Add BUILD_MODE argument

This expands the number of modes from two to four, allowing one to
separate running the build from running tests.

This change is backwards-compatible. No existing behavior is changed.

ci/config.sh

@@ -115,20 +115,53 @@ extra_package pandoc
 build_tool_package alex
 build_tool_package happy
 
-# Quick build configuration
-# =========================
+# $BUILD_MODE controls how head.hackage runs.
+# ===========================================
 #
-# If $QUICK is defined we build the "quick" configuration, which builds a small
-# subset of the overall package set. We do this during the merge request
-# validation pipeline.
+# Four build modes exist: FULL, QUICK, TEST, and COMPAT.
+#
+# FULL.
+# ------
+# Build all patched + extra packages.
+#
+# QUICK.
+# ------
+# Build the "quick" configuration, which builds a small subset of the overall
+# package set. We do this during the merge request validation pipeline. Note: If
+# "$QUICK" is non-null, it is used as a backwards-compat synonym for
+# BUILD_MODE=QUICK.
+#
+# TEST.
+# -----
+# Just build the local test packages and run the tests.
+#
+# COMPAT: FULL + TEST.
+# --------------------
+# Backwards-compat default build mode.
+#
+: ${BUILD_MODE:=COMPAT}
 if [ -n "$QUICK" ]; then
-  only_package Cabal
-  only_package microlens
-  only_package free
-  only_package optparse-applicative
+    BUILD_MODE=QUICK
 fi
-
-# Testing specific local packages
-
-test_package system-test "$(pwd)/../tests/ghc-debug/test/"
-test_package ghc-tests "$(pwd)/../tests/ghc-tests"
+case "$BUILD_MODE" in
+    FULL) ;;
+    QUICK)
+        only_package tasty
+        only_package Cabal
+        only_package microlens
+        only_package free
+        only_package optparse-applicative
+        ;;
+    TEST)
+        # FIXME: I specify a single "only_package" to prevent all the other
+        # packages from being built. Morally, I really want to say "build
+        # nothing at all besides the tests".
+        only_package tasty
+        test_package system-test "$(pwd)/../tests/ghc-debug/test/"
+        test_package ghc-tests "$(pwd)/../tests/ghc-tests"
+        ;;
+    COMPAT)
+        test_package system-test "$(pwd)/../tests/ghc-debug/test/"
+        test_package ghc-tests "$(pwd)/../tests/ghc-tests"
+        ;;
+esac