don't download/unpack core unnecessarily

scripts/download-core.sh

@@ -5,6 +5,7 @@ set -o pipefail
 
 : ${REALM_CORE_VERSION:=0.95.6} # set to "current" to always use the current build
 
+download_core() {
 	echo "Downloading dependency: core ${REALM_CORE_VERSION}"
 
 	TMP_DIR="$TMPDIR/core_bin"
@@ -16,6 +17,8 @@ if [ ! -f "${CORE_TAR}" ]; then
 		curl -f -L -s "https://static.realm.io/downloads/core/realm-core-${REALM_CORE_VERSION}.tar.bz2" -o "${CORE_TMP_TAR}" ||
 			(echo "Downloading core failed. Please try again once you have an Internet connection." && exit 1)
 		mv "${CORE_TMP_TAR}" "${CORE_TAR}"
+    else
+		echo "Using cached core from TEMPDIR"
 	fi
 
 	(
@@ -28,3 +31,26 @@ fi
 	rm -rf core-${REALM_CORE_VERSION} core
 	mv ${TMP_DIR}/core-${REALM_CORE_VERSION} .
 	ln -s core-${REALM_CORE_VERSION} core
+}
+if [ -d core -a -d ../realm-core -a ! -L core ]; then
+    # Allow newer versions than expected for local builds as testing
+    # with unreleased versions is one of the reasons to use a local build
+    if ! $(grep -i "${REALM_CORE_VERSION} Release notes" core/release_notes.txt >/dev/null); then
+	    echo "Local build of core is out of date."
+	    exit 1
+    else
+	    echo "The core library seems to be up to date."
+    fi
+elif ! [ -L core ]; then
+	echo "core is not a symlink. Deleting..."
+	rm -rf core
+	download_core
+	# With a prebuilt version we only want to check the first non-empty
+	# line so that checking out an older commit will download the
+	# appropriate version of core if the already-present version is too new
+	elif ! $(grep -m 1 . core/release_notes.txt | grep -i "${REALM_CORE_VERSION} RELEASE NOTES" >/dev/null); then
+		download_core
+	else
+		echo "The core library seems to be up to date."
+fi
+