Fix potential erroneous load of assembly

This change makes the assembly resolve handler be more strict when
checking whether or not to load the YamlDotNet assembly.

This should prevent accidentally loading the YamlDotNet assembly as
a result of a request coming from another assembly that is looking
to load a different assembly.

Signed-off-by: Gabriel Adrian Samfira <gsamfira@cloudbasesolutions.com>

powershell-yaml.psm1

@@ -34,33 +34,40 @@ function Invoke-LoadFile {
         [string]$assemblyPath
     )
 
-    $global:powershellYamlDotNetAssemblyPath = Join-Path $assemblyPath "YamlDotNet.dll"
+    $powershellYamlDotNetAssemblyPath = Join-Path $assemblyPath "YamlDotNet.dll"
     $serializerAssemblyPath = Join-Path $assemblyPath "PowerShellYamlSerializer.dll"
     $yamlAssembly = [Reflection.Assembly]::LoadFile($powershellYamlDotNetAssemblyPath)
     $serializerAssembly = [Reflection.Assembly]::LoadFile($serializerAssemblyPath)
 
     if ($PSVersionTable['PSEdition'] -eq 'Core') {
         # Register the AssemblyResolve event to load dependencies manually. This seems to be needed only on
         # PowerShell Core.
-        [System.AppDomain]::CurrentDomain.add_AssemblyResolve({
+        $resolver = {
             param ($snd, $e)
+            # This event only needs to run once when the Invoke-LoadFile function is called.
+            # If it's called again, the variables defined in this functions will not be available,
+            # so we can safely ignore the event.
+            if (-not $serializerAssemblyPath -or -not $powershellYamlDotNetAssemblyPath) {
+                return $null
+            }
             # Load YamlDotNet if it's requested by PowerShellYamlSerializer. Ignore other requests as they might
             # originate from other assemblies that are not part of this module and which might have different
             # versions of the module that they need to load.
-            if ($e.Name -like "*YamlDotNet*" -and $e.RequestingAssembly -like "*PowerShellYamlSerializer*" ) {
+            if ($e.Name -match "^YamlDotNet,*" -and $e.RequestingAssembly.Location -eq  $serializerAssemblyPath) {
                 return [System.Reflection.Assembly]::LoadFile($powershellYamlDotNetAssemblyPath)
             }
 
             return $null
-        })
+        }
+        [System.AppDomain]::CurrentDomain.add_AssemblyResolve($resolver)
         # Load the StringQuotingEmitter from PowerShellYamlSerializer to force the resolver handler to fire once.
-        # This will load the YamlDotNet assembly and expand the global variable $powershellYamlDotNetAssemblyPath.
-        # We then remove it to avoid polluting the global scope.
         # This is an ugly hack I am not happy with.
         $serializerAssembly.GetType("StringQuotingEmitter") | Out-Null
+
+        # Remove the resolver handler after it has been used.
+        [System.AppDomain]::CurrentDomain.remove_AssemblyResolve($resolver)
     }
 
-    Remove-Variable -Name powershellYamlDotNetAssemblyPath -Scope Global
     return @{ "yaml"= $yamlAssembly; "quoted" = $serializerAssembly }
 }