Fix DSC_RESOURCE_PATH env var to also limit exe search

Author: SteveL-MSFT

dsc/tests/dsc_discovery.tests.ps1

@@ -220,4 +220,32 @@ Describe 'tests for resource discovery' {
             $env:DSC_RESOURCE_PATH = $oldPath
         }
     }
+
+    It 'DSC_RESOURCE_PATH should be used for executable lookup' {
+        $dscTest = Get-Command dscecho -ErrorAction Stop
+        $target = if ($IsWindows) {
+            'echoIt.exe'
+        } else {
+            'echoIt'
+        }
+        Copy-Item -Path "$($dscTest.Source)" -Destination "$testdrive\$target"
+        $manifest = Get-Content -Raw -Path "$(Split-Path -Path $dscTest.Source -Parent)\echo.dsc.resource.json" | ConvertFrom-Json
+        $manifest.type = 'Test/MyEcho'
+        $manifest.get.executable = $target
+        $manifest.set = $null
+        $manifest.test = $null
+        $manifest.schema.command.executable = $target
+        Set-Content -Path "$testdrive/test.dsc.resource.json" -Value ($manifest | ConvertTo-Json -Depth 10)
+
+        $oldPath = $env:DSC_RESOURCE_PATH
+        try {
+            $env:DSC_RESOURCE_PATH = $testdrive
+            $out = dsc resource get -r 'Test/MyEcho' -i '{"output":"Custom"}' 2> "$testdrive/error.txt" | ConvertFrom-Json
+            $LASTEXITCODE | Should -Be 0
+            $out.actualState.output | Should -BeExactly 'Custom'
+        }
+        finally {
+            $env:DSC_RESOURCE_PATH = $oldPath
+        }
+    }
 }

dsc_lib/src/discovery/command_discovery.rs

@@ -149,8 +149,16 @@ impl CommandDiscovery {
         let mut uniques: HashSet<PathBuf> = HashSet::new();
         paths.retain(|e|uniques.insert((*e).clone()));
 
-        // if exe home is not already in PATH env var then add it to env var and list of searched paths
-        if !using_custom_path {
+        if using_custom_path {
+            // when using custom path, intent is to isolate the search of manifests and executables to the custom path
+            // so we replace the PATH with the custom path
+            if let Ok(new_path) = env::join_paths(paths.clone()) {
+                env::set_var("PATH", new_path);
+            } else {
+                return Err(DscError::Operation(t!("discovery.commandDiscovery.failedSetEnvPath").to_string()));
+            }
+        } else {
+            // if exe home is not already in PATH env var then add it to env var and list of searched paths
             if let Some(exe_home) = get_exe_path()?.parent() {
                 let exe_home_pb = exe_home.to_path_buf();
                 if paths.contains(&exe_home_pb) {