mesh-catalog/modules/mounts/cmd/mounts/data.go

package main

// What the module may not mount over (the review of 2026-10-08, findings 4a–4d): the mesh's own data on
// this machine, a mount point that something else uses, a folder that holds files, and a share this
// machine serves itself. And what the bundle leaves for the process so that it can tell: the paths the
// mesh keeps data in on this machine, from the controller's `data` verb over the bus.

import (
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"os"
	"path/filepath"
	"strings"
)

// DataFile is what the bundle left from mesh-controller.data for this machine.
const DataFile = BusDir + "/data.json"

// DataPaths is where the mesh keeps data on this machine, as its controller says.
type DataPaths struct {
	Machine string   `json:"machine"`
	Asked   string   `json:"asked"`
	Paths   []string `json:"paths"`
	// Error is why the last ask got no answer; the paths are then the last answer's, or none.
	Error    string `json:"error,omitempty"`
	Answered string `json:"answered,omitempty"`
}

// Want is one share this machine asks of one server, put on the bus so that the server exports the share
// to this machine and to nobody who does not ask (finding 5). The server decides the access it gives; this
// is only what is asked.
type Want struct {
	Server string `json:"server"`
	Share  string `json:"share"`
	Access string `json:"access"`
}

// Wants are what a config asks of its server, by share name.
func (c Config) Wants() []Want {
	out := []Want{}
	server := c.ServerNode()
	if server == "" {
		return out
	}
	for _, w := range c.Shares {
		access := "rw"
		if w.ReadOnly {
			access = "ro"
		}
		out = append(out, Want{Server: server, Share: w.Name, Access: access})
	}
	sortWants(out)
	return out
}

func sortWants(w []Want) {
	for i := 1; i < len(w); i++ {
		for j := i; j > 0 && w[j].Share < w[j-1].Share; j-- {
			w[j], w[j-1] = w[j-1], w[j]
		}
	}
}

// DataFrom reads the paths out of mesh-controller.data's answer: `answer.data`, `data`, or the same as
// text in `output`. An answer with no data list is an error, never an empty list: an empty list would
// let every mount point through.
func DataFrom(raw json.RawMessage) ([]string, error) {
	type item struct {
		Path string `json:"path"`
	}
	var whole struct {
		Answer *struct {
			Data *[]item `json:"data"`
		} `json:"answer"`
		Data   *[]item `json:"data"`
		Output string  `json:"output"`
	}
	if err := json.Unmarshal(raw, &whole); err != nil {
		return nil, fmt.Errorf("the controller's data answer is not JSON: %v", err)
	}
	var items *[]item
	switch {
	case whole.Answer != nil && whole.Answer.Data != nil:
		items = whole.Answer.Data
	case whole.Data != nil:
		items = whole.Data
	case whole.Output != "":
		var inner struct {
			Data *[]item `json:"data"`
		}
		if err := json.Unmarshal([]byte(whole.Output), &inner); err == nil && inner.Data != nil {
			items = inner.Data
		}
	}
	if items == nil {
		return nil, errors.New("the controller's data answer names no data list")
	}
	paths := []string{}
	for _, it := range *items {
		if strings.HasPrefix(it.Path, "/") {
			paths = append(paths, filepath.Clean(it.Path))
		}
	}
	return paths, nil
}

// nearData refuses a mount point at, under or above a path the mesh keeps data in here: at or under, the
// mount would hide the data; above, it would hide it too, and everything beside it.
func nearData(point string, paths []string) error {
	for _, d := range paths {
		switch {
		case point == d:
			return fmt.Errorf("the mount point %s is at %s, where the mesh keeps data on this machine; it is never mounted over", point, d)
		case strings.HasPrefix(point, d+"/"):
			return fmt.Errorf("the mount point %s is under %s, where the mesh keeps data on this machine; it is never mounted over", point, d)
		case strings.HasPrefix(d, point+"/"):
			return fmt.Errorf("the mount point %s is above %s, where the mesh keeps data on this machine; mounting there would hide it", point, d)
		}
	}
	return nil
}

// sourceIs says whether what is mounted at a point is a pair's own source. For a network share the source
// is compared as the kernel writes it; for a disk, the device its by-uuid link resolves to.
func (m *Machine) sourceIs(kind, what string, mt Mounted) bool {
	switch kind {
	case "nfs":
		return strings.HasPrefix(mt.Type, "nfs") && strings.TrimSuffix(mt.Source, "/") == strings.TrimSuffix(what, "/")
	case "smb":
		return (mt.Type == "cifs" || mt.Type == "smb3") && strings.EqualFold(strings.TrimSuffix(mt.Source, "/"), strings.TrimSuffix(what, "/"))
	case "disk":
		dev, err := filepath.EvalSymlinks(filepath.Join(m.Root, what))
		if err != nil {
			return false
		}
		src, err := filepath.EvalSymlinks(filepath.Join(m.Root, mt.Source))
		return err == nil && src == dev
	}
	return false
}

// kindOf is a unit's kind from what it mounts.
func kindOf(what, fstype string) string {
	switch {
	case strings.HasPrefix(fstype, "nfs"):
		return "nfs"
	case fstype == "cifs" || strings.HasPrefix(what, "//"):
		return "smb"
	case strings.HasPrefix(what, "/dev/"):
		return "disk"
	}
	return ""
}

// notOwnSource says why a mounted filesystem at a point is not the source a unit of the module mounts
// there, or nothing when it is, or nothing is mounted there but an automount.
func (m *Machine) notOwnSource(kind, what, point string, mounted map[string]Mounted) error {
	mt, there := mounted[point]
	if !there || mt.IsAutomount() {
		return nil
	}
	if kind != "" && m.sourceIs(kind, what, mt) {
		return nil
	}
	return fmt.Errorf("%s is mounted at %s (%s), which is not what the module mounts there (%s)", mt.Source, point, mt.Type, what)
}

// mountpointFree refuses a mount point another mount uses, or a folder that holds files. A point in
// mountinfo is never opened, so a network server that is away cannot hold the look.
func (m *Machine) mountpointFree(p Pair, own map[string]bool, mounted map[string]Mounted) error {
	ours := own[p.MountUnit] && own[p.AutoUnit]
	if mt, there := mounted[p.Mountpoint]; there {
		if mt.IsAutomount() {
			if ours {
				return nil
			}
			return fmt.Errorf("%s is already an automount that is not the module's (%s)", p.Mountpoint, mt.Source)
		}
		if ours && m.sourceIs(p.Kind, p.What, mt) {
			return nil
		}
		return fmt.Errorf("%s is already mounted: %s (%s) is there, so this share is not mounted over it", p.Mountpoint, mt.Source, mt.Type)
	}
	dir, err := os.Open(filepath.Join(m.Root, p.Mountpoint))
	if err != nil {
		return nil // missing: it is made
	}
	defer dir.Close()
	info, err := dir.Stat()
	if err != nil {
		return fmt.Errorf("%s cannot be looked at: %v", p.Mountpoint, err)
	}
	if !info.IsDir() {
		return fmt.Errorf("%s is a file, not a folder", p.Mountpoint)
	}
	if names, err := dir.Readdirnames(1); err == nil && len(names) > 0 {
		return fmt.Errorf("%s is not empty (it holds %s); a mount would hide what is in it, so it is not mounted there", p.Mountpoint, names[0])
	} else if err != nil && err != io.EOF {
		return fmt.Errorf("%s cannot be read: %v", p.Mountpoint, err)
	}
	return nil
}

// unitField reads one Key= of a unit file.
func unitField(text, key string) string {
	for _, line := range strings.Split(text, "\n") {
		if v, ok := strings.CutPrefix(strings.TrimSpace(line), key+"="); ok {
			return strings.TrimSpace(v)
		}
	}
	return ""
}