Fix TarWriter.TarEntry exception when adding file whose Unix group owner does not exist in the system

src/libraries/System.Formats.Tar/tests/TarWriter/TarWriter.File.Base.Unix.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Diagnostics;
 using System.IO;
 using Xunit;
 
@@ -51,5 +52,68 @@ protected void VerifyPlatformSpecificMetadata(string filePath, TarEntry entry)
                 }
             }
         }
+
+        protected int CreateGroup(string groupName)
+        {
+            int exitCode = Execute("groupadd", groupName, out string standardOutput, out string standardError);
+            if (exitCode != 0)
+            {
+                ThrowOnError(exitCode, "groupadd", groupName, standardError);
+            }
+            return GetGroupId(groupName);
+        }
+
+        protected int GetGroupId(string groupName)
+        {
+            int exitCode = Execute("getent", $"group {groupName}", out string standardOutput, out string standardError);
+            if (exitCode != 0)
+            {
+                ThrowOnError(exitCode, "getent", "group", standardError);
+            }
+
+            string[] values = standardOutput.Split(':', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries);
+
+            return int.Parse(values[^1]);
+        }
+
+        protected void SetGroupAsOwnerOfFile(string groupName, string filePath)
+        {
+            int exitCode = Execute("chgrp", $"{groupName} {filePath}", out string standardOutput, out string standardError);
+            if (exitCode != 0)
+            {
+                ThrowOnError(exitCode, "chgroup", $"{groupName} {filePath}", standardError);
+            }
+        }
+
+        protected void DeleteGroup(string groupName)
+        {
+            int exitCode = Execute("groupdel", groupName, out string standardOutput, out string standardError);
+            if (exitCode != 0)
+            {
+                ThrowOnError(exitCode, "groupdel", groupName, standardError);
+            }
+        }
+
+        private int Execute(string command, string arguments, out string standardOutput, out string standardError)
+        {
+            using Process p = new Process();
+
+            p.StartInfo.UseShellExecute = false;
+            p.StartInfo.FileName = command;
+            p.StartInfo.Arguments = arguments;
+            p.StartInfo.RedirectStandardOutput = true;
+            p.StartInfo.RedirectStandardError = true;
+            p.Start();
+            p.WaitForExit();
+
+            standardOutput = p.StandardOutput.ReadToEnd();
+            standardError = p.StandardError.ReadToEnd();
+            return p.ExitCode;
+        }
+
+        private void ThrowOnError(int code, string command, string arguments, string message)
+        {
+            throw new IOException($"Error '{code}' when executing '{command} {arguments}'. Message: {message}");
+        }
     }
 }

src/libraries/System.Formats.Tar/tests/TarWriter/TarWriter.WriteEntry.File.Tests.Unix.cs

@@ -139,5 +139,40 @@ public void Add_CharacterDevice(TarEntryFormat format)
 
             }, format.ToString(), new RemoteInvokeOptions { RunAsSudo = true }).Dispose();
         }
+
+        [ConditionalFact(nameof(IsRemoteExecutorSupportedAndPrivilegedProcess))]
+        public void CreateEntryFromFileOwnedByNonExistentGroup()
+        {
+            RemoteExecutor.Invoke(() =>
+            {
+                string groupName = Path.GetRandomFileName()[0..6];
+                int groupId = CreateGroup(groupName);
+
+                using TempDirectory root = new TempDirectory();
+
+                string fileName = "file.txt";
+                string filePath = Path.Join(root.Path, fileName);
+                File.Create(filePath).Dispose();
+
+                SetGroupAsOwnerOfFile(groupName, filePath);
+
+                DeleteGroup(groupName);
+
+                using MemoryStream archive = new MemoryStream();
+                using (TarWriter writer = new TarWriter(archive, TarEntryFormat.Ustar, leaveOpen: true))
+                {
+                    writer.WriteEntry(filePath, fileName); // Should not throw
+                }
+                archive.Seek(0, SeekOrigin.Begin);
+
+                using (TarReader reader = new TarReader(archive, leaveOpen: false))
+                {
+                    UstarTarEntry entry = reader.GetNextEntry() as UstarTarEntry;
+                    Assert.NotNull(entry);
+                    Assert.Equal(entry.GroupName, string.Empty);
+                    Assert.Equal(groupId, entry.Gid);
+                }
+            }, new RemoteInvokeOptions { RunAsSudo = true }).Dispose();
+        }
     }
 }

src/libraries/System.Formats.Tar/tests/TarWriter/TarWriter.WriteEntryAsync.File.Tests.Unix.cs

@@ -149,5 +149,40 @@ public void Add_CharacterDevice_Async(TarEntryFormat format)
                 }
             }, format.ToString(), new RemoteInvokeOptions { RunAsSudo = true }).Dispose();
         }
+
+        [ConditionalFact(nameof(IsRemoteExecutorSupportedAndPrivilegedProcess))]
+        public void CreateEntryFromFileOwnedByNonExistentGroup_Async()
+        {
+            RemoteExecutor.Invoke(async () =>
+            {
+                string groupName = Path.GetRandomFileName()[0..6];
+                int groupId = CreateGroup(groupName);
+
+                using TempDirectory root = new TempDirectory();
+
+                string fileName = "file.txt";
+                string filePath = Path.Join(root.Path, fileName);
+                File.Create(filePath).Dispose();
+
+                SetGroupAsOwnerOfFile(groupName, filePath);
+
+                DeleteGroup(groupName);
+
+                await using MemoryStream archive = new MemoryStream();
+                await using (TarWriter writer = new TarWriter(archive, TarEntryFormat.Ustar, leaveOpen: true))
+                {
+                    await writer.WriteEntryAsync(filePath, fileName); // Should not throw
+                }
+                archive.Seek(0, SeekOrigin.Begin);
+
+                await using (TarReader reader = new TarReader(archive, leaveOpen: false))
+                {
+                    UstarTarEntry entry = await reader.GetNextEntryAsync() as UstarTarEntry;
+                    Assert.NotNull(entry);
+                    Assert.Equal(entry.GroupName, string.Empty);
+                    Assert.Equal(groupId, entry.Gid);
+                }
+            }, new RemoteInvokeOptions { RunAsSudo = true }).Dispose();
+        }
     }
 }