Added a test to prevent a regression of the issue seen in #1874. This issue was fixed in PR#2240

MediaBrowser.sln

@@ -1,4 +1,4 @@
-Microsoft Visual Studio Solution File, Format Version 12.00
+Microsoft Visual Studio Solution File, Format Version 12.00
 # Visual Studio 15
 VisualStudioVersion = 15.0.26730.3
 MinimumVisualStudioVersion = 10.0.40219.1
@@ -58,6 +58,8 @@ Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Jellyfin.Naming.Tests", "te
 EndProject
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Jellyfin.Api.Tests", "tests\Jellyfin.Api.Tests\Jellyfin.Api.Tests.csproj", "{A2FD0A10-8F62-4F9D-B171-FFDF9F0AFA9D}"
 EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Emby.Server.Implementations.Tests", "tests\Emby.Server.Implementations.Tests\Emby.Server.Implementations.Tests.csproj", "{2E3A1B4B-4225-4AAA-8B29-0181A84E7AEE}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -164,6 +166,10 @@ Global
 		{A2FD0A10-8F62-4F9D-B171-FFDF9F0AFA9D}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{A2FD0A10-8F62-4F9D-B171-FFDF9F0AFA9D}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{A2FD0A10-8F62-4F9D-B171-FFDF9F0AFA9D}.Release|Any CPU.Build.0 = Release|Any CPU
+		{2E3A1B4B-4225-4AAA-8B29-0181A84E7AEE}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{2E3A1B4B-4225-4AAA-8B29-0181A84E7AEE}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{2E3A1B4B-4225-4AAA-8B29-0181A84E7AEE}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{2E3A1B4B-4225-4AAA-8B29-0181A84E7AEE}.Release|Any CPU.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE
@@ -194,5 +200,6 @@ Global
 		{28464062-0939-4AA7-9F7B-24DDDA61A7C0} = {FBBB5129-006E-4AD7-BAD5-8B7CA1D10ED6}
 		{3998657B-1CCC-49DD-A19F-275DC8495F57} = {FBBB5129-006E-4AD7-BAD5-8B7CA1D10ED6}
 		{A2FD0A10-8F62-4F9D-B171-FFDF9F0AFA9D} = {FBBB5129-006E-4AD7-BAD5-8B7CA1D10ED6}
+		{2E3A1B4B-4225-4AAA-8B29-0181A84E7AEE} = {FBBB5129-006E-4AD7-BAD5-8B7CA1D10ED6}
 	EndGlobalSection
 EndGlobal

tests/Emby.Server.Implementations.Tests/Emby.Server.Implementations.Tests.csproj

@@ -0,0 +1,23 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <TargetFramework>netcoreapp3.1</TargetFramework>
+
+        <IsPackable>false</IsPackable>
+    </PropertyGroup>
+
+    <ItemGroup>
+        <PackageReference Include="AutoFixture" Version="4.11.0" />
+        <PackageReference Include="AutoFixture.AutoMoq" Version="4.11.0" />
+        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.2.0" />
+        <PackageReference Include="Moq" Version="4.13.1" />
+        <PackageReference Include="xunit" Version="2.4.0" />
+        <PackageReference Include="xunit.runner.visualstudio" Version="2.4.0" />
+        <PackageReference Include="coverlet.collector" Version="1.0.1" />
+    </ItemGroup>
+
+    <ItemGroup>
+      <ProjectReference Include="..\..\Emby.Server.Implementations\Emby.Server.Implementations.csproj" />
+    </ItemGroup>
+
+</Project>

tests/Emby.Server.Implementations.Tests/IO/ManagedFileSystemTests.cs

@@ -0,0 +1,31 @@
+using AutoFixture;
+using AutoFixture.AutoMoq;
+using Emby.Server.Implementations.IO;
+using Xunit;
+
+namespace Emby.Server.Implementations.Tests.IO
+{
+    public class ManagedFileSystemTests
+    {
+        private readonly IFixture _fixture;
+        private readonly ManagedFileSystem _sut;
+
+        public ManagedFileSystemTests()
+        {
+            _fixture = new Fixture().Customize(new AutoMoqCustomization { ConfigureMembers = true });
+            _sut = _fixture.Create<ManagedFileSystem>();
+        }
+
+        [Theory]
+        [InlineData("/Volumes/Library/Sample/Music/Playlists/", "../Beethoven/Misc/Moonlight Sonata.mp3", "/Volumes/Library/Sample/Music/Beethoven/Misc/Moonlight Sonata.mp3")]
+        [InlineData("/Volumes/Library/Sample/Music/Playlists/", "../../Beethoven/Misc/Moonlight Sonata.mp3", "/Volumes/Library/Sample/Beethoven/Misc/Moonlight Sonata.mp3")]
+        public void MakeAbsolutePathCorrectlyHandlesRelativeFilePaths(
+            string folderPath,
+            string filePath,
+            string expectedAbsolutePath)
+        {
+            var generatedPath = _sut.MakeAbsolutePath(folderPath, filePath);
+            Assert.Equal(expectedAbsolutePath, generatedPath);
+        }
+    }
+}