pin the unit to its absolute path.
+java_version_of() { "$1" -version 2>&1 | head -1 | grep -oE '"[0-9]+' | tr -d '"'; }
 
-jver="$(java -version 2>&1 | head -1 | grep -oE '"[0-9]+' | tr -d '"' || echo 0)"
-[ "${jver:-0}" -ge 21 ] || warn "java looks like version $jver; NeoForge for 1.21.x needs 21"
+find_java21() {
+    local c
+    for c in /usr/lib/jvm/java-21-openjdk*/bin/java \
+             /usr/lib/jvm/java-21*/bin/java \
+             /usr/lib/jvm/*-21-*/bin/java \
+             /usr/lib/jvm/*21*/bin/java; do
+        [ -x "$c" ] || continue
+        [ "$(java_version_of "$c")" = "21" ] && { printf '%s' "$c"; return 0; }
+    done
+    if command -v java >/dev/null && [ "$(java_version_of "$(command -v java)")" = "21" ]; then
+        command -v java
+        return 0
+    fi
+    return 1
+}
+
+JAVA_BIN="$(find_java21 || true)"
+if [ -z "$JAVA_BIN" ]; then
+    have="none"
+    command -v java >/dev/null && have="$(java_version_of "$(command -v java)")"
+    die "no Java 21 found (default java is version $have).
+     NeoForge for Minecraft 1.21.x needs Java 21 specifically — a newer JDK
+     fails inside ModLauncher rather than refusing to start.
+       sudo dnf install java-21-openjdk-headless
+     then re-run."
+fi
+info "java 21: $JAVA_BIN"
 
 # ------------------------------------------------------------------ user ----
 
@@ -355,7 +383,7 @@ StandardError=journal
 RuntimeDirectory=minecraft
 RuntimeDirectoryMode=0755
 ExecStartPre=/bin/sh -c 'rm -f $FIFO && mkfifo -m 0660 $FIFO'
-ExecStart=/bin/bash -c 'exec 3<>$FIFO; exec /usr/bin/java @$SHARE/user_jvm_args.txt @$ARGS_FILE nogui <&3'
+ExecStart=/bin/bash -c 'exec 3<>$FIFO; exec $JAVA_BIN @$SHARE/user_jvm_args.txt @$ARGS_FILE nogui <&3'
 ExecStop=/bin/sh -c '/bin/echo stop > $FIFO'
 
 # Generous: a big world with many chunks loaded can take a while to save.