factor out npm-install-checks

This removes the following checks from install.js, which are run
during installation:

- checkEngine
- checkPlatform
- checkCycle
- checkGit

lib/install.js

@@ -73,6 +73,7 @@ var npm = require("./npm.js")
   , lifecycle = require("./utils/lifecycle.js")
   , archy = require("archy")
   , isGitUrl = require("./utils/is-git-url.js")
+  , npmInstallChecks = require("npm-install-checks")
 
 function install (args, cb_) {
   var hasArguments = !!args.length
@@ -842,12 +843,16 @@ function installOne_ (target, where, context, cb) {
   }
   installOnesInProgress[target.name].push(where)
   var indexOfIOIP = installOnesInProgress[target.name].length - 1
+    , force = npm.config.get("force")
+    , nodeVersion = npm.config.get("node-version")
+    , strict = npm.config.get("engine-strict")
+    , c = npmInstallChecks
 
   chain
-    ( [ [checkEngine, target]
-      , [checkPlatform, target]
-      , [checkCycle, target, context.ancestors]
-      , [checkGit, targetFolder]
+    ( [ [c.checkEngine, target, npm.version, nodeVersion, force, strict]
+      , [c.checkPlatform, target, force]
+      , [c.checkCycle, target, context.ancestors]
+      , [c.checkGit, targetFolder]
       , [write, target, targetFolder, context] ]
     , function (er, d) {
         installOnesInProgress[target.name].splice(indexOfIOIP, 1)
@@ -860,146 +865,6 @@ function installOne_ (target, where, context, cb) {
     )
 }
 
-function checkEngine (target, cb) {
-  var npmv = npm.version
-    , force = npm.config.get("force")
-    , nodev = force ? null : npm.config.get("node-version")
-    , strict = npm.config.get("engine-strict") || target.engineStrict
-    , eng = target.engines
-  if (!eng) return cb()
-  if (nodev && eng.node && !semver.satisfies(nodev, eng.node)
-      || eng.npm && !semver.satisfies(npmv, eng.npm)) {
-    if (strict) {
-      var er = new Error("Unsupported")
-      er.code = "ENOTSUP"
-      er.required = eng
-      er.pkgid = target._id
-      return cb(er)
-    } else {
-      log.warn( "engine", "%s: wanted: %j (current: %j)"
-              , target._id, eng, {node: nodev, npm: npm.version} )
-    }
-  }
-  return cb()
-}
-
-function checkPlatform (target, cb) {
-  var platform = process.platform
-    , arch = process.arch
-    , osOk = true
-    , cpuOk = true
-    , force = npm.config.get("force")
-
-  if (force) {
-    return cb()
-  }
-
-  if (target.os) {
-    osOk = checkList(platform, target.os)
-  }
-  if (target.cpu) {
-    cpuOk = checkList(arch, target.cpu)
-  }
-  if (!osOk || !cpuOk) {
-    var er = new Error("Unsupported")
-    er.code = "EBADPLATFORM"
-    er.os = target.os || ['any']
-    er.cpu = target.cpu || ['any']
-    er.pkgid = target._id
-    return cb(er)
-  }
-  return cb()
-}
-
-function checkList (value, list) {
-  var tmp
-    , match = false
-    , blc = 0
-  if (typeof list === "string") {
-    list = [list]
-  }
-  if (list.length === 1 && list[0] === "any") {
-    return true
-  }
-  for (var i = 0; i < list.length; ++i) {
-    tmp = list[i]
-    if (tmp[0] === '!') {
-      tmp = tmp.slice(1)
-      if (tmp === value) {
-        return false
-      }
-      ++blc
-    } else {
-      match = match || tmp === value
-    }
-  }
-  return match || blc === list.length
-}
-
-function checkCycle (target, ancestors, cb) {
-  // there are some very rare and pathological edge-cases where
-  // a cycle can cause npm to try to install a never-ending tree
-  // of stuff.
-  // Simplest:
-  //
-  // A -> B -> A' -> B' -> A -> B -> A' -> B' -> A -> ...
-  //
-  // Solution: Simply flat-out refuse to install any name@version
-  // that is already in the prototype tree of the ancestors object.
-  // A more correct, but more complex, solution would be to symlink
-  // the deeper thing into the new location.
-  // Will do that if anyone whines about this irl.
-  //
-  // Note: `npm install foo` inside of the `foo` package will abort
-  // earlier if `--force` is not set.  However, if it IS set, then
-  // we need to still fail here, but just skip the first level. Of
-  // course, it'll still fail eventually if it's a true cycle, and
-  // leave things in an undefined state, but that's what is to be
-  // expected when `--force` is used.  That is why getPrototypeOf
-  // is used *twice* here: to skip the first level of repetition.
-
-  var p = Object.getPrototypeOf(Object.getPrototypeOf(ancestors))
-    , name = target.name
-    , version = target.version
-  while (p && p !== Object.prototype && p[name] !== version) {
-    p = Object.getPrototypeOf(p)
-  }
-  if (p[name] !== version) return cb()
-
-  var er = new Error("Unresolvable cycle detected")
-  var tree = [target._id, JSON.parse(JSON.stringify(ancestors))]
-    , t = Object.getPrototypeOf(ancestors)
-  while (t && t !== Object.prototype) {
-    if (t === p) t.THIS_IS_P = true
-    tree.push(JSON.parse(JSON.stringify(t)))
-    t = Object.getPrototypeOf(t)
-  }
-  log.verbose("unresolvable dependency tree", tree)
-  er.pkgid = target._id
-  er.code = "ECYCLE"
-  return cb(er)
-}
-
-function checkGit (folder, cb) {
-  // if it's a git repo then don't touch it!
-  fs.lstat(folder, function (er, s) {
-    if (er || !s.isDirectory()) return cb()
-    else checkGit_(folder, cb)
-  })
-}
-
-function checkGit_ (folder, cb) {
-  fs.stat(path.resolve(folder, ".git"), function (er, s) {
-    if (!er && s.isDirectory()) {
-      var e = new Error("Appears to be a git repo or submodule.")
-      e.path = folder
-      e.code = "EISGIT"
-      return cb(e)
-    }
-    cb()
-  })
-}
-
 function write (target, targetFolder, context, cb_) {
   var up = npm.config.get("unsafe-perm")
     , user = up ? null : npm.config.get("user")