Support changing RUBY_BUILD_ROOT

This is to allow specifying an alternate path where `share/ruby-build/*`
definitions reside. Useful in tests as well.

README.md

@@ -118,6 +118,8 @@ You can set certain environment variables to control the build process.
   choosing.
 * `RUBY_BUILD_SKIP_MIRROR`, if set, forces ruby-build to download packages from
   their original source URLs instead of using a mirror.
+* `RUBY_BUILD_ROOT` overrides the default location from where build definitions
+  in `share/ruby-build/` are looked up.
 * `CC` sets the path to the C compiler.
 * `RUBY_CFLAGS` lets you pass additional options to the default `CFLAGS`. Use
   this to override, for instance, the `-O3` option.

bin/ruby-build

@@ -907,10 +907,12 @@ usage() {
 }
 
 list_definitions() {
+  shopt -s nullglob
   { for definition in "${RUBY_BUILD_ROOT}/share/ruby-build/"*; do
       echo "${definition##*/}"
     done
   } | sort
+  shopt -u nullglob
 }
 
 
@@ -918,7 +920,10 @@ list_definitions() {
 unset VERBOSE
 unset KEEP_BUILD_PATH
 unset HAS_PATCH
-RUBY_BUILD_ROOT="$(abs_dirname "$0")/.."
+
+if [ -z "$RUBY_BUILD_ROOT" ]; then
+  RUBY_BUILD_ROOT="$(abs_dirname "$0")/.."
+fi
 
 parse_options "$@"
 

test/definitions.bats

@@ -11,6 +11,21 @@ NUM_DEFINITIONS="$(ls "$BATS_TEST_DIRNAME"/../share/ruby-build | wc -l)"
   assert [ "${#lines[*]}" -eq "$NUM_DEFINITIONS" ]
 }
 
+@test "custom RUBY_BUILD_ROOT: nonexistent" {
+  export RUBY_BUILD_ROOT="$TMP"
+  assert [ ! -e "${RUBY_BUILD_ROOT}/share/ruby-build" ]
+  run ruby-build --definitions
+  assert_success ""
+}
+
+@test "custom RUBY_BUILD_ROOT: single definition" {
+  export RUBY_BUILD_ROOT="$TMP"
+  mkdir -p "${RUBY_BUILD_ROOT}/share/ruby-build"
+  touch "${RUBY_BUILD_ROOT}/share/ruby-build/1.9.3-test"
+  run ruby-build --definitions
+  assert_success "1.9.3-test"
+}
+
 @test "installing nonexistent definition" {
   run ruby-build "nonexistent" "${TMP}/install"
   assert [ "$status" -eq 2 ]