make bazel cache track host tools

experiment/nursery/create_bazel_cache_rcs.sh

@@ -19,16 +19,44 @@
 # TODO(bentheelder): verify that this works and move it into the images
 CACHE_HOST="http://bazel-cache.default:8080"
 
+# get the installed version of a debian package
+package_to_version () {
+    dpkg-query --showformat='${Version}' --show $1
+}
+
+# look up a binary with which and return the debian package it belongs to
+command_to_package () {
+    BINARY_PATH=$(readlink -f $(which $1))
+    # `dpkg -S $package` spits out lines with the format: "package: file"
+    dpkg -S $1 | grep $BINARY_PATH | cut -d':' -f1
+}
+
+# get the installed package version relating to a binary
+command_to_version () {
+    PACKAGE=$(command_to_package $1)
+    package_to_version $PACKAGE
+}
+
 make_bazel_rc () {
     # this is the default for recent releases but we set it explicitly
     # since this is the only hash our cache supports
-    echo "startup --host_jvm_args=-Dbazel.DigestFunction=sha256" >> $1
+    echo "startup --host_jvm_args=-Dbazel.DigestFunction=sha256"
     # use remote caching for all the things
-    echo "build --experimental_remote_spawn_cache" >> $1
+    echo "build --experimental_remote_spawn_cache"
     # point it at our http cache ...
-    echo "build --remote_http_cache=${CACHE_HOST}" >> $1
+    echo "build --remote_http_cache=${CACHE_HOST}"
     # don't fail if the cache is unavailable
-    echo "build --remote_local_fallback" >> $1
+    echo "build --remote_local_fallback"
+    # make sure the cache considers host toolchain versions
+    # NOTE: these assume nobody installs new host toolchains afterwards
+    # if $CC is set bazel will use this to detect c/c++ toolchains, otherwise gcc
+    # https://blog.bazel.build/2016/03/31/autoconfiguration.html
+    CC="${CC:-gcc}"
+    CC_VERSION=$(command_to_version $CC)
+    echo "build --action_env=CACHE_GCC_VERSION=${CC_VERSION}"
+    # NOTE: IIRC some rules call python internally, this can't hurt
+    PYTHON_VERSION=$(command_to_version python)
+    echo "build --action_env=CACHE_PYTHON_VERSION=${PYTHON_VERSION}"
 }
 
 # https://docs.bazel.build/versions/master/user-manual.html#bazelrc
@@ -37,8 +65,8 @@ make_bazel_rc () {
 # - The path specified by the --bazelrc=file startup option. If specified, this option must appear before the command name (e.g. build)
 # - A file named .bazelrc in your base workspace directory
 # - A file named .bazelrc in your home directory
-make_bazel_rc "${HOME}/.bazelrc"
+make_bazel_rc >> "${HOME}/.bazelrc"
 # Aside from the optional configuration file described above, Bazel also looks for a master rc file next to the binary, in the workspace at tools/bazel.rc or system-wide at /etc/bazel.bazelrc.
 # These files are here to support installation-wide options or options shared between users. Reading of this file can be disabled using the --nomaster_bazelrc option.
-make_bazel_rc "/etc/bazel.bazelrc"
+make_bazel_rc >> "/etc/bazel.bazelrc"
 # hopefully no repos create *both* of these ...