Make cache directory configurable (#717)

Read the cache directory from the environment, falling back to a location under `N_PREFIX`

README.md

@@ -255,6 +255,9 @@ To change the location to say `$HOME/.n`, add lines like the following to your s
     export N_PREFIX=$HOME/.n
     export PATH=$N_PREFIX/bin:$PATH
 
+If you want to store the downloads under a different location, use `N_CACHE_PREFIX`. This does *not* affect the currently active
+node version.
+
 `n` defaults to using xz compressed Node.js tarballs for the download if it is likely tar on the system supports xz decompression.
 You can override the automatic choice by setting an environment variable to zero or non-zero:
 

bin/n

@@ -66,7 +66,11 @@ VERSION="v8.0.3-0"
 N_PREFIX="${N_PREFIX-/usr/local}"
 N_PREFIX=${N_PREFIX%/}
 readonly N_PREFIX
-readonly CACHE_DIR=$N_PREFIX/n/versions
+
+N_CACHE_PREFIX="${N_CACHE_PREFIX-${N_PREFIX}}"
+N_CACHE_PREFIX=${N_CACHE_PREFIX%/}
+CACHE_DIR="${N_CACHE_PREFIX}/n/versions"
+readonly N_CACHE_PREFIX CACHE_DIR
 
 N_NODE_MIRROR=${N_NODE_MIRROR:-${NODE_MIRROR:-https://nodejs.org/dist}}
 N_NODE_MIRROR=${N_NODE_MIRROR%/}

test/tests/install-contents.bats

@@ -42,3 +42,21 @@ function setup() {
 
   rm -rf "${TMP_PREFIX_DIR}"
 }
+
+@test "install: cache prefix" {
+  readonly N_CACHE_PREFIX="$(mktemp -d)"
+  readonly TARGET_VERSION="4.9.1"
+  setup_tmp_prefix
+  export N_CACHE_PREFIX
+
+  [ ! -d "${N_CACHE_PREFIX}/n/versions/node/${TARGET_VERSION}" ]
+  [ ! -d "${N_PREFIX}/n/versions/node/${TARGET_VERSION}" ]
+
+  n ${TARGET_VERSION}
+
+  # Cached version
+  [ -d "${N_CACHE_PREFIX}/n/versions/node/${TARGET_VERSION}" ]
+  [ ! -d "${N_PREFIX}/n/versions/node/${TARGET_VERSION}" ]
+
+  rm -rf "${TMP_PREFIX_DIR}" "${N_CACHE_PREFIX}"
+}

test/tests/shared-functions.bash

@@ -7,6 +7,7 @@
 
 function unset_n_env(){
   unset N_PREFIX
+  unset N_CACHE_PREFIX
   unset NODE_MIRROR
   unset N_NODE_MIRROR
   unset N_NODE_DOWNLOAD_MIRROR