Fix emsdk_env.sh for multiple shells (#594)

This change allows sourcing emsdk_env.sh from bash, zsh and
ksh.

The script works out the true location of the emsdk directory,
even if it is a symlink or the script itself is a symlink.

Added a test in scripts/test_source_env.sh to try sourcing via
all the shells and with various paths.

Co-authored-by: Bob Tolbert <bob@tolbert.org>

emsdk_env.sh

@@ -1,26 +1,71 @@
 # This script is sourced by the user and uses
-# their shell. Try not to use bashisms.
-
+# their shell.
+#
+# This script tries to find its location but
+# this does not work in every shell.
+#
+# It is known to work in bash, zsh and ksh
+#
 # Do not execute this script without sourcing,
 # because it won't have any effect then.
 # That is, always run this script with
 #
-#     . ./emsdk_env.sh
+#     . /path/to/emsdk_env.sh
+#
 # or
-#     source ./emsdk_env.sh
+#
+#     source /path/to/emsdk_env.sh
 #
 # instead of just plainly running with
 #
 #     ./emsdk_env.sh
 #
 # which won't have any effect.
-if [ -z "$BASH_SOURCE" ]; then
-  if [ ! -f "./emsdk.py" ]; then
-    echo "error: You must be in the same directory as emsdk_env.sh when sourcing it (or switch to the bash shell)" 1>&2
+
+CURRENT_SCRIPT=
+DIR="."
+
+# use shell specific method to get the path
+# to the current file being source'd.
+#
+# To add a shell, add another conditional below,
+# then add tests to scripts/test_source_env.sh
+
+if [ -n "$BASH_SOURCE" ]; then
+  CURRENT_SCRIPT="$BASH_SOURCE"
+elif [ -n "$ZSH_VERSION" ]; then
+  CURRENT_SCRIPT="${(%):-%x}"
+elif [ -n "$KSH_VERSION" ]; then
+  CURRENT_SCRIPT=${.sh.file}
+fi
+
+if [ -n "$CURRENT_SCRIPT" ]; then
+  DIR=$(dirname "$CURRENT_SCRIPT")
+  if [ -h "$CURRENT_SCRIPT" ]; then
+    # Now work out actual DIR since this is part of a symlink.
+    # Since we can't be sure that readlink or realpath
+    # are available, use tools more likely to be installed.
+    # (This will still fail if sed is not available.)
+    SYMDIR=$(dirname "$(ls -l "$CURRENT_SCRIPT" | sed -n "s/.*-> //p")")
+    if [ -z "$SYMDIR" ]; then
+      SYMDIR="."
+    fi
+    FULLDIR="$DIR/$SYMDIR"
+    DIR=$(cd "$FULLDIR" > /dev/null 2>&1; /bin/pwd)
+    unset SYMDIR
+    unset FULLDIR
   fi
-  DIR="."
-else
-  DIR="$(dirname "$BASH_SOURCE")"
+fi
+unset CURRENT_SCRIPT
+
+if [ ! -f "$DIR/emsdk.py" ]; then
+  echo "Error: unable to determine 'emsdk' directory. Perhaps you are using a shell or" 1>&2
+  echo "       environment that this script does not support." 1>&2
+  echo 1>&2
+  echo "A possible solution is to source this script while in the 'emsdk' directory." 1>&2
+  echo 1>&2
+  unset DIR
+  return
 fi
 
 # Force emsdk to use bash syntax so that this works in windows + bash too