scan: skip OS default excludes, and record nothing on filesystems that are not persistent (closes #124)

scan never records or reads anything on a filesystem that is not
persistent (tmpfs, proc, sysfs and the like), however it reaches it; no
flag turns this off. It notes each such filesystem's device ID as it
starts: on Linux from /proc/self/mountinfo, on macOS by lstat of each
listed mount point. Below an operand the walk also skips the default
excludes, a path list per system (Linux, macOS, and Windows, not used
yet); --no-default-excludes turns off only those. A default exclude that
is itself an operand gets no skip line. The tests stop at once, saying
why, when TMPDIR is not on a persistent filesystem.

Judgement call: an unreadable mount list fails the scan, and no flag gets round it.
Judgement call: a file on such a filesystem below an operand gets the same line as a directory.
Judgement call: such an operand that another operand's walk also reaches is named twice.
Unverified: whether macOS's mount list gives the device lstat reports, so macOS keeps lstat, and a hidden mount there can still add another filesystem's device.
Unverified: the macOS code is never built or run here; bind mounts need root to test.

Model: opus-5-5

main_test.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"context"
 	"errors"
+	"fmt"
 	"io"
 	"io/fs"
 	"os"
@@ -18,6 +19,36 @@ import (
 // error prints and a runtime failure must not.
 const usageMarker = "Usage:"
 
+// TestMain stops the tests at once, saying why, when their temporary
+// directory is not on a persistent filesystem: scan records nothing
+// there (README "Rules for the walk"), so most scan tests would fail
+// without saying why.
+func TestMain(m *testing.M) {
+	nonPersistent, err := nonPersistentFilesystems()
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "list the mounted filesystems: %v\n", err)
+		os.Exit(1)
+	}
+
+	dir := os.TempDir()
+
+	fi, err := os.Stat(dir)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+
+	dev, ok := deviceOfInfo(fi)
+	if fsType, found := nonPersistent[dev]; ok && found {
+		fmt.Fprintf(os.Stderr, "%s is on %s, not a persistent filesystem, "+
+			"and scan records nothing there: set TMPDIR to a directory on a "+
+			"persistent filesystem\n", dir, fsType)
+		os.Exit(1)
+	}
+
+	os.Exit(m.Run())
+}
+
 // walSuffixes are the SQLite sidecar files a WAL-mode database keeps
 // while it is open. A clean close checkpoints the WAL and removes
 // both; finding either afterwards means the database was never closed.
@@ -535,7 +566,7 @@ func TestRunNamesDefaultDatabase(t *testing.T) {
 	}{
 		cmdScan: {
 			run: func() error {
-				return runScan(t.Context(), []string{missing}, 1, false)
+				return runScan(t.Context(), []string{missing}, 1, false, false)
 			},
 			want: scanWant,
 		},