Add support for repository url relative URLs in readme

This allows permalinks at pack time by turning non-absolute URLs to repository-relative blob+commit URLs.

We don't use the branch/tag since that isn't necessarily permanent. Since the repository metadata for the package is immutable, we map the URL to an equally immutable full path URL.

Fixes #641

Author: kzu

src/NuGetizer.Tasks/CreatePackage.cs

@@ -44,6 +44,7 @@ public class CreatePackage : Task
         Manifest manifest;
         Dictionary<string, string> tokens;
         Regex tokensExpr;
+        Regex linkExpr;
 
         public override bool Execute()
         {
@@ -230,6 +231,26 @@ void GeneratePackage(Stream output = null)
             {
                 // replace readme with includes replaced.
                 var replaced = ReplaceTokens(IncludesResolver.Process(readmeFile.Source, message => Log.LogWarningCode("NG001", message)));
+
+                if (manifest.Metadata.Repository?.Type == "git" &&
+                    !string.IsNullOrEmpty(manifest.Metadata.Repository?.Commit) &&
+                    Uri.TryCreate(manifest.Metadata.Repository.Url, UriKind.Absolute, out var uri) &&
+                    uri.Host.EndsWith("github.com"))
+                {
+                    // expr to match markdown links. use named groups to capture the link text and url.
+                    linkExpr ??= new Regex(@"\[(?<text>[^\]]+)\]\((?<url>[^)]+)\)", RegexOptions.None);
+                    var repoUrl = manifest.Metadata.Repository.Url.TrimEnd('/');
+                    replaced = linkExpr.Replace(replaced, match =>
+                    {
+                        var url = match.Groups["url"].Value;
+                        if (Uri.IsWellFormedUriString(url, UriKind.Absolute))
+                            return match.Value;
+
+                        var newUrl = $"{repoUrl}/blob/{manifest.Metadata.Repository.Commit}/{url.TrimStart('/')}";
+                        return $"[{match.Groups["text"].Value}]({newUrl})";
+                    });
+                }
+
                 if (!replaced.Equals(File.ReadAllText(readmeFile.Source), StringComparison.Ordinal))
                 {
                     var temp = Path.GetTempFileName();

src/NuGetizer.Tests/CreatePackageTests.cs

@@ -300,6 +300,42 @@ public void when_readme_has_include_and_tokens_then_replacements_applied()
             Assert.Contains("NuGetizer", readme);
         }
 
+        [Fact]
+        public void when_readme_has_relativeurl_then_expands_github_url()
+        {
+            var content = Path.GetTempFileName();
+            File.WriteAllText(content, "See [license](license.txt).");
+            task.Contents = new[]
+            {
+                new TaskItem(content, new Metadata
+                {
+                    { MetadataName.PackageId, task.Manifest.GetMetadata("Id") },
+                    { MetadataName.PackFolder, PackFolderKind.None },
+                    { MetadataName.PackagePath, "readme.md" }
+                }),
+            };
+
+            task.Manifest.SetMetadata("Readme", "readme.md");
+            task.Manifest.SetMetadata("RepositoryType", "git");
+            task.Manifest.SetMetadata("RepositoryUrl", "https://github.com/devlooped/nugetizer");
+            task.Manifest.SetMetadata("RepositorySha", "9dc2cb5de");
+
+            createPackage = true;
+            ExecuteTask(out var manifest);
+
+            Assert.NotNull(manifest);
+
+            Assert.Equal("readme.md", manifest.Metadata.Readme);
+
+            var file = manifest.Files.FirstOrDefault(f => Path.GetFileName(f.Target) == manifest.Metadata.Readme);
+            Assert.NotNull(file);
+            Assert.True(File.Exists(file.Source));
+
+            var readme = File.ReadAllText(file.Source);
+
+            Assert.Contains("[license](https://github.com/devlooped/nugetizer/blob/9dc2cb5de/license.txt)", readme);
+        }
+
         [Fact]
         public void when_creating_package_with_simple_dependency_then_contains_dependency_group()
         {