Patch the file a symlinked source names

src/DiffEngine.Tests/InlineApplierUnixTests.cs

@@ -0,0 +1,80 @@
+#if NET10_0
+/// <summary>
+/// What the applier does to the file itself, rather than to the text in it. The patch is written
+/// through a temporary and swapped in, and on Linux and macOS that swap is a rename - so what
+/// survives it is the temporary, with the temporary's identity.
+/// </summary>
+public class InlineApplierUnixTests :
+    IDisposable
+{
+    const string source = "class C\n{\n    void M() => Verify(value).Snapshot(\"old\");\n}";
+
+    /// <summary>
+    /// A source file reached through a symlink - a worktree, a vendored copy, a checkout shared
+    /// between two trees. The rename replaced the link with a regular file: the link stopped being
+    /// one, and the file it pointed at still held the old literal, so the next run reported the
+    /// same snapshot again and the patched copy was invisible to the compiler.
+    /// </summary>
+    [Test]
+    // A symlink on Windows needs elevation or developer mode, so this cannot be arranged there.
+    [RunOn(TUnit.Core.Enums.OS.Linux | TUnit.Core.Enums.OS.MacOs)]
+    public async Task A_symlinked_source_is_followed_to_the_file_it_names()
+    {
+        var real = Path.Combine(directory, "Real.cs");
+        File.WriteAllText(real, source);
+        var link = Path.Combine(directory, "Link.cs");
+        File.CreateSymbolicLink(link, real);
+
+        var result = InlineApplier.Apply(Patch(link));
+
+        await Assert.That(result.Status).IsEqualTo(InlineApplyStatus.Applied);
+        await Assert.That(File.ReadAllText(real)).Contains("\"new\"");
+        // Still a link, rather than a regular file holding the patch while the real one holds the
+        // snapshot that failed
+        await Assert.That(new FileInfo(link).LinkTarget).IsNotNull();
+    }
+
+    /// <summary>
+    /// The temporary is created with this process's umask, so without carrying the mode across, a
+    /// source file that was executable - or group writable, or read only to the world - came back
+    /// as whatever the umask said.
+    /// </summary>
+    [Test]
+    // A Unix file mode is not a thing Windows has.
+    [RunOn(TUnit.Core.Enums.OS.Linux | TUnit.Core.Enums.OS.MacOs)]
+    public async Task The_file_keeps_the_permissions_it_had()
+    {
+        var path = Path.Combine(directory, "Sample.cs");
+        File.WriteAllText(path, source);
+        const UnixFileMode mode = UnixFileMode.UserRead | UnixFileMode.UserWrite | UnixFileMode.UserExecute |
+                                  UnixFileMode.GroupRead | UnixFileMode.GroupExecute |
+                                  UnixFileMode.OtherRead | UnixFileMode.OtherExecute;
+        File.SetUnixFileMode(path, mode);
+
+        var result = InlineApplier.Apply(Patch(path));
+
+        await Assert.That(result.Status).IsEqualTo(InlineApplyStatus.Applied);
+        await Assert.That(File.GetUnixFileMode(path)).IsEqualTo(mode);
+    }
+
+    /// <summary>
+    /// Nothing here queues a patch, so none of them has a reviewable identity.
+    /// </summary>
+    static InlinePatch Patch(string sourceFile) =>
+        new(sourceFile, 3, "\"old\"", "new")
+        {
+            TestName = null
+        };
+
+    public InlineApplierUnixTests()
+    {
+        directory = Path.Combine(Path.GetTempPath(), $"InlineApplierUnixTests_{Guid.NewGuid():N}");
+        Directory.CreateDirectory(directory);
+    }
+
+    public void Dispose() =>
+        Directory.Delete(directory, true);
+
+    readonly string directory;
+}
+#endif

src/DiffEngine/Inline/InlineApplier.cs

@@ -1,4 +1,4 @@
-namespace DiffEngine;
+namespace DiffEngine;
 
 /// <summary>
 /// Applies an <see cref="InlinePatch"/> to a source file, preserving the file's
@@ -56,6 +56,10 @@ static InlineApplyResult Run(InlinePatch patch, bool write)
             return InlineApplyResult.Failed($"Invalid InlinePatch.SourceFile: {patch.SourceFile}", exception);
         }
 
+        // Followed before anything else, so the lock, the mutex, the read and the swap all name
+        // the file that actually holds the source
+        fullPath = ResolveLink(fullPath);
+
         var newContent = SourceLanguage.NormalizeNewlines(patch.NewContent);
         var normalizedPath = fullPath.ToLowerInvariant();
         lock (gates.GetOrAdd(normalizedPath, static _ => new()))
@@ -199,6 +203,65 @@ static InlineApplyResult LockedApply(string fullPath, InlinePatch patch, string
     /// framework this targets.
     /// </para>
     /// </summary>
+    /// <summary>
+    /// The file a symlinked source points at, which is the file to patch.
+    /// <para>
+    /// The whole file is rewritten through a temporary and swapped in, and on Linux and macOS that
+    /// swap is a rename: it replaces the link itself with a regular file, leaving the target still
+    /// holding the old literal and the link no longer a link. Following it first puts the patch on
+    /// the real file, and gives two links to one file the same lock into the bargain.
+    /// </para>
+    /// <para>
+    /// The final target rather than one hop, since a chain has the same problem, and the path as
+    /// it stands when nothing resolves: a broken link is a file that cannot be read, which the
+    /// read reports better than this could.
+    /// </para>
+    /// </summary>
+    static string ResolveLink(string path)
+    {
+#if NET6_0_OR_GREATER
+        try
+        {
+            return File.ResolveLinkTarget(path, true)?.FullName ?? path;
+        }
+        catch (Exception exception)
+            when (exception is IOException or UnauthorizedAccessException)
+        {
+            return path;
+        }
+#else
+        return path;
+#endif
+    }
+
+    /// <summary>
+    /// The destination's Unix permissions onto the temporary, because the swap is a rename and the
+    /// file that survives it is the temporary - created with this process's umask. A source file
+    /// that was executable, or group writable, or anything else out of the ordinary, came back as
+    /// whatever the umask happened to say. Windows keeps the destination's ACLs across a Replace,
+    /// so there is nothing to carry there.
+    /// </summary>
+    static void CopyMode(string destination, string temporary)
+    {
+#if NET7_0_OR_GREATER
+        if (OperatingSystem.IsWindows())
+        {
+            return;
+        }
+
+        try
+        {
+            File.SetUnixFileMode(temporary, File.GetUnixFileMode(destination));
+        }
+        catch (Exception exception)
+            when (exception is IOException or UnauthorizedAccessException)
+        {
+            // Best effort. The content is the point, and a mode that could not be read or set is
+            // not worth failing a patch that otherwise applied.
+        }
+#endif
+    }
+
     static void WriteThroughTemporary(string fullPath, byte[] output)
     {
         var directory = Path.GetDirectoryName(fullPath)!;
@@ -208,6 +271,7 @@ static void WriteThroughTemporary(string fullPath, byte[] output)
         try
         {
             File.WriteAllBytes(temporary, output);
+            CopyMode(fullPath, temporary);
             File.Replace(temporary, fullPath, null);
         }
         finally