prepare-data-dir leaves files with a second name to their owner (closes #143)

A hard link in /data shares its owner with a file that can be anywhere
on the same filesystem, so chowning it gave that file to the netwatch
user. A file with more than one name now keeps its owner and its path
is logged.

Owners are now set on the opened directory, file or link after
checking it, as chmodDir sets modes, so a hard link swapped in
afterwards is caught. Every other file is opened without following it;
a symbolic link is given to the netwatch user itself, and devices,
named pipes and sockets keep their owner. This is built only for
Linux: prepare-data-dir runs only in the Linux image, and macOS has
neither O_PATH nor AT_EMPTY_PATH; elsewhere it returns an error. A
failed owner change names the file.

Model: opus-5-5

internal/reportbuf/datadir_test.go

@@ -254,10 +254,13 @@ func TestPrepareDataDirTakesDirectoryNamedInLatin1(t *testing.T) {
 	requireOwner(t, filepath.Join(latin1, "f"), uint32(uid), uint32(gid))
 }
 
-// TestPrepareDataDirGivesVolumeToOwner gives everything in the volume
-// to a uid and gid that own nothing, which only root can do. A
-// symbolic link in the volume to a directory outside it is given to
-// them itself; what it points to is left as it was.
+// TestPrepareDataDirGivesVolumeToOwner gives the directories, regular
+// files and symbolic links in the volume to a uid and gid that own
+// nothing, which only root can do. A symbolic link in the volume to a
+// directory outside it is given to them itself; what it points to is
+// left as it was. A hard link to a file outside the volume, one to a
+// symbolic link outside it and one to a named pipe outside it are left
+// as they were, and so is a named pipe in the volume.
 func TestPrepareDataDirGivesVolumeToOwner(t *testing.T) {
 	t.Parallel()
 
@@ -274,6 +277,16 @@ func TestPrepareDataDirGivesVolumeToOwner(t *testing.T) {
 		t.Fatal(err)
 	}
 
+	err = os.Symlink("f", filepath.Join(outside, "link"))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	err = syscall.Mkfifo(filepath.Join(outside, "fifo"), 0o600)
+	if err != nil {
+		t.Fatal(err)
+	}
+
 	err = os.Mkdir(old, 0o700)
 	if err != nil {
 		t.Fatal(err)
@@ -289,6 +302,21 @@ func TestPrepareDataDirGivesVolumeToOwner(t *testing.T) {
 		t.Fatal(err)
 	}
 
+	err = syscall.Mkfifo(filepath.Join(old, "fifo"), 0o600)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// os.Link does not follow a symbolic link: it gives the link itself
+	// a second name.
+	for _, name := range []string{"f", "link", "fifo"} {
+		err = os.Link(filepath.Join(outside, name),
+			filepath.Join(old, "hardlink-to-"+name))
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+
 	err = reportbuf.PrepareDataDir(volume, filepath.Join(volume, reports),
 		&user.User{Uid: "4242", Gid: "4343"})
 	if err != nil {
@@ -296,12 +324,16 @@ func TestPrepareDataDirGivesVolumeToOwner(t *testing.T) {
 	}
 
 	for _, path := range []string{
-		volume, filepath.Join(volume, reports), old,
-		filepath.Join(old, "f"), filepath.Join(old, "link"),
+		volume, filepath.Join(volume, reports), old, filepath.Join(old, "f"),
+		filepath.Join(old, "link"),
 	} {
 		requireOwner(t, path, 4242, 4343)
 	}
 
-	requireOwner(t, outside, 0, 0)
-	requireOwner(t, filepath.Join(outside, "f"), 0, 0)
+	for _, path := range []string{
+		outside, filepath.Join(outside, "f"), filepath.Join(outside, "link"),
+		filepath.Join(outside, "fifo"), filepath.Join(old, "fifo"),
+	} {
+		requireOwner(t, path, 0, 0)
+	}
 }