ConfigurationBuilder.AddJsonFile can't fire OnChange event using relative paths (#111101)

Fixes #71386

---------

Co-authored-by: Jeff Handley <[email protected]>

Author: pedrobsaila

src/libraries/Microsoft.Extensions.Configuration/tests/FunctionalTests/ConfigurationTests.cs

@@ -1017,5 +1017,31 @@ private sealed class MyOptions
 
             public string XmlKey1 { get; set; }
         }
+
+        private async Task WatchOverConfigJsonFileAndUpdateIt(string filePath)
+        {
+            var builder = new ConfigurationBuilder().AddJsonFile(filePath, true, true).Build();
+            bool reloaded = false;
+            ChangeToken.OnChange(builder.GetReloadToken, () =>
+            {
+                reloaded = true;
+            });
+            File.WriteAllText(filePath, "{\"Prop2\":\"Value2\"}");
+            await WaitForChange(
+                () => reloaded,
+                "on file change event handler did not get executed");
+        }
+
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsWindows))]
+        public async Task OnChangeGetFiredForRelativeWindowsPath()
+        {
+            await WatchOverConfigJsonFileAndUpdateIt(".\\testFileToReload.json");
+        }
+
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsLinux))]
+        public async Task OnChangeGetFiredForRelativeLinuxPath()
+        {
+            await WatchOverConfigJsonFileAndUpdateIt("./testFileToReload.json");
+        }
     }
 }

src/libraries/Microsoft.Extensions.Configuration/tests/FunctionalTests/Microsoft.Extensions.Configuration.Functional.Tests.csproj

@@ -1,13 +1,18 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
     <TargetFrameworks>$(NetCoreAppCurrent);$(NetFrameworkCurrent)</TargetFrameworks>
     <EnableDefaultItems>true</EnableDefaultItems>
   </PropertyGroup>
 
   <ItemGroup>
-    <Compile Include="$(LibrariesProjectRoot)Microsoft.Extensions.Configuration\tests\Common\ConfigurationProviderExtensions.cs"
-             Link="Common\ConfigurationProviderExtensions.cs" />
+    <Compile Include="$(LibrariesProjectRoot)Microsoft.Extensions.Configuration\tests\Common\ConfigurationProviderExtensions.cs" Link="Common\ConfigurationProviderExtensions.cs" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Content Include="testFileToReload.json">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </Content>
   </ItemGroup>
 
   <ItemGroup>

src/libraries/Microsoft.Extensions.Configuration/tests/FunctionalTests/testFileToReload.json

@@ -0,0 +1,3 @@
+{
+    "Prop1": "Value1"
+}

src/libraries/Microsoft.Extensions.FileProviders.Physical/src/PhysicalFilesWatcher.cs

@@ -165,12 +165,18 @@ private IChangeToken GetOrAddChangeToken(string pattern)
             }
             else
             {
-                changeToken = GetOrAddFilePathChangeToken(pattern);
+                // get rid of \. in Windows and ./ in UNIX's at the start of path file
+                var filePath = RemoveRelativePathSegment(pattern);
+                changeToken = GetOrAddFilePathChangeToken(filePath);
             }
 
             return changeToken;
         }
 
+        private static string RemoveRelativePathSegment(string pattern) =>
+            // The pattern has already been normalized to unix directory separators
+            pattern.StartsWith("./", StringComparison.Ordinal) ? pattern.Substring(2) : pattern;
+
         internal IChangeToken GetOrAddFilePathChangeToken(string filePath)
         {
             if (!_filePathTokenLookup.TryGetValue(filePath, out ChangeTokenInfo tokenInfo))