add a wasm bazel toolchain (#603)

scripts/update_bazel_workspace.sh

@@ -0,0 +1,63 @@
+#!/bin/bash
+# This script will update emsdk/bazel/WORKSPACE to the latest version of
+# emscripten. It reads emsdk/emscripten-releases-tags.txt to get the latest
+# version number. Then, it downloads the prebuilts for that version and computes
+# the sha256sum for the archive. It then puts all this information into the
+# emsdk/bazel/WORKSPACE file.
+
+ERR=0
+# Attempt to change to the emsdk root directory
+cd $(dirname $0)/..
+
+# If the previous command succeeded. We are in the emsdk root. Check to make
+# sure the files and directories we need are present.
+if [[ $? = 0 ]]; then
+  if [[ ! -f emscripten-releases-tags.txt ]]; then
+    echo "Cannot find emscripten-releases-tags.txt."
+    ERR=1
+  fi
+
+  if [[ ! -d bazel ]]; then
+    echo "Cannot find the bazel directory."
+    ERR=1
+  elif [[ ! -f bazel/WORKSPACE ]]; then
+    echo "Cannot find bazel/WORKSPACE."
+    ERR=1
+  fi
+else
+  ERR=1
+fi
+
+if [[ $ERR = 1 ]]; then
+  echo "Unable to cd into the emsdk root directory."
+  exit 1
+fi
+
+URL1=https://storage.googleapis.com/webassembly/emscripten-releases-builds/linux/
+URL2=/wasm-binaries.tbz2
+
+# Get the latest version number from emscripten-releases-tag.txt.
+VER=$(grep -oP '(?<=latest\": \")([\d\.]+)(?=\")' \
+        emscripten-releases-tags.txt \
+      | sed --expression "s/\./\\\./g")
+# Based on the latest version number, get the commit hash for that version.
+HASH=$(grep "${VER}" emscripten-releases-tags.txt \
+      | grep -v latest \
+      | cut -f4 -d\")
+# Download and compute the sha256sum for the archive with the prebuilts.
+SHA=$(curl "${URL1}${HASH}${URL2}" 2>/dev/null \
+      | sha256sum \
+      | awk '{print $1}')
+# Get the line number on which the sha256 sum lives for emscripten.
+# This will always be one line after the name of the rule.
+SHALINE=$(($(grep -n 'name = "emscripten"' bazel/WORKSPACE \
+      | sed 's/^\([[:digit:]]*\).*$/\1/')+1))
+
+# Insert the new commit hash into the url.
+sed -i "s!\(${URL1}\)\([[:alnum:]]*\)\(${URL2}\)!\1${HASH}\3!" bazel/WORKSPACE
+# Insert the new version number.
+sed -i "s!\(# emscripten \)\(.*\)!\1${VER}!" bazel/WORKSPACE
+# Insert the new sha256 sum.
+sed -i "${SHALINE}s!\"[[:alnum:]]*\"!\"${SHA}\"!" bazel/WORKSPACE
+
+echo "Done!"