canary: cmuxTests full-recompile probe (do not merge)

.github/workflows/ci-macos.yml

@@ -579,6 +579,9 @@ jobs:
       - name: Compile app-host test product
         id: hosted-compile
         if: steps.reuse-products.outputs.hit != 'true'
+        # CANARY: swift-driver explains each incremental decision.
+        env:
+          CMUX_CI_EXTRA_SWIFT_FLAGS: -driver-show-incremental
         run: |
           set -uo pipefail
           started="$(python3 -c 'import time; print(time.monotonic())')"
@@ -593,6 +596,49 @@ jobs:
           echo "seconds=$seconds" >> "$GITHUB_OUTPUT"
           exit "$status"
 
+      # CANARY (do not merge): why does cmuxTests recompile in full after the
+      # cmux scheme builds, when a cmux-unit null rebuild is cheap?
+      - name: Canary probe cmuxTests rebuild cause
+        if: always() && steps.hosted-compile.outcome == 'success'
+        continue-on-error: true
+        timeout-minutes: 50
+        run: |
+          set +e -u
+          dd="$CMUX_COMPILE_ADMISSION_DERIVED_DATA"; src=/private/tmp/cmux-ci/src
+          out="$RUNNER_TEMP/canary"; mkdir -p "$out"
+          probe() {
+            local name="$1" flags="$2" log="$out/probe-$1.txt" started=$SECONDS
+            CMUX_CI_EXTRA_SWIFT_FLAGS="$flags" CMUX_CI_BUILD_SCHEMES=cmux-unit \
+              scripts/ci/compile-app-host-test-product.sh canonical-build \
+              "$dd" "$PWD/.ci-source-packages" "$CMUX_COMPILE_ADMISSION_CAS" "$log" > /dev/null 2>&1 \
+              || echo "probe $name build failed: $(grep -m3 -E 'error:' "$log")"
+            local line="probe $name [$flags]: $((SECONDS - started))s"
+            line+=" cmuxTests=$(grep -cE "^SwiftCompile normal arm64 .*\(in target 'cmuxTests'" "$log")"
+            line+=" cmux=$(grep -cE "^SwiftCompile normal arm64 .*\(in target 'cmux'" "$log")"
+            line+=" newerChained=$(grep -c 'Newer: cmuxTests-.*ChainedBridgingHeader' "$log")"
+            line+=" $(grep -m1 -E 'SwiftCompile \([0-9]+ tasks\)' "$log")"
+            echo "$line"; echo "- $line" >> "$GITHUB_STEP_SUMMARY"
+          }
+          edit() { printf '\n// canary edit %s\n' "$1" >> "$src/cmuxTests/CmuxSSHURLRequestTests.swift"; }
+          edit baseline; probe baseline-test-edit "-driver-show-incremental"
+          export CMUX_CI_EXTRA_BUILD_SETTING=SWIFT_PRECOMPILE_BRIDGING_HEADER=NO
+          probe nopch-prime "-driver-show-incremental"
+          find "$dd" -name '*ChainedBridgingHeader*' -newer "$src/cmuxTests/CmuxSSHURLRequestTests.swift" -exec stat -f '%Sm %N' {} + 2>/dev/null | head -3
+          edit nopch1; probe nopch-test-edit "-driver-show-incremental"
+          edit nopch2; probe nopch-test-edit-2 "-driver-show-incremental"
+          probe nopch-null "-driver-show-incremental"
+          unset CMUX_CI_EXTRA_BUILD_SETTING
+          exit 0
+
+      - name: Upload canary probe logs
+        if: always() && steps.hosted-compile.outcome == 'success'
+        continue-on-error: true
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
+        with:
+          name: canary-cmuxtests-rebuild-${{ github.run_attempt }}
+          path: ${{ runner.temp }}/canary
+          retention-days: 5
+
       # Adoption sets a per-user Xcode default. Only a build whose inputs were
       # just replayed should ignore inode changes, so a reused runner does not
       # carry it into an unrelated job.

.github/workflows/ci.yml

@@ -602,7 +602,11 @@ jobs:
           RUN_CLAUDE_WRAPPER: ${{ steps.standalone.outputs.claude_wrapper }}
           RUN_CLI: ${{ steps.detect.outputs.cli }}
           RUN_REMOTE_DAEMON: ${{ steps.standalone.outputs.remote_daemon }}
-        run: python3 scripts/ci/pr_runner_pool.py
+        # CANARY (do not merge): pin an owned mini, which reuses its own build
+        # state under one Xcode install, so Xcode mtimes cannot differ.
+        run: |
+          python3 scripts/ci/pr_runner_pool.py
+          printf 'runner=blacksmith-12vcpu-macos-26\nxcode_app=\npersistent=false\nretry_runner=blacksmith-12vcpu-macos-26\n' >> "$GITHUB_OUTPUT"
 
       # A job queued on a persistent pool (owned Macs) waits for it however long
       # it stays busy. This marker tells ci-owned-pool-rescue.yml, running from

cmuxTests/CmuxSSHURLRequestTests.swift

@@ -1004,3 +1004,5 @@ final class CmuxNavigationURLRequestTests: XCTestCase {
         return request.target
     }
 }
+
+// canary: test-only change for the cmuxTests rebuild probe (do not merge)

scripts/ci/compile-app-host-test-product.sh

@@ -150,6 +150,7 @@ build() {
   [ "${#schemes[@]}" -gt 0 ] || { echo "empty product profile scheme list" >&2; exit 1; }
   # shellcheck disable=SC2016 # Xcode expands $(inherited), not the shell
   for scheme in "${schemes[@]}"; do
+    case " ${CMUX_CI_BUILD_SCHEMES:-$scheme} " in *" $scheme "*) ;; *) continue ;; esac
     FileSystemMode="$XCBUILD_FILE_SYSTEM_MODE" xcodebuild -project cmux.xcodeproj -scheme "$scheme" -configuration Debug \
       -derivedDataPath "$derived_data" \
       -clonedSourcePackagesDirPath "$source_packages" \
@@ -161,7 +162,7 @@ build() {
       "COMPILATION_CACHE_CAS_PATH=$cas_path" \
       "COMPILATION_CACHE_LIMIT_SIZE=$cache_limit_bytes" \
       ${module_cache_setting[@]+"${module_cache_setting[@]}"} \
-      -showBuildTimingSummary \
+      ${CMUX_CI_EXTRA_SWIFT_FLAGS:+"OTHER_SWIFT_FLAGS=\$(inherited) $CMUX_CI_EXTRA_SWIFT_FLAGS"} ${CMUX_CI_EXTRA_BUILD_SETTING:+"$CMUX_CI_EXTRA_BUILD_SETTING"} -showBuildTimingSummary \
       build-for-testing 2>&1 | tee "$derived_data/$scheme-build.log" | tee -a "$log"
   done
 }