lib/config: Add file inside folder marker directory (#9525)

### Purpose

Avoid the issue where the folder marker is deleted by overzealous
cleanup tools because it's just a useless, empty directory.

We create a small file containing a an admonishment to not delete the
directory, and some metadata that is just for human consumption at the
moment. (But it would parse as a valid yaml file if we wanted to read
this, at some point.)

This will only apply when _creating_ a folder marker, that is, existing
setups will not gain the file automatically. Obviously, when using a
custom folder marker none of this applies.

Also, slightly adjust the permission bits for the folder marker directory and file on Unixes, making sure the group & write bits are unset.

### Testing

I've created and deleted a few folders and it appears to behave as I
expect.

### Screenshots

```
jb@ok:~/somefolder % ls -la
total 0
drwxr-xr-x   3 jb  staff   96 May  1 08:52 ./
drwx------  12 jb  staff  384 May  1 08:52 ../
drwxr-xr-x   3 jb  staff   96 May  1 08:52 .stfolder/
jb@ok:~/somefolder % ls -l .stfolder
total 8
-rw-r--r--  1 jb  staff  122 May  1 08:52 syncthing-folder-39a4b0.txt
jb@ok:~/somefolder % cat .stfolder/syncthing-folder-39a4b0.txt
# This directory is a Syncthing folder marker.
# Do not delete.

folderID: xtdca-cudyf
created: 2024-05-01T08:52:49+02:00
```

2 files changed, 38 insertions, 12 deletions

diff --git a/lib/config/folderconfiguration.go b/lib/config/folderconfiguration.go
index 3079c0f4cf..8765fc4e4e 100644
--- a/lib/config/folderconfiguration.go
+++ b/lib/config/folderconfiguration.go
@@ -7,6 +7,8 @@
 package config
 
 import (
+	"bytes"
+	"crypto/sha256"
 	"errors"
 	"fmt"
 	"path"
@@ -90,27 +92,51 @@ func (f *FolderConfiguration) CreateMarker() error {
 		return nil
 	}
 
-	permBits := fs.FileMode(0o777)
-	if build.IsWindows {
-		// Windows has no umask so we must chose a safer set of bits to
-		// begin with.
-		permBits = 0o700
-	}
-	fs := f.Filesystem(nil)
-	err := fs.Mkdir(DefaultMarkerName, permBits)
+	ffs := f.Filesystem(nil)
+
+	// Create the marker as a directory
+	err := ffs.Mkdir(DefaultMarkerName, 0o755)
 	if err != nil {
 		return err
 	}
-	if dir, err := fs.Open("."); err != nil {
+
+	// Create a file inside it, reducing the risk of the marker directory
+	// being removed by automated cleanup tools.
+	markerFile := filepath.Join(DefaultMarkerName, f.markerFilename())
+	if err := fs.WriteFile(ffs, markerFile, f.markerContents(), 0o644); err != nil {
+		return err
+	}
+
+	// Sync & hide the containing directory
+	if dir, err := ffs.Open("."); err != nil {
 		l.Debugln("folder marker: open . failed:", err)
 	} else if err := dir.Sync(); err != nil {
 		l.Debugln("folder marker: fsync . failed:", err)
 	}
-	fs.Hide(DefaultMarkerName)
+	ffs.Hide(DefaultMarkerName)
 
 	return nil
 }
 
+func (f *FolderConfiguration) RemoveMarker() error {
+	ffs := f.Filesystem(nil)
+	_ = ffs.Remove(filepath.Join(DefaultMarkerName, f.markerFilename()))
+	return ffs.Remove(DefaultMarkerName)
+}
+
+func (f *FolderConfiguration) markerFilename() string {
+	h := sha256.Sum256([]byte(f.ID))
+	return fmt.Sprintf("syncthing-folder-%x.txt", h[:3])
+}
+
+func (f *FolderConfiguration) markerContents() []byte {
+	var buf bytes.Buffer
+	buf.WriteString("# This directory is a Syncthing folder marker.\n# Do not delete.\n\n")
+	fmt.Fprintf(&buf, "folderID: %s\n", f.ID)
+	fmt.Fprintf(&buf, "created: %s\n", time.Now().Format(time.RFC3339))
+	return buf.Bytes()
+}
+
 // CheckPath returns nil if the folder root exists and contains the marker file
 func (f *FolderConfiguration) CheckPath() error {
 	return f.checkFilesystemPath(f.Filesystem(nil), ".")
diff --git a/lib/model/model.go b/lib/model/model.go
index 487dff7dca..f88118eb9c 100644
--- a/lib/model/model.go
+++ b/lib/model/model.go
@@ -464,9 +464,9 @@ func (m *model) removeFolder(cfg config.FolderConfiguration) {
 	if isPathUnique {
 		// Remove (if empty and removable) or move away (if non-empty or
 		// otherwise not removable) Syncthing-specific marker files.
-		fs := cfg.Filesystem(nil)
-		if err := fs.Remove(config.DefaultMarkerName); err != nil {
+		if err := cfg.RemoveMarker(); err != nil && !errors.Is(err, os.ErrNotExist) {
 			moved := config.DefaultMarkerName + time.Now().Format(".removed-20060102-150405")
+			fs := cfg.Filesystem(nil)
 			_ = fs.Rename(config.DefaultMarkerName, moved)
 		}
 	}