Avoid re-downloading if there's a valid tarball in build location

If there already exists a valid tarball in the build location, e.g. as
artefact of a previous install using `--keep`, don't re-download the
file, as there is no need.

References #487

bin/ruby-build

@@ -261,7 +261,7 @@ fetch_tarball() {
     tar_args="${tar_args/z/j}"
   fi
 
-  if ! symlink_tarball_from_cache "$package_filename" "$checksum"; then
+  if ! reuse_existing_tarball "$package_filename" "$checksum"; then
     echo "Downloading ${package_filename}..." >&2
     http head "$mirror_url" &&
     download_tarball "$mirror_url" "$package_filename" "$checksum" ||
@@ -278,13 +278,19 @@ fetch_tarball() {
   } >&4 2>&1
 }
 
-symlink_tarball_from_cache() {
-  [ -n "$RUBY_BUILD_CACHE_PATH" ] || return 1
-
+reuse_existing_tarball() {
   local package_filename="$1"
-  local cached_package_filename="${RUBY_BUILD_CACHE_PATH}/$package_filename"
   local checksum="$2"
 
+  # Reuse existing file in build location
+  if [ -e "$package_filename" ] && verify_checksum "$package_filename" "$checksum"; then
+    return 0
+  fi
+
+  # Reuse previously downloaded file in cache location
+  [ -n "$RUBY_BUILD_CACHE_PATH" ] || return 1
+  local cached_package_filename="${RUBY_BUILD_CACHE_PATH}/$package_filename"
+
   [ -e "$cached_package_filename" ] || return 1
   verify_checksum "$cached_package_filename" "$checksum" >&4 2>&1 || return 1
   ln -s "$cached_package_filename" "$package_filename" >&4 2>&1 || return 1

test/checksum.bats

@@ -68,3 +68,46 @@ export RUBY_BUILD_CACHE_PATH=
   unstub curl
   unstub md5
 }
+
+@test "existing tarball in build location is reused" {
+  stub md5 true "echo 83e6d7725e20166024a1eb74cde80677"
+  stub curl false
+  stub wget false
+
+  export -n RUBY_BUILD_CACHE_PATH
+  export RUBY_BUILD_BUILD_PATH="${TMP}/build"
+
+  mkdir -p "$RUBY_BUILD_BUILD_PATH"
+  ln -s "${FIXTURE_ROOT}/package-1.0.0.tar.gz" "$RUBY_BUILD_BUILD_PATH"
+
+  run_inline_definition <<DEF
+install_package "package-1.0.0" "http://example.com/packages/package-1.0.0.tar.gz#83e6d7725e20166024a1eb74cde80677" copy
+DEF
+
+  assert_success
+  [ -x "${INSTALL_ROOT}/bin/package" ]
+
+  unstub md5
+}
+
+@test "existing tarball in build location is discarded if not matching checksum" {
+  stub md5 true \
+    "echo invalid" \
+    "echo 83e6d7725e20166024a1eb74cde80677"
+  stub curl "-q -o * -*S* http://example.com/* : cp $FIXTURE_ROOT/\${5##*/} \$3"
+
+  export -n RUBY_BUILD_CACHE_PATH
+  export RUBY_BUILD_BUILD_PATH="${TMP}/build"
+
+  mkdir -p "$RUBY_BUILD_BUILD_PATH"
+  touch "${RUBY_BUILD_BUILD_PATH}/package-1.0.0.tar.gz"
+
+  run_inline_definition <<DEF
+install_package "package-1.0.0" "http://example.com/packages/package-1.0.0.tar.gz#83e6d7725e20166024a1eb74cde80677" copy
+DEF
+
+  assert_success
+  [ -x "${INSTALL_ROOT}/bin/package" ]
+
+  unstub md5
+}