Allow plugins to run code before and after the installation process

bin/rbenv-install

@@ -65,25 +65,62 @@ for option in "${OPTIONS[@]}"; do
   esac
 done
 
+unset VERSION_NAME
 DEFINITION="${ARGUMENTS[0]}"
 [ -n "$DEFINITION" ] || usage 1
 
+
+# Define `before_install` and `after_install` functions that allow
+# plugin hooks to register a string of code for execution before or
+# after the installation process.
+declare -a before_hooks after_hooks
+
+before_install() {
+  local hook="$1"
+  before_hooks["${#before_hooks[@]}"]="$hook"
+}
+
+after_install() {
+  local hook="$1"
+  after_hooks["${#after_hooks[@]}"]="$hook"
+}
+
+# Load plugin hooks.
 for script in $(rbenv-hooks install); do
   source "$script"
 done
 
-VERSION_NAME="${DEFINITION##*/}"
+
+# Set VERSION_NAME from $DEFINITION, if it is not already set. Then
+# compute the installation prefix.
+[ -n "$VERSION_NAME" ] || VERSION_NAME="${DEFINITION##*/}"
 PREFIX="${RBENV_ROOT}/versions/${VERSION_NAME}"
 
-# If RBENV_BUILD_ROOT is set, then always pass keep options to ruby-build
+# If RBENV_BUILD_ROOT is set, always pass keep options to ruby-build.
 if [ -n "${RBENV_BUILD_ROOT}" ]; then
   export RUBY_BUILD_BUILD_PATH="${RBENV_BUILD_ROOT}/${VERSION_NAME}"
   KEEP="-k"
 fi
 
+# Set RUBY_BUILD_CACHE_PATH to $RBENV_ROOT/cache, if the directory
+# exists and the variable is not already set.
 if [ -z "${RUBY_BUILD_CACHE_PATH}" ] && [ -d "${RBENV_ROOT}/cache" ]; then
   export RUBY_BUILD_CACHE_PATH="${RBENV_ROOT}/cache"
 fi
 
-ruby-build $KEEP $VERBOSE "$DEFINITION" "$PREFIX"
-rbenv rehash
+
+# Execute `before_install` hooks.
+for hook in "${before_hooks[@]}"; do eval "$hook"; done
+
+# Invoke `ruby-build` and record the exit status in $STATUS. Run
+# `rbenv rehash` after a successful installation.
+STATUS=0
+ruby-build $KEEP $VERBOSE "$DEFINITION" "$PREFIX" || STATUS="$?"
+
+# Execute `after_install` hooks.
+for hook in "${after_hooks[@]}"; do eval "$hook"; done
+
+# Run `rbenv-rehash` after a successful installation.
+[ "$STATUS" != "0" ] || rbenv rehash
+
+exit "$STATUS"