From 26cafe3e8066d9ae65230ff0aa17a5533cfa92f8 Mon Sep 17 00:00:00 2001
From: Ophestra <cat@gensokyo.uk>
Date: Mon, 25 Aug 2025 21:31:45 +0900
Subject: [PATCH] hst/fs: implement link fstype

Symlinks do not require special treatment, and doing this allows placing links in order.

Signed-off-by: Ophestra <cat@gensokyo.uk>
---
 hst/fs.go          |  9 +++++++
 hst/fs_test.go     | 10 ++++++++
 hst/fslink.go      | 58 ++++++++++++++++++++++++++++++++++++++++++++++
 hst/fslink_test.go | 49 +++++++++++++++++++++++++++++++++++++++
 4 files changed, 126 insertions(+)
 create mode 100644 hst/fslink.go
 create mode 100644 hst/fslink_test.go

diff --git a/hst/fs.go b/hst/fs.go
index a2a75a7..1d33095 100644
--- a/hst/fs.go
+++ b/hst/fs.go
@@ -95,6 +95,12 @@ func (f *FilesystemConfigJSON) MarshalJSON() ([]byte, error) {
 			*FSOverlay
 		}{fsType{FilesystemOverlay}, cv}
 
+	case *FSLink:
+		v = &struct {
+			fsType
+			*FSLink
+		}{fsType{FilesystemLink}, cv}
+
 	default:
 		return nil, FSImplError{f.FilesystemConfig}
 	}
@@ -120,6 +126,9 @@ func (f *FilesystemConfigJSON) UnmarshalJSON(data []byte) error {
 	case FilesystemOverlay:
 		*f = FilesystemConfigJSON{new(FSOverlay)}
 
+	case FilesystemLink:
+		*f = FilesystemConfigJSON{new(FSLink)}
+
 	default:
 		return FSTypeError(t.Type)
 	}
diff --git a/hst/fs_test.go b/hst/fs_test.go
index af8738f..00186d2 100644
--- a/hst/fs_test.go
+++ b/hst/fs_test.go
@@ -70,6 +70,16 @@ func TestFilesystemConfigJSON(t *testing.T) {
 		}, nil,
 			`{"type":"overlay","dst":"/nix/store","lower":["/mnt-root/nix/.ro-store"],"upper":"/mnt-root/nix/.rw-store/upper","work":"/mnt-root/nix/.rw-store/work"}`,
 			`{"fs":{"type":"overlay","dst":"/nix/store","lower":["/mnt-root/nix/.ro-store"],"upper":"/mnt-root/nix/.rw-store/upper","work":"/mnt-root/nix/.rw-store/work"},"magic":3236757504}`},
+
+		{"link", hst.FilesystemConfigJSON{
+			FilesystemConfig: &hst.FSLink{
+				Target:      m("/run/current-system"),
+				Linkname:    "/run/current-system",
+				Dereference: true,
+			},
+		}, nil,
+			`{"type":"link","dst":"/run/current-system","linkname":"/run/current-system","dereference":true}`,
+			`{"fs":{"type":"link","dst":"/run/current-system","linkname":"/run/current-system","dereference":true},"magic":3236757504}`},
 	}
 
 	for _, tc := range testCases {
diff --git a/hst/fslink.go b/hst/fslink.go
new file mode 100644
index 0000000..8bba9b4
--- /dev/null
+++ b/hst/fslink.go
@@ -0,0 +1,58 @@
+package hst
+
+import (
+	"encoding/gob"
+	"path"
+
+	"hakurei.app/container"
+)
+
+func init() { gob.Register(new(FSLink)) }
+
+// FilesystemLink is the type string of a symbolic link.
+const FilesystemLink = "link"
+
+// FSLink represents a symlink in the container filesystem.
+type FSLink struct {
+	// link path in container
+	Target *container.Absolute `json:"dst"`
+	// linkname the symlink points to
+	Linkname string `json:"linkname"`
+	// whether to dereference linkname before creating the link
+	Dereference bool `json:"dereference,omitempty"`
+}
+
+func (l *FSLink) Valid() bool {
+	if l == nil || l.Target == nil || l.Linkname == "" {
+		return false
+	}
+	return !l.Dereference || path.IsAbs(l.Linkname)
+}
+
+func (l *FSLink) Path() *container.Absolute {
+	if !l.Valid() {
+		return nil
+	}
+	return l.Target
+}
+
+func (l *FSLink) Host() []*container.Absolute { return nil }
+
+func (l *FSLink) Apply(z *ApplyState) {
+	if !l.Valid() {
+		return
+	}
+	z.Link(l.Target, l.Linkname, l.Dereference)
+}
+
+func (l *FSLink) String() string {
+	if !l.Valid() {
+		return "<invalid>"
+	}
+
+	dereference := ""
+	if l.Dereference {
+		dereference = "*"
+	}
+	return "&" + l.Target.String() + ":" + dereference + l.Linkname
+}
diff --git a/hst/fslink_test.go b/hst/fslink_test.go
new file mode 100644
index 0000000..c858354
--- /dev/null
+++ b/hst/fslink_test.go
@@ -0,0 +1,49 @@
+package hst_test
+
+import (
+	"testing"
+
+	"hakurei.app/container"
+	"hakurei.app/hst"
+)
+
+func TestFSLink(t *testing.T) {
+	checkFs(t, []fsTestCase{
+		{"nil", (*hst.FSLink)(nil), false, nil, nil, nil, "<invalid>"},
+		{"zero", new(hst.FSLink), false, nil, nil, nil, "<invalid>"},
+
+		{"deref rel", &hst.FSLink{Target: m("/"), Linkname: ":3", Dereference: true}, false, nil, nil, nil, "<invalid>"},
+		{"deref", &hst.FSLink{
+			Target:      m("/run/current-system"),
+			Linkname:    "/run/current-system",
+			Dereference: true,
+		}, true, container.Ops{
+			&container.SymlinkOp{
+				Target:      m("/run/current-system"),
+				LinkName:    "/run/current-system",
+				Dereference: true,
+			},
+		}, m("/run/current-system"), nil,
+			"&/run/current-system:*/run/current-system"},
+
+		{"direct", &hst.FSLink{
+			Target:   m("/etc/mtab"),
+			Linkname: "/proc/mounts",
+		}, true, container.Ops{
+			&container.SymlinkOp{
+				Target:   m("/etc/mtab"),
+				LinkName: "/proc/mounts",
+			},
+		}, m("/etc/mtab"), nil, "&/etc/mtab:/proc/mounts"},
+
+		{"direct rel", &hst.FSLink{
+			Target:   m("/etc/mtab"),
+			Linkname: "../proc/mounts",
+		}, true, container.Ops{
+			&container.SymlinkOp{
+				Target:   m("/etc/mtab"),
+				LinkName: "../proc/mounts",
+			},
+		}, m("/etc/mtab"), nil, "&/etc/mtab:../proc/mounts"},
+	})
+}