bin/n: Port auto-xz feature from nvh

Includes platform/feature detection to guess if the user has
the ability to decompress xz tarballs using `tar`.

Doesn't attempt to fetch/decompress xz tarballs
for Node versions less than 4. (KISS).

The feature is set up to use xz instead of gz by default,
but the default behavior is easily configurable.

Co-authored-by: John Gee <[email protected]>

bin/n

@@ -55,6 +55,18 @@ N_NODE_DOWNLOAD_MIRROR=${N_NODE_DOWNLOAD_MIRROR:-https://nodejs.org/download}
 N_NODE_DOWNLOAD_MIRROR=${N_NODE_DOWNLOAD_MIRROR%/}
 readonly N_NODE_DOWNLOAD_MIRROR
 
+# Using xz instead of gzip is enabled by default, if xz compatibility checks pass.
+# User may set N_USE_XZ to 0 to disable, or set to anything else to enable.
+# May also be overridden by command line flags.
+
+# Normalise external values to true/false
+if [[ "${N_USE_XZ}" = "0" ]]; then
+  N_USE_XZ="false"
+elif [[ -n "${N_USE_XZ+defined}" ]]; then
+  N_USE_XZ="true"
+fi
+# Not setting to readonly. Overriden by CLI flags, and update_xz_settings_for_version.
+
 N_MAX_REMOTE_MATCHES=${N_MAX_REMOTE_MATCHES:-20}
 # modified by update_mirror_settings_for_version
 g_mirror_url=${N_NODE_MIRROR}
@@ -165,6 +177,23 @@ function update_mirror_settings_for_version() {
   fi
 }
 
+#
+# Synopsis: update_xz_settings_for_version version
+# Globals modified:
+# - N_USE_XZ
+#
+
+function update_xz_settings_for_version() {
+  # tarballs in xz format were available in later version of iojs, but KISS and only use xz from v4.
+  if [[ "${N_USE_XZ}" = "true" ]]; then
+    local resolved_major_version
+    resolved_major_version="$(echo "${1#v}" | cut -d '.' -f '1')"
+    if [[ "${resolved_major_version}" -lt 4 ]]; then
+      N_USE_XZ="false"
+    fi
+  fi
+}
+
 #
 # Synopsis: is_lts_codename version
 #
@@ -276,13 +305,14 @@ Commands:
 
 Options:
 
-  -V, --version   Output version of n
-  -h, --help      Display help information
-  -q, --quiet     Disable curl output (if available)
-  -d, --download  Download only
-  -a, --arch      Override system architecture
-  --all           ls-remote displays all matches instead of last 20
-  --insecure      Turn off certificate checking for https requests (may be needed from behind a proxy server)
+  -V, --version         Output version of n
+  -h, --help            Display help information
+  -q, --quiet           Disable curl output (if available)
+  -d, --download        Download only
+  -a, --arch            Override system architecture
+  --all                 ls-remote displays all matches instead of last 20
+  --insecure            Turn off certificate checking for https requests (may be needed from behind a proxy server)
+  --use-xz/--no-use-xz  Override automatic detection of xz support and enable/disable use of xz compressed node downloads.
 
 Aliases:
 
@@ -589,10 +619,11 @@ install() {
   version="$(display_latest_resolved_version "$1")" || return 2
   [[ -n "${version}" ]] || abort "no version found for '$1'"
   update_mirror_settings_for_version "$1"
+  update_xz_settings_for_version "${version}"
 
   local dir="${CACHE_DIR}/${g_mirror_folder_name}/${version}"
 
-  if test -n "$N_USE_XZ"; then
+  if test "$N_USE_XZ" = "true"; then
     local tarflag="-Jx"
   else
     local tarflag="-zx"
@@ -775,6 +806,25 @@ function is_ok() {
   fi
 }
 
+#
+# Synopsis: can_use_xz
+# Test system to see if xz decompression is supported by tar.
+#
+
+function can_use_xz() {
+  # Be conservative and only enable if xz is likely to work. Unfortunately we can't directly query tar itself.
+  # For research, see https://github.com/shadowspawn/nvh/issues/8
+  local uname_s="$(uname -s)"
+  if [[ "${uname_s}" = "Linux" ]] && command -v xz &> /dev/null ; then
+    # tar on linux is likely to support xv if it is available separately
+    return 0
+  elif [[ "${uname_s}" = "Darwin" && "$(sw_vers -productVersion | cut -d '.' -f 2)" -gt "8" ]]; then
+    # tar on recent Darwin has xv support built-in
+    return 0
+  fi
+  return 2 # not supported
+}
+
 #
 # Synopsis: display_tarball_platform
 #
@@ -830,7 +880,7 @@ function display_compatible_file_field {
 function tarball_url() {
   local version="$1"
   local ext=gz
-  [ -n "$N_USE_XZ" ] && ext="xz"
+  [ "$N_USE_XZ" = "true" ] && ext="xz"
   echo "${g_mirror_url}/v${version}/node-v${version}-$(display_tarball_platform).tar.${ext}"
 }
 
@@ -1191,6 +1241,8 @@ while [[ $# -ne 0 ]]; do
     -q|--quiet) set_quiet ;;
     -d|--download) ACTIVATE=false ;;
     --insecure) set_insecure ;;
+    --use-xz) N_USE_XZ="true" ;;
+    --no-use-xz) N_USE_XZ="false" ;;
     --latest) display_remote_versions latest; exit ;;
     --stable) display_remote_versions lts; exit ;; # [sic] old terminology
     --lts) display_remote_versions lts; exit ;;
@@ -1202,6 +1254,11 @@ while [[ $# -ne 0 ]]; do
   shift
 done
 
+if [[ -z "${N_USE_XZ+defined}" ]]; then
+  N_USE_XZ="true" # Default to using xz
+  can_use_xz || N_USE_XZ="false"
+fi
+
 set -- "${unprocessed_args[@]}"
 
 if test $# -eq 0; then