Name the scan lock file from the database's resolved path (closes #101)

The lock file was the configured database path with .lock appended, so
a scan reaching the database through a symlink to it locked a different
file and ran alongside a scan holding the lock through the real path.
The lock file is now named from the database's path with symlinks
resolved. Before the database exists, its directory is resolved and the
file name joined to it, so the error names the same lock file either
way. testDBPath resolves its temp dir, so lock errors name the expected
path where the temp dir is itself reached through a symlink.

Model: opus-5-5

main_test.go

@@ -854,6 +854,64 @@ func TestRunSecondScanFails(t *testing.T) {
 	}
 }
 
+// assertSecondScanFails holds the lock on the database at path and
+// runs a scan against the database SFDUPES_DATABASE names, another
+// path to the same database. It fails the test unless that scan fails
+// at once, naming the lock file beside path.
+func assertSecondScanFails(t *testing.T, path string) {
+	t.Helper()
+
+	holdScanLock(t, path)
+
+	var stderr bytes.Buffer
+
+	code := run([]string{cmdScan, t.TempDir()}, io.Discard, &stderr)
+	if code != exitFatal {
+		t.Errorf("run(scan) = %d, want %d", code, exitFatal)
+	}
+
+	want := "sfdupes: another scan is running (lock held on " +
+		path + ".lock)\n"
+	if got := stderr.String(); got != want {
+		t.Errorf("stderr = %q, want %q", got, want)
+	}
+}
+
+func TestRunSecondScanThroughSymlinkFails(t *testing.T) {
+	// README §Database: every path to one database names the same lock
+	// file, so while a scan holds it through the database's real path, a
+	// scan through a symlink fails, naming the real path's lock file.
+	t.Run("symlink to the database", func(t *testing.T) {
+		path := testDBPath(t)
+		t.Setenv(databaseEnv, path)
+
+		scanFixture(t)
+
+		link := filepath.Join(t.TempDir(), "link.sqlite")
+
+		err := os.Symlink(path, link)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		t.Setenv(databaseEnv, link)
+		assertSecondScanFails(t, path)
+	})
+
+	t.Run("symlink to its directory, no database yet", func(t *testing.T) {
+		path := testDBPath(t)
+		link := filepath.Join(t.TempDir(), "link")
+
+		err := os.Symlink(filepath.Dir(path), link)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		t.Setenv(databaseEnv, filepath.Join(link, filepath.Base(path)))
+		assertSecondScanFails(t, path)
+	})
+}
+
 func TestRunScanReleasesLock(t *testing.T) {
 	// README §Database: a scan releases the lock however it ends.
 	t.Run("success", func(t *testing.T) {