fix: treat top-level global packages as "top" nodes

When working with the global install space, such as
/usr/local/lib/node_modules, we create a "project root" at the parent of
the node_modules directory, and then more or less proceed normally, with
the globalStyle install strategy.

The main difference, of course, is that in such a case, we should not
ever place *peers* of a top-level global dependency in that same global
space.  This is part of the root cause of npm/cli#3457, where a peer
dependency of the globally installed package was placed as a top-level
globally installed package, instead of being nested under its dependent.

Note that this _also_ could result in an `ERESOLVE` if multiple
globally-installed packages depended upon conflicting versions of a peer
dep, or worse yet, quietly replacing that peerDep in such a way that it
breaks a previously-installed globally installed package.

With this change, peer dependencies of a global top-level package will
always be nested under their dependent, removing any chance that they
might collide with anything else in the top level of the global install
space.

Fix: npm/cli#3457

lib/arborist/build-ideal-tree.js

@@ -1025,6 +1025,15 @@ This is a one-time fix-up, please be patient...
     // keep track of the thing that caused this node to be included.
     const src = parent.sourceReference
     this[_peerSetSource].set(node, src)
+
+    // do not load the peers along with the set if this is a global top pkg
+    // otherwise we'll be tempted to put peers as other top-level installed
+    // things, potentially clobbering what's there already, which is not
+    // what we want.  the missing edges will be picked up on the next pass.
+    if (this[_global] && edge.from.isProjectRoot)
+      return node
+
+    // otherwise, we have to make sure that our peers can go along with us.
     return this[_loadPeerSet](node, required)
   }
 

lib/deepest-nesting-target.js

@@ -5,7 +5,7 @@
 const deepestNestingTarget = (start, name) => {
   for (const target of start.ancestry()) {
     // note: this will skip past the first target if edge is peer
-    if (target.isProjectRoot || !target.resolveParent)
+    if (target.isProjectRoot || !target.resolveParent || target.globalTop)
       return target
     const targetEdge = target.edgesOut.get(name)
     if (!targetEdge || !targetEdge.peer)

lib/node.js

@@ -249,7 +249,7 @@ class Node {
 
   // true for packages installed directly in the global node_modules folder
   get globalTop () {
-    return this.global && this.parent.isProjectRoot
+    return this.global && this.parent && this.parent.isProjectRoot
   }
 
   get workspaces () {
@@ -479,6 +479,9 @@ class Node {
   }
 
   get isProjectRoot () {
+    // only treat as project root if it's the actual link that is the root,
+    // or the target of the root link, but NOT if it's another link to the
+    // same root that happens to be somewhere else.
     return this === this.root || this === this.root.target
   }
 
@@ -773,9 +776,15 @@ class Node {
     this[_loadDepType](this.package.dependencies, 'prod')
     this[_loadDepType](this.package.optionalDependencies, 'optional')
 
-    const { isTop, path, sourceReference } = this
-    const { isTop: srcTop, path: srcPath } = sourceReference || {}
-    if (isTop && path && (!sourceReference || srcTop && srcPath))
+    const { globalTop, isTop, path, sourceReference } = this
+    const {
+      globalTop: srcGlobalTop,
+      isTop: srcTop,
+      path: srcPath,
+    } = sourceReference || {}
+    const thisDev = isTop && !globalTop && path
+    const srcDev = !sourceReference || srcTop && !srcGlobalTop && srcPath
+    if (thisDev && srcDev)
       this[_loadDepType](this.package.devDependencies, 'dev')
   }
 
@@ -1223,7 +1232,7 @@ class Node {
   }
 
   get isTop () {
-    return !this.parent
+    return !this.parent || this.globalTop
   }
 
   get top () {