feat(build): add --incremental flag to xce-build.sh (bug-1565)

setup_build() unconditionally rm -rf'd the whole profile build
directory on every single-component invocation -- running
/xce-build xceadmin after /xce-build controller silently deleted
xce_controller/xce_test/etc. from the same build dir.

Added an opt-in --incremental flag (meson setup --reconfigure in
place instead of wiping) rather than changing the default: this
project's capnp-generation custom_target has known missing-
dependency-edge races that a full wipe currently papers over as a
loud, retry-fixable failure. Making incremental the default risked
turning that same gap into silent stale-generated-code instead, so
the safer full-wipe behavior is preserved unless explicitly opted
into.

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

src/tools/xce-build.sh

@@ -1,6 +1,36 @@
 #!/usr/bin/env bash
 set -euo pipefail
 
+# --incremental (bug-1565 escape hatch): opt-in, NOT the default. Every
+# invocation normally does a full `rm -rf "$BUILD_DIR"` + fresh `meson
+# setup` (see setup_build below) -- correct in isolation, but since
+# `meson compile` afterward only rebuilds the ONE requested component, a
+# single-component build silently deletes any OTHER already-built binary
+# sharing that same dir (e.g. `/xce-build xceadmin` after `/xce-build
+# controller` in xce.build/ wipes xce_controller). --incremental instead
+# reconfigures the EXISTING dir in place (`meson setup --reconfigure`),
+# preserving whatever else was already built there.
+#
+# Deliberately NOT made the default: this project's capnp-generation
+# custom_target (lib/controller.capnp.h/.c) has documented, unfixed
+# missing-dependency-edge races (buglog.json, "retry once before
+# debugging"). A full wipe papers over that gap -- worst case it's a loud,
+# self-evident, retry-fixable failure on a clean build. Without the wipe,
+# the SAME gap can instead cause silent staleness: a `.capnp` schema edit
+# not triggering regeneration, so the build reports success while linking
+# against stale generated code. Use --incremental only when you specifically
+# want a sibling component alongside an already-built one in the same dir;
+# rebuild without it (a real clean build) if anything looks off afterward.
+INCREMENTAL=0
+ARGS=()
+for arg in "$@"; do
+    case "$arg" in
+        --incremental) INCREMENTAL=1 ;;
+        *) ARGS+=("$arg") ;;
+    esac
+done
+set -- "${ARGS[@]}"
+
 COMPONENT="${1:-all}"
 
 # Build PROFILE controls the meson buildtype + sanitizer (and the default build
@@ -48,16 +78,25 @@ SRC_DIR="$(cd "$SCRIPT_DIR/.." && pwd)"
 cd "$SRC_DIR"
 
 usage() {
-    echo "Usage: $0 [component] [profile|build_dir] [build_dir]"
+    echo "Usage: $0 [component] [profile|build_dir] [build_dir] [--incremental]"
     echo "Components: all (default), lib, controller, scheduler, worker, apiserver, xcecli, xceadmin"
     echo "Profiles:   asan (default), debug (non-ASAN -O0 -g3, for gdb/cores), release"
+    echo "--incremental: reconfigure the existing build dir in place instead of a full"
+    echo "  rm -rf + fresh setup -- preserves other already-built components in the same"
+    echo "  dir (bug-1565). Not the default: see the comment at the top of this script"
+    echo "  for the staleness tradeoff. Rebuild without it if anything looks wrong."
     exit 1
 }
 
 setup_build() {
-    echo "=== Clean build: $BUILD_DIR (profile=$PROFILE) ==="
-    rm -rf "$BUILD_DIR"
-    meson setup "$BUILD_DIR" "${MESON_OPTS[@]}"
+    if [ "$INCREMENTAL" = 1 ] && [ -f "$BUILD_DIR/build.ninja" ]; then
+        echo "=== Incremental build: $BUILD_DIR (profile=$PROFILE) — reconfiguring in place, other components' binaries preserved. If anything looks wrong, rebuild without --incremental. ==="
+        meson setup --reconfigure "$BUILD_DIR" "${MESON_OPTS[@]}"
+    else
+        echo "=== Clean build: $BUILD_DIR (profile=$PROFILE) ==="
+        rm -rf "$BUILD_DIR"
+        meson setup "$BUILD_DIR" "${MESON_OPTS[@]}"
+    fi
 }
 
 case "$COMPONENT" in
@@ -139,6 +178,11 @@ case "$COMPONENT" in
         setup_build
         echo "=== Building + testing xceadmin (via meson) ==="
         meson compile -C "$BUILD_DIR" xceadmin
+        WHEEL_FILE=$(ls "$BUILD_DIR"/xceadmin/dist/xceadmin-*.whl 2>/dev/null || true)
+        if [ -n "$WHEEL_FILE" ]; then
+            echo "=== xceadmin wheel: $WHEEL_FILE ==="
+            echo "    Install it directly with: pip install \"$WHEEL_FILE\""
+        fi
         ;;
     all)
         setup_build