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 needs Linux, the
image's system; elsewhere it returns an error. A failed owner change
names the file.

Model: opus-5-5

internal/reportbuf/datadir_test.go

@@ -277,6 +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)
@@ -292,38 +302,19 @@ func TestPrepareDataDirGivesVolumeToOwner(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	err = os.Link(filepath.Join(outside, "f"), filepath.Join(old, "hardlink"))
+	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.
-	err = os.Symlink("f", filepath.Join(outside, "link"))
+	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 = os.Link(filepath.Join(outside, "link"),
-		filepath.Join(old, "hardlink-to-link"))
-	if err != nil {
-		t.Fatal(err)
-	}
-
-	err = syscall.Mkfifo(filepath.Join(outside, "fifo"), 0o600)
-	if err != nil {
-		t.Fatal(err)
-	}
-
-	err = os.Link(filepath.Join(outside, "fifo"),
-		filepath.Join(old, "hardlink-to-fifo"))
-	if err != nil {
-		t.Fatal(err)
-	}
-
-	err = syscall.Mkfifo(filepath.Join(old, "fifo"), 0o600)
-	if err != nil {
-		t.Fatal(err)
 	}
 
 	err = reportbuf.PrepareDataDir(volume, filepath.Join(volume, reports),