#9298: Add debugging messages and explanations to the code

lib/vagrant/util/platform.rb

@@ -1,6 +1,8 @@
 require "rbconfig"
 require "shellwords"
+require "tempfile"
 require "tmpdir"
+require "log4r"
 
 require "vagrant/util/subprocess"
 require "vagrant/util/powershell"
@@ -300,30 +302,57 @@ module Vagrant
           @_wsl_rootfs = nil
 
           if wsl?
+            # Mark our filesystem with a temporary file having an unique name.
+            marker = Tempfile.new(Time.now.to_i.to_s)
+            logger = Log4r::Logger.new("vagrant::util::platfowm::wsl")
+
+            logger.debug("Querying installed WSL from Windows registry.")
+
             PowerShell.execute_cmd('(Get-ChildItem HKCU:\Software\Microsoft\Windows\CurrentVersion\Lxss | ForEach-Object {Get-ItemProperty $_.PSPath}).BasePath').split(" ").each do |path|
               # Lowercase the drive letter, skip the next symbol (which is a
               # colon from a Windows path) and convert path to UNIX style.
-              path = "/mnt/" + path[0, 1].downcase + path[2..-1].tr('\\', '/') + "/rootfs"
+              path = "/mnt/#{path[0, 1].downcase}#{path[2..-1].tr('\\', '/')}/rootfs"
 
               begin
-                fs = Dir.open(path)
-              rescue Errno::EACCES
+                # https://blogs.msdn.microsoft.com/wsl/2016/06/15/wsl-file-system-support
+                logger.debug("Checking whether the \"#{path}\" is a root VolFS mount of current WSL instance.")
                 # Current WSL instance doesn't have an access to its mount from
                 # within itself despite all others are available. That's the
                 # hacky way we're using to determine current instance.
+                # For example we have three WSL instances:
+                # A -> C:\User\USER\AppData\Local|Packages\A\LocalState\rootfs
+                # B -> C:\User\USER\AppData\Local|Packages\B\LocalState\rootfs
+                # C -> C:\User\USER\AppData\Local|Packages\C\LocalState\rootfs
+                # If we're in "A" WSL at the moment, then its path will not be
+                # accessible since it's mounted for exactly the instance we're
+                # in. All others can be opened.
+                Dir.open(path) do |fs|
+                  # A fallback for a case if our trick will stop working. For
+                  # that we've created a temporary file with an unique name in
+                  # a current WSL and now seeking it among all WSL.
+                  @_wsl_rootfs = path if File.exist?("#{fs.path}/#{marker.path}")
+                end
+              rescue Errno::EACCES
                 @_wsl_rootfs = path
                 # You can create and simultaneously run multiple WSL instances,
                 # comment out the "break", run this script within each one and
                 # it'll return only single value.
                 break
-              else
-                fs.close
+              rescue Errno::ENOENT
+                # Warn about data discrepancy between Winreg and file system
+                # states. For the sake of justice, it's worth mentioning that
+                # it is possible only when someone will manually break WSL by
+                # removing a directory of its base path (kinda "stupid WSL
+                # uninstallation by removing hidden and system directory").
+                logger.warn("Windows registry has an information about WSL instance with the \"#{path}\" base path that is no longer exist or broken.")
               end
+              # All other exceptions have to be raised since they will mean
+              # something unpredictably terrible.
             end
 
-            if @_wsl_rootfs.nil?
-              raise Vagrant::Errors::WSLRootFsNotFoundError
-            end
+            marker.close!
+
+            raise Vagrant::Errors::WSLRootFsNotFoundError if @_wsl_rootfs.nil?
           end
 
           @_wsl_rootfs