[et] Simplify path canonicalisation logic

In order to canonicalise paths, previously we were doing an iterative
computation to resolve symlinks to a canonical path, directory by
directory. This was because on macOS and other BSDs, readlink doesn't
support the `-f` (follow symlinks) option. However, macOS and other
BSD-based systems *do* bundle the `realpath` utility, which resolves
symlinks.

Author: cbracken

bin/et

@@ -9,25 +9,22 @@ set -e
 # Needed because if it is set, cd may print the path it changed to.
 unset CDPATH
 
-# On Mac OS, readlink -f doesn't work, so follow_links traverses the path one
-# link at a time, and then cds into the link destination and find out where it
-# ends up.
-#
-# The function is enclosed in a subshell to avoid changing the working directory
-# of the caller.
-function follow_links() (
-  cd -P "$(dirname -- "$1")"
-  file="$PWD/$(basename -- "$1")"
-  while [[ -h "$file" ]]; do
-    cd -P "$(dirname -- "$file")"
-    file="$(readlink -- "$file")"
-    cd -P "$(dirname -- "$file")"
-    file="$PWD/$(basename -- "$file")"
-  done
-  echo "$file"
-)
-
-SCRIPT_DIR="$(dirname -- "$(follow_links "${BASH_SOURCE[0]}")")"
+# Returns the canonical path for its argument, with any symlinks resolved.
+function canonical_path() {
+  case "$(uname -s)" in
+    Linux)
+      readlink -f -- "$1"
+      ;;
+    Darwin)
+      realpath -q -- "$1"
+      ;;
+    *)
+      echo "The host platform is not supported by this tool"
+      exit 1
+  esac
+}
+
+SCRIPT_DIR="$(dirname -- "$(canonical_path "${BASH_SOURCE[0]}")")"
 ENGINE_DIR="$(cd "$SCRIPT_DIR/.."; pwd -P)"
 
 case "$(uname -s)" in