Hold a lock so a second scan fails at once (closes #53)

scan takes an exclusive flock(2) on a lock file beside the database
(its path with .lock appended) before it walks anything or opens the
database, and holds it until it returns. A second scan against the
same database fails at once with a one-line error naming the lock
file and exits 1. report and trees never take the lock. The lock ends
with the process, so a fatal error or an interrupt releases it; the
file is never deleted. golang.org/x/sys becomes a direct dependency.

The README smoke test now keeps the database outside the scanned
tree, where its empty lock file would have joined the empty-file
group.

Model: opus-5-5

main_test.go

@@ -412,6 +412,95 @@ func TestRunReportsNeedOnlyReadAccess(t *testing.T) {
 	}
 }
 
+// holdScanLock takes the lock on the database at path, as a running
+// scan does, and holds it until the test ends. It fails the test when
+// the lock is already held.
+func holdScanLock(t *testing.T, path string) {
+	t.Helper()
+
+	lock, err := lockScanDatabase(path)
+	if err != nil {
+		t.Fatalf("lock %s: %v", path, err)
+	}
+
+	t.Cleanup(func() { _ = lock.Close() })
+}
+
+func TestRunSecondScanFails(t *testing.T) {
+	// README §Database: while one scan holds the lock, a second scan
+	// fails at once, naming the lock file, without creating the
+	// database.
+	path := testDBPath(t)
+	t.Setenv(databaseEnv, path)
+
+	holdScanLock(t, path)
+
+	var stdout, stderr bytes.Buffer
+
+	code := run([]string{cmdScan, t.TempDir()}, &stdout, &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)
+	}
+
+	if got := stdout.String(); got != "" {
+		t.Errorf("stdout = %q, want nothing (data only)", got)
+	}
+
+	_, err := os.Stat(path)
+	if !errors.Is(err, fs.ErrNotExist) {
+		t.Errorf("stat %s = %v, want the database not created", path, err)
+	}
+}
+
+func TestRunScanReleasesLock(t *testing.T) {
+	// README §Database: a scan releases the lock however it ends.
+	t.Run("success", func(t *testing.T) {
+		path := testDBPath(t)
+		t.Setenv(databaseEnv, path)
+
+		scanFixture(t)
+		holdScanLock(t, path)
+	})
+
+	t.Run("fatal error", func(t *testing.T) {
+		path := brokenDatabase(t)
+		t.Setenv(databaseEnv, path)
+
+		code := run([]string{cmdScan, t.TempDir()}, io.Discard, io.Discard)
+		if code != exitFatal {
+			t.Fatalf("run(scan) = %d, want %d", code, exitFatal)
+		}
+
+		holdScanLock(t, path)
+	})
+}
+
+func TestRunReportsDuringScan(t *testing.T) {
+	// README §Database: report and trees never take the lock, so they
+	// run while a scan holds it.
+	path := testDBPath(t)
+	t.Setenv(databaseEnv, path)
+
+	scanFixture(t)
+	holdScanLock(t, path)
+
+	for _, name := range []string{cmdReport, cmdTrees} {
+		var stderr bytes.Buffer
+
+		code := run([]string{name}, io.Discard, &stderr)
+		if code != exitOK {
+			t.Errorf("run(%s) = %d, want %d; stderr: %s",
+				name, code, exitOK, stderr.String())
+		}
+	}
+}
+
 func TestRunStdoutClosedIsFatal(t *testing.T) {
 	// README §Error handling: a stdout write failure exits 1, reported
 	// in one line on stderr.