Enable ccache, with its cache outside the build env

.gitignore

@@ -2,6 +2,7 @@
 .*.sw[po]
 
 # Directories created during the build, containing build artifacts
+ccache
 dist
 download
 envs

cygwin-gap-build.list

@@ -1,5 +1,6 @@
 autoconf
 automake
+ccache
 cddlib-devel
 gcc-core
 gcc-core

release_gap.sh

@@ -37,9 +37,28 @@ download/setup-x86_64.exe --site ${CYGWIN_MIRROR} \
     --arch x86_64 --no-admin --no-shortcuts --quiet-mode
 
 # Install symlinks for CCACHE
-if [ -x envs/build-${GAP_VERSION}-x86_64/usr/bin/ccache ]; then
+# (the build env mounts its bin/ as /usr/bin, so that is where setup puts it)
+if [ -f envs/build-${GAP_VERSION}-x86_64/bin/ccache.exe ]; then
+    mkdir -p envs/build-${GAP_VERSION}-x86_64/usr/local/bin
     ln -s /usr/bin/ccache envs/build-${GAP_VERSION}-x86_64/usr/local/bin/gcc
     ln -s /usr/bin/ccache envs/build-${GAP_VERSION}-x86_64/usr/local/bin/g++
+
+    # The cache lives outside the build env, whose path contains the GAP
+    # version, so that CI can keep it across builds. subcyg clears the
+    # environment, hence a profile script; it receives a Windows path because
+    # a POSIX path means something else in the build env's Cygwin.
+    # CCACHE_BASEDIR makes the versioned paths below /opt relative, and the
+    # compiler is reinstalled for every build, so check its content, not its
+    # mtime.
+    mkdir -p ccache
+    cat > envs/build-${GAP_VERSION}-x86_64/etc/profile.d/ccache.sh <<EOF
+export CCACHE_DIR="\$(cygpath -u '$(cygpath -w -a ccache)')"
+export CCACHE_BASEDIR=/opt
+export CCACHE_COMPILERCHECK=content
+export CCACHE_COMPRESS=1
+EOF
+    tools/subcyg "${ENV_BUILD_DIR}" "ccache -M 2G && ccache -z"
+    USE_CCACHE=yes
 fi
 # A bit of cleanup
 rm -f envs/build-${GAP_VERSION}-x86_64/Cygwin*.{bat,ico}
@@ -89,6 +108,12 @@ echo "::group::Build Packages"
 tools/subcyg "${ENV_BUILD_DIR}" "cd ${GAP_ROOT}/pkg && (../bin/BuildPackages.sh --parallel || true)"
 echo "::endgroup::"
 
+if [ "${USE_CCACHE}" = yes ]; then
+    echo "::group::ccache statistics"
+    tools/subcyg "${ENV_BUILD_DIR}" "ccache -s"
+    echo "::endgroup::"
+fi
+
 # gap-prep-runtime
 echo "::group::gap-prep-runtime"
 mkdir -p $(dirname ${GAP_ROOT_RUNTIME})

tools/subcyg

@@ -53,7 +53,7 @@ pid=$!
 # sometimes there can be a delay.
 sleep 1
 if [ -f "$subcyg_root$outtmp" ]; then
-    tail -F --pid=$pid "$subcyg_root$outtmp" 2>/dev/null
+    tail -n +1 -F --pid=$pid "$subcyg_root$outtmp" 2>/dev/null
     if [ $? -eq 130 ]; then
         # tail was interrupted by Ctrl-C, so treat this as though the
         # wrapped command were terminated as well.