Allow PATCH method to apply partial updates.

Gated behind the `patch` feature.

src/patch.go

@@ -0,0 +1,128 @@
+package git_pages
+
+import (
+	"archive/tar"
+	"errors"
+	"fmt"
+	"io"
+	"maps"
+	"slices"
+	"strings"
+)
+
+var ErrMalformedPatch = errors.New("malformed patch")
+
+// Mutates `manifest` according to a tar stream and the following rules:
+//   - A character device with major 0 and minor 0 is a "whiteout marker".  When placed
+//     at a given path, this path and its entire subtree (if any) are removed from the manifest.
+//   - When a directory is placed at a given path, this path and its entire subtree (if any) are
+//     removed from the manifest and replaced with the contents of the directory.
+func ApplyTarPatch(manifest *Manifest, reader io.Reader) error {
+	type Node struct {
+		entry    *Entry
+		children map[string]*Node
+	}
+
+	// Extract the manifest contents (which is using a flat hash map) into a directory tree
+	// so that recursive delete operations have O(1) complexity. s
+	var root *Node
+	sortedNames := slices.Sorted(maps.Keys(manifest.GetContents()))
+	for _, name := range sortedNames {
+		entry := manifest.Contents[name]
+		node := &Node{entry: entry}
+		if entry.GetType() == Type_Directory {
+			node.children = map[string]*Node{}
+		}
+		if name == "" {
+			root = node
+		} else {
+			segments := strings.Split(name, "/")
+			fileName := segments[len(segments)-1]
+			iter := root
+			for _, segment := range segments[:len(segments)-1] {
+				if iter.children == nil {
+					panic("malformed manifest")
+				} else if _, exists := iter.children[segment]; !exists {
+					panic("malformed manifest")
+				} else {
+					iter = iter.children[segment]
+				}
+			}
+			iter.children[fileName] = node
+		}
+	}
+	manifest.Contents = map[string]*Entry{}
+
+	// Process the archive as a patch operation.
+	archive := tar.NewReader(reader)
+	for {
+		header, err := archive.Next()
+		if err == io.EOF {
+			break
+		} else if err != nil {
+			return err
+		}
+
+		segments := strings.Split(strings.TrimRight(header.Name, "/"), "/")
+		fileName := segments[len(segments)-1]
+		node := root
+		for index, segment := range segments[:len(segments)-1] {
+			if node.children == nil {
+				dirName := strings.Join(segments[:index], "/")
+				return fmt.Errorf("%w: %s: not a directory", ErrMalformedPatch, dirName)
+			}
+			if _, exists := node.children[segment]; !exists {
+				nodeName := strings.Join(segments[:index+1], "/")
+				return fmt.Errorf("%w: %s: path not found", ErrMalformedPatch, nodeName)
+			} else {
+				node = node.children[segment]
+			}
+		}
+		if node.children == nil {
+			dirName := strings.Join(segments[:len(segments)-1], "/")
+			return fmt.Errorf("%w: %s: not a directory", ErrMalformedPatch, dirName)
+		}
+
+		switch header.Typeflag {
+		case tar.TypeReg:
+			fileData, err := io.ReadAll(archive)
+			if err != nil {
+				return fmt.Errorf("tar: %s: %w", header.Name, err)
+			}
+			node.children[fileName] = &Node{
+				entry: NewManifestEntry(Type_InlineFile, fileData),
+			}
+		case tar.TypeSymlink:
+			node.children[fileName] = &Node{
+				entry: NewManifestEntry(Type_Symlink, []byte(header.Linkname)),
+			}
+		case tar.TypeDir:
+			node.children[fileName] = &Node{
+				entry:    NewManifestEntry(Type_Directory, nil),
+				children: map[string]*Node{},
+			}
+		case tar.TypeChar:
+			if header.Devmajor == 0 && header.Devminor == 0 {
+				delete(node.children, fileName)
+			} else {
+				AddProblem(manifest, header.Name,
+					"tar: unsupported chardev %d,%d", header.Devmajor, header.Devminor)
+			}
+		default:
+			AddProblem(manifest, header.Name,
+				"tar: unsupported type '%c'", header.Typeflag)
+			continue
+		}
+	}
+
+	// Repopulate manifest contents with the updated directory tree.
+	var traverse func([]string, *Node)
+	traverse = func(segments []string, node *Node) {
+		manifest.Contents[strings.Join(segments, "/")] = node.entry
+		for fileName, childNode := range node.children {
+			traverse(append(segments, fileName), childNode)
+		}
+	}
+	traverse([]string{}, root)
+	return nil
+}