tailfs: initial implementation

Add a WebDAV-based folder sharing mechanism that is exposed to local clients at
100.100.100.100:8080 and to remote peers via a new peerapi endpoint at
/v0/tailfs.

Add the ability to manage folder sharing via the new 'share' CLI sub-command.

Updates tailscale/corp#16827

Signed-off-by: Percy Wegmann <percy@tailscale.com>

tailfs/compositefs/openfile.go

@@ -0,0 +1,65 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package compositefs
+
+import (
+	"context"
+	"io/fs"
+	"os"
+
+	"github.com/tailscale/xnet/webdav"
+	"tailscale.com/tailfs/shared"
+)
+
+// OpenFile implements interface webdav.Filesystem.
+func (cfs *CompositeFileSystem) OpenFile(ctx context.Context, name string, flag int, perm os.FileMode) (webdav.File, error) {
+	if !shared.IsRoot(name) {
+		pathInfo, err := cfs.pathInfoFor(name)
+		if err != nil {
+			return nil, err
+		}
+
+		if pathInfo.refersToChild {
+			// this is the child itself, ask it to open its root
+			return pathInfo.child.FS.OpenFile(ctx, "/", flag, perm)
+		}
+
+		return pathInfo.child.FS.OpenFile(ctx, pathInfo.pathOnChild, flag, perm)
+	}
+
+	// the root directory contains one directory for each child
+	di, err := cfs.Stat(ctx, name)
+	if err != nil {
+		return nil, err
+	}
+
+	return &shared.DirFile{
+		Info: di,
+		LoadChildren: func() ([]fs.FileInfo, error) {
+			cfs.childrenMu.Lock()
+			children := cfs.children
+			cfs.childrenMu.Unlock()
+
+			childInfos := make([]fs.FileInfo, 0, len(cfs.children))
+			for _, c := range children {
+				if c.isAvailable() {
+					var childInfo fs.FileInfo
+					if cfs.statChildren {
+						fi, err := c.FS.Stat(ctx, "/")
+						if err != nil {
+							return nil, err
+						}
+						// we use the full name
+						childInfo = shared.RenamedFileInfo(ctx, c.Name, fi)
+					} else {
+						// always use now() as the modified time to bust caches
+						childInfo = shared.ReadOnlyDirInfo(c.Name, cfs.now())
+					}
+					childInfos = append(childInfos, childInfo)
+				}
+			}
+			return childInfos, nil
+		},
+	}, nil
+}