use the idealtree target path if it exists

This ensures relpaths are from the correct root

PR-URL: #224
Credit: @nlf
Close: #224
Reviewed-by: @wraithgar, @ruyadorno

lib/arborist/build-ideal-tree.js

@@ -491,7 +491,8 @@ module.exports = cls => class IdealTreeBuilder extends cls {
     /* istanbul ignore else - should also be covered by realpath failure */
     if (filepath) {
       const { name } = spec
-      spec = npa(`file:${relpath(this.path, filepath)}`, this.path)
+      const tree = this.idealTree.target || this.idealTree
+      spec = npa(`file:${relpath(tree.path, filepath)}`, tree.path)
       spec.name = name
     }
     return spec

tap-snapshots/test-arborist-build-ideal-tree.js-TAP.test.js

@@ -315,6 +315,46 @@ ArboristNode {
 }
 `
 
+exports[`test/arborist/build-ideal-tree.js TAP adding tarball to global prefix that is a symlink at a different path depth > must match snapshot 1`] = `
+ArboristLink {
+  "location": "../another-path/global-root",
+  "name": "global-root",
+  "path": "{CWD}/test/arborist/build-ideal-tree-adding-tarball-to-global-prefix-that-is-a-symlink-at-a-different-path-depth/another-path/global-root",
+  "realpath": "{CWD}/test/arborist/build-ideal-tree-adding-tarball-to-global-prefix-that-is-a-symlink-at-a-different-path-depth/real-root",
+  "resolved": "file:../real-root",
+  "target": ArboristNode {
+    "children": Map {
+      "mkdirp" => ArboristNode {
+        "edgesIn": Set {
+          EdgeIn {
+            "from": "",
+            "name": "mkdirp",
+            "spec": "file:../../../fixtures/registry-mocks/content/mkdirp/-/mkdirp-1.0.2.tgz",
+            "type": "prod",
+          },
+        },
+        "location": "node_modules/mkdirp",
+        "name": "mkdirp",
+        "path": "{CWD}/test/arborist/build-ideal-tree-adding-tarball-to-global-prefix-that-is-a-symlink-at-a-different-path-depth/real-root/node_modules/mkdirp",
+        "resolved": "file:{CWD}/test/fixtures/registry-mocks/content/mkdirp/-/mkdirp-1.0.2.tgz",
+        "version": "1.0.2",
+      },
+    },
+    "edgesOut": Map {
+      "mkdirp" => EdgeOut {
+        "name": "mkdirp",
+        "spec": "file:../../../fixtures/registry-mocks/content/mkdirp/-/mkdirp-1.0.2.tgz",
+        "to": "node_modules/mkdirp",
+        "type": "prod",
+      },
+    },
+    "location": "",
+    "name": "real-root",
+    "path": "{CWD}/test/arborist/build-ideal-tree-adding-tarball-to-global-prefix-that-is-a-symlink-at-a-different-path-depth/real-root",
+  },
+}
+`
+
 exports[`test/arborist/build-ideal-tree.js TAP allow updating when peer outside of explicit update set conflict, but resolves appropriately with --force > succeed if force applied 1`] = `
 ArboristNode {
   "children": Map {

test/arborist/build-ideal-tree.js

@@ -1,4 +1,4 @@
-const {basename, resolve} = require('path')
+const {basename, resolve, relative} = require('path')
 const t = require('tap')
 const Arborist = require('../..')
 const fixtures = resolve(__dirname, '../fixtures')
@@ -1236,6 +1236,32 @@ t.test('workspaces', t => {
   t.end()
 })
 
+t.test('adding tarball to global prefix that is a symlink at a different path depth', async t => {
+  const fixt = t.testdir({
+    'real-root': {},
+    'another-path': {
+      'global-root': t.fixture('symlink', '../real-root'),
+    },
+  })
+  const path = resolve(fixt, 'another-path/global-root')
+  const arb = new Arborist({
+    path,
+    global: true,
+    ...OPT,
+  })
+
+  const tarballpath = resolve(__dirname, '../fixtures/registry-mocks/content/mkdirp/-/mkdirp-1.0.2.tgz')
+  const tree = await arb.buildIdealTree({
+    path,
+    global: true,
+    add: [
+      // this will be a relative path to the tarball above
+      relative(process.cwd(), tarballpath),
+    ],
+  })
+  t.matchSnapshot(printTree(tree))
+})
+
 t.test('add symlink that points to a symlink', t => {
   const fixt = t.testdir({
     'global-prefix': {