FunctionalTests: support variable local cache root

We update the CloneWithDefaultLocalCacheLocation() functional
test so it accommodates variable local cache paths on Linux,
depending on whether $XDG_CACHE_HOME is set or not.  This allows
the test to succeed on developer systems where we want to avoid
polluting the default ~/.cache/scalar directory during test runs
and so set $XDG_CACHE_HOME explicitly.

Note that the new ScalarTestConfig.DefaultLocalCacheRoot accessor
returns a path on Windows which should be "C:\.scalarCache",
to match what would be supplied by the TryGetDefaultLocalCacheRoot()
method of the WindowsPlatform if it were passed the default
functional test enlistment root path.  However, this Windows-
specific DefaultLocalCacheRoot value is unused, as the only
functional test which uses it is CloneWithDefaultLocalCacheLocation()
and that test belongs to the POSIXOnly category.

Scalar.FunctionalTests/ScalarTestConfig.cs

@@ -1,4 +1,6 @@
+using System;
 using System.IO;
+using System.Runtime.InteropServices;
 
 namespace Scalar.FunctionalTests
 {
@@ -10,6 +12,40 @@ public static class ScalarTestConfig
 
         public static string LocalCacheRoot { get; set; }
 
+        public static string DefaultLocalCacheRoot {
+            get
+            {
+                string homeDirectory = null;
+                string cachePath = ".scalarCache";
+
+                if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
+                {
+                    homeDirectory = Path.GetPathRoot(Properties.Settings.Default.EnlistmentRoot);
+                }
+                else if (RuntimeInformation.IsOSPlatform(OSPlatform.OSX))
+                {
+                    homeDirectory = Environment.GetEnvironmentVariable("HOME");
+                }
+                else
+                {
+                    // On Linux we use a local cache path per the XDG Base Directory Specification.
+                    homeDirectory = Environment.GetEnvironmentVariable("XDG_CACHE_HOME");
+                    if (!string.IsNullOrEmpty(homeDirectory))
+                    {
+                        cachePath = "scalar";
+                    }
+                    else
+                    {
+                        homeDirectory = Environment.GetEnvironmentVariable("HOME");
+                        cachePath = Path.Combine(".cache", "scalar");
+                    }
+
+                }
+
+                return Path.Combine(homeDirectory, cachePath);
+            }
+        }
+
         public static object[] FileSystemRunners { get; set; }
 
         public static object[] GitRepoTestsValidateWorkTree { get; set; }

Scalar.FunctionalTests/Tests/EnlistmentPerFixture/CloneTests.cs

@@ -61,8 +61,9 @@ public void SparseCloneWithNoFetchOfCommitsAndTreesSucceeds()
         public void CloneWithDefaultLocalCacheLocation()
         {
             FileSystemRunner fileSystem = FileSystemRunner.DefaultRunner;
-            string homeDirectory = Environment.GetEnvironmentVariable("HOME");
-            homeDirectory.ShouldBeADirectory(fileSystem);
+            string defaultLocalCacheRoot = ScalarTestConfig.DefaultLocalCacheRoot;
+            fileSystem.CreateDirectory(defaultLocalCacheRoot);
+            defaultLocalCacheRoot.ShouldBeADirectory(fileSystem);
 
             string newEnlistmentRoot = ScalarFunctionalTestEnlistment.GetUniqueEnlistmentRoot();
 
@@ -80,8 +81,7 @@ public void CloneWithDefaultLocalCacheLocation()
 
             string gitObjectsRoot = ScalarHelpers.GetObjectsRootFromGitConfig(Path.Combine(newEnlistmentRoot, "src"));
 
-            string defaultScalarCacheRoot = Path.Combine(homeDirectory, ".scalarCache");
-            gitObjectsRoot.StartsWith(defaultScalarCacheRoot, StringComparison.Ordinal).ShouldBeTrue($"Git objects root did not default to using {homeDirectory}");
+            gitObjectsRoot.StartsWith(defaultLocalCacheRoot, StringComparison.Ordinal).ShouldBeTrue($"Git objects root did not default to using {defaultLocalCacheRoot}");
 
             RepositoryHelpers.DeleteTestDirectory(newEnlistmentRoot);
         }