provision: Use a more efficient approach for getting yarn version.

Since yarn has a package.json conveniently available, we can parse
that with jq, saving the expensive operation of starting up yarn.

This saves ~300ms in a no-op provision.

scripts/lib/install

@@ -163,7 +163,7 @@ fi
 
 apt-get -y dist-upgrade "${APT_OPTIONS[@]}"
 if ! apt-get install -y \
-    puppet git curl wget \
+    puppet git curl wget jq \
     python python3 python-six python3-six crudini \
     "${ADDITIONAL_PACKAGES[@]}"; then
     set +x

scripts/lib/install-node

@@ -6,7 +6,7 @@ ZULIP_SRV="/srv"
 if [ "$TRAVIS" ] ; then
     ZULIP_SRV="/home/travis"
 fi
-YARN_BIN="$ZULIP_SRV/zulip-yarn/bin/yarn"
+YARN_PACKAGE_JSON="$ZULIP_SRV/zulip-yarn/package.json"
 node_version=8.11.1
 yarn_version=1.7.0
 
@@ -20,7 +20,12 @@ if node_wrapper_path="$(command -v node)"; then
     current_node_version="$(node --version)"
 fi
 
-if [ "$($YARN_BIN --version 2>/dev/null)" = "$yarn_version" ] && [ "$current_node_version" = "v$node_version" ] && [ -L "$node_wrapper_path" ]; then
+current_yarn_version="none"
+if [ -e "$YARN_PACKAGE_JSON" ]; then
+    current_yarn_version=$(jq -r '.version' "$YARN_PACKAGE_JSON")
+fi
+
+if [ "$current_yarn_version" = "$yarn_version" ] && [ "$current_node_version" = "v$node_version" ] && [ -L "$node_wrapper_path" ]; then
     echo "Node version $node_version and yarn version $yarn_version are already installed."
     exit 0
 fi

scripts/lib/setup_venv.py

@@ -37,6 +37,12 @@ VENV_DEPENDENCIES = [
     "libxslt1-dev",         # Used for installing talon
     "libpq-dev",            # Needed by psycopg2
     "libssl-dev",           # Needed to build pycurl and other libraries
+
+    # This is technically a node dependency, but we add it here
+    # because we don't have another place that we install apt packages
+    # on upgrade of a production server, and it's not worth adding
+    # another call to `apt install` for.
+    "jq",                   # Used by scripts/lib/install-node to check yarn version
 ]
 
 codename = parse_lsb_release()["DISTRIB_CODENAME"]