Keep a non-root user's database in the XDG state directory (closes #171)

Without SFDUPES_DATABASE, a user other than root now uses
sfdupes/db.sqlite in the XDG state directory (~/.local/state by
default) instead of failing on /var/lib/sfdupes. scan creates the
missing directories there with mode 0700, as the XDG Base Directory
specification asks. An XDG_STATE_HOME that is not an absolute path is
a fatal error for every subcommand. Root keeps
/var/lib/sfdupes/db.sqlite. The help of sfdupes and of scan, and
README, say where the database is. Adds github.com/adrg/xdg, the
decided XDG library.

Model: opus-5-5

README.md

@@ -29,7 +29,6 @@ first look at a new machine with one command.
 
 ```sh
 make build
-export SFDUPES_DATABASE="$HOME/.local/share/sfdupes/db.sqlite"
 ./sfdupes scan /srv
 ./sfdupes report > dupes.tsv
 ./sfdupes trees > dupetrees.tsv
@@ -47,8 +46,10 @@ Both cover the whole database, or only the paths given after them, as in
 `sfdupes report /srv/photos`. `info` prints the database's path, size and record
 count.
 
-The database is `/var/lib/sfdupes/db.sqlite` unless `SFDUPES_DATABASE` names
-another file. The first `scan` creates it.
+Root keeps the database in `/var/lib/sfdupes/db.sqlite`. Any other user keeps it
+in `sfdupes/db.sqlite` in the XDG state directory, `~/.local/state` by default
+on Linux. `SFDUPES_DATABASE` names another file (see "Location"). The first
+`scan` creates the database.
 
 `report`, `trees` and `info` write their data to stdout. Progress, warnings and
 errors go to stderr. stdout and stderr are a program's two output streams. The
@@ -116,15 +117,20 @@ docker run --rm -v /srv:/srv:ro -v /var/lib/sfdupes:/var/lib/sfdupes \
 Run `scan` as root, so that it can read every file. `scan` warns about a
 directory it cannot read, and about a new or changed file it cannot read. It
 skips the path and deletes the records at or below it (see "Rules for the
-walk"). As the file `/etc/cron.d/sfdupes`:
+walk").
+
+An unmounted disk leaves its mount point empty, so `scan` deletes every record
+under it, and the next scan reads every file on the disk again. Test the mount
+point of each disk at or below `/srv` with its own `mountpoint -q`, joined with
+`&&`, and test `/srv` only when it is one. As the file `/etc/cron.d/sfdupes`:
 
 ```
-30 3 * * * root /usr/local/bin/sfdupes scan /srv 2>>/var/log/sfdupes.log || tail -n 3 /var/log/sfdupes.log
+30 3 * * * root if mountpoint -q /srv; then /usr/local/bin/sfdupes scan /srv 2>>/var/log/sfdupes.log || tail -n 3 /var/log/sfdupes.log; else echo "a disk is not mounted; scan of /srv skipped"; fi
 ```
 
 - The first scan creates `/var/lib/sfdupes/db.sqlite` and its directory. To keep
-  the database elsewhere, put `SFDUPES_DATABASE=/path/to/db.sqlite` before the
-  command on the same line.
+  the database elsewhere, put `SFDUPES_DATABASE=/path/to/db.sqlite` directly
+  before `/usr/local/bin/sfdupes`.
 - `scan` writes nothing to stdout. Its stderr, appended here to
   `/var/log/sfdupes.log`, holds:
     - one line naming the database;
@@ -150,8 +156,9 @@ walk"). As the file `/etc/cron.d/sfdupes`:
 - `report`, `trees` and `info` need only read access to the database. The umask
   is the setting that removes permissions from the files a process creates.
   Under the usual umask of `022`, the first scan creates the database readable
-  by every user. An unprivileged user can then run those three commands against
-  root's database.
+  by every user. Another user can then run those three commands against root's
+  database, with `SFDUPES_DATABASE=/var/lib/sfdupes/db.sqlite` before the
+  command.
 
 ### Reading the reports
 
@@ -290,9 +297,9 @@ The reference below is also the program's specification.
 ### Settings
 
 - `SFDUPES_DATABASE` names the database file. When it is unset or empty, the
-  database is `/var/lib/sfdupes/db.sqlite`. There is no command-line flag for
-  it. The path names the file exactly, whatever characters it holds. A relative
-  path is relative to the working directory.
+  database is at its default path (see "Location"). There is no command-line
+  flag for it. The path names the file exactly, whatever characters it holds. A
+  relative path is relative to the working directory.
 - `--workers N` (`scan`, `dwim`) sets how many directories `scan` reads at once
   while it lists the trees. It also sets how many files `scan` reads at once
   while it hashes them. The default is the number of CPUs. `N` must be at
@@ -880,7 +887,8 @@ The summary line on stderr gives:
   operands;
 - the duplicate-tree groups;
 - the dupe trees, which is the number of rows;
-- the reclaimable bytes, the sum of `size` over all rows.
+- the reclaimable bytes, the sum of `size` over all rows, leaving out a row
+  whose `dupe` lies inside another row's `dupe`.
 
 ```
 trees: 14 records read, 1 duplicate tree groups, 1 dupe trees, 3.0 KiB reclaimable
@@ -971,7 +979,22 @@ All five subcommands work on one SQLite database file.
 #### Location
 
 The database is the file named by `SFDUPES_DATABASE`, when that is set and not
-empty, and otherwise `/var/lib/sfdupes/db.sqlite`.
+empty. Otherwise it is at its default path, which depends on the user:
+
+- For root, it is `/var/lib/sfdupes/db.sqlite`.
+- For any other user, it is `sfdupes/db.sqlite` in the XDG state directory. That
+  directory is `$XDG_STATE_HOME` when that is set and not empty. Otherwise it is
+  `~/.local/state` on Linux and `~/Library/Application Support` on macOS. An
+  `XDG_STATE_HOME` that is not an absolute path is a fatal error, exit 1, for
+  every subcommand. The error names `XDG_STATE_HOME`.
+
+`scan` creates each missing directory on the way to the database with mode 755.
+At the default path of a user other than root, it uses mode 700 instead, as the
+XDG Base Directory specification asks. Other users cannot enter a directory with
+mode 700, so they cannot read a database in it.
+
+All five subcommands use the same default, so a user's `report` reads the
+database that user's `scan` wrote.
 
 Each subcommand prints one line on stderr naming the database it uses, before
 anything else and whether or not stderr is a terminal. The line gives the path
@@ -1011,9 +1034,9 @@ Only one `scan` at a time can hold the lock.
 - For its whole run, `scan` holds a lock on a lock file beside the database. The
   lock is taken with the `flock(2)` system call, a request a program makes to
   the kernel, and only one process can hold it. The lock file's name is the
-  database's path, with symlinks resolved, plus `.lock`. By default that is
-  `/var/lib/sfdupes/db.sqlite.lock`. A path through a symlink names the same
-  lock file.
+  database's path, with symlinks resolved, plus `.lock`. For root, by default,
+  that is `/var/lib/sfdupes/db.sqlite.lock`. A path through a symlink names the
+  same lock file.
 - `scan` takes the lock before it walks the filesystem or opens the database.
   Just before that, it creates the database file, empty, if it is missing. So
   the lock file is named the same way before the first scan as after. A symlink
@@ -1235,6 +1258,14 @@ On a terminal:
     ⠹ [3/4] hash: [12345/98765], 92 files/s, elapsed 2m32s, remain 17m44s            12% |█         |
     ```
 
+- During `scan`'s `walk`, the line below the display shows the path of the last
+  file the walk found. Each draw shows the path of that moment, cut short before
+  the terminal's last column. The path is escaped as in the reports (see "Report
+  output format"). Each control character left after that is written as `\x` and
+  its code in two hexadecimal digits, such as `\x1b` for the escape character. A
+  control character is one that a terminal acts on instead of showing. The line
+  is erased when the walk ends, and the next phase is drawn on it.
+
 - No line reaches the terminal's last column. A line too wide for the terminal
   first leaves out its rate, the words of a stalled phase, or what the phase is
   waiting for. It leaves out its bar next. It is cut short at the edge only when

cmd/sfdupes/main.go

@@ -5,8 +5,10 @@
 // KiB, and of the content. The content hash covers the whole file under
 // 100 MiB, and 1 MiB at each gigabyte at 100 MiB or more, so a large
 // file is never read in full. scan maintains a persistent SQLite
-// database of file signatures (SFDUPES_DATABASE, default
-// /var/lib/sfdupes/db.sqlite) that the other subcommands read.
+// database of file signatures that the other subcommands read. The
+// database is SFDUPES_DATABASE, or by default /var/lib/sfdupes/db.sqlite
+// for root and sfdupes/db.sqlite in the XDG state directory for any
+// other user.
 //
 // Usage:
 //

go.mod

@@ -3,6 +3,8 @@ module sneak.berlin/go/sfdupes
 go 1.25.7
 
 require (
+	github.com/adrg/xdg v0.5.3
+	github.com/rivo/uniseg v0.4.7
 	github.com/schollz/progressbar/v3 v3.19.1
 	github.com/spf13/cobra v1.10.2
 	golang.org/x/sys v0.46.0
@@ -18,7 +20,6 @@ require (
 	github.com/mitchellh/colorstring v0.0.0-20190213212951-d06e56a500db // indirect
 	github.com/ncruces/go-strftime v1.0.0 // indirect
 	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
-	github.com/rivo/uniseg v0.4.7 // indirect
 	github.com/spf13/pflag v1.0.9 // indirect
 	modernc.org/libc v1.74.1 // indirect
 	modernc.org/mathutil v1.7.1 // indirect

go.sum

@@ -1,3 +1,5 @@
+github.com/adrg/xdg v0.5.3 h1:xRnxJXne7+oWDatRhR1JLnvuccuIeCoBu2rtuLqQB78=
+github.com/adrg/xdg v0.5.3/go.mod h1:nlTsY+NNiCBGCK2tpm09vRqfVzrc2fLmXGpBLF0zlTQ=
 github.com/chengxilo/virtualterm v1.0.4 h1:Z6IpERbRVlfB8WkOmtbHiDbBANU7cimRIof7mk9/PwM=
 github.com/chengxilo/virtualterm v1.0.4/go.mod h1:DyxxBZz/x1iqJjFxTFcr6/x+jSpqN0iwWCOK1q10rlY=
 github.com/cpuguy83/go-md2man/v2 v2.0.6/go.mod h1:oOW0eioCTA6cOiMLiUPZOpcVxMig6NIQQ7OS05n1F4g=

internal/sfdupes/cli.go

@@ -34,10 +34,14 @@ const (
 
 // databaseHelp is the paragraph of the root and scan help that names
 // the database (README "Location").
-const databaseHelp = "scan and dwim store what they find in a database " +
-	"file. report, trees\nand info read it. The file is $" + databaseEnv +
-	" when that is set and\nnot empty, and otherwise " +
-	defaultDatabasePath + ", where creating it\nneeds root.\n"
+const databaseHelp = `scan and dwim store what they find in a database file. report,
+trees and info read it. The file is $SFDUPES_DATABASE when that is set
+and not empty. Otherwise root uses /var/lib/sfdupes/db.sqlite. Any other
+user uses sfdupes/db.sqlite in the XDG state directory: $XDG_STATE_HOME,
+or ~/.local/state when that is unset or empty
+(~/Library/Application Support on macOS). An XDG_STATE_HOME that is
+not an absolute path is an error.
+`
 
 // escapesHelp is the paragraph of the report and trees help on how
 // paths are escaped (README "Report output format").

internal/sfdupes/cli_test.go

@@ -15,6 +15,7 @@ import (
 	"strings"
 	"testing"
 
+	"github.com/adrg/xdg"
 	"golang.org/x/sys/unix"
 )
 
@@ -559,13 +560,17 @@ func TestRunNamesDefaultDatabase(t *testing.T) {
 	// line.
 	t.Setenv(databaseEnv, "")
 
-	// Whether scan calls the database new depends on that machine too.
-	scanWant := "scan: database " + defaultDatabasePath + " (default)\n"
+	dbPath, err := defaultDatabasePath(os.Geteuid())
+	if err != nil {
+		t.Fatal(err)
+	}
 
-	_, err := os.Stat(defaultDatabasePath)
+	// Whether scan calls the database new depends on that machine too.
+	scanWant := "scan: database " + dbPath + " (default)\n"
+
+	_, err = os.Stat(dbPath)
 	if errors.Is(err, fs.ErrNotExist) {
-		scanWant = "scan: new database " + defaultDatabasePath +
-			" (default)\n"
+		scanWant = "scan: new database " + dbPath + " (default)\n"
 	}
 
 	missing := filepath.Join(t.TempDir(), "nope")
@@ -585,21 +590,21 @@ func TestRunNamesDefaultDatabase(t *testing.T) {
 			run: func() error {
 				return runReport(cancelledContext(t), nil, io.Discard)
 			},
-			want: "report: database " + defaultDatabasePath + " (default)\n" +
+			want: "report: database " + dbPath + " (default)\n" +
 				reportOpenLine + "\n" + reportOpenLine + "\n",
 		},
 		cmdTrees: {
 			run: func() error {
 				return runTrees(cancelledContext(t), nil, io.Discard)
 			},
-			want: "trees: database " + defaultDatabasePath + " (default)\n" +
+			want: "trees: database " + dbPath + " (default)\n" +
 				treesOpenLine + "\n" + treesOpenLine + "\n",
 		},
 		cmdInfo: {
 			run: func() error {
 				return runInfo(cancelledContext(t), io.Discard)
 			},
-			want: "info: database " + defaultDatabasePath + " (default)\n" +
+			want: "info: database " + dbPath + " (default)\n" +
 				infoOpenLine + "\n" + infoOpenLine + "\n",
 		},
 	}
@@ -716,6 +721,19 @@ func TestRunHelp(t *testing.T) {
 			t.Errorf("run(%v) stdout = %q, want nothing (data only)",
 				args, got)
 		}
+
+		// README §Location: the help of sfdupes and of scan says where
+		// the database is, and that XDG_STATE_HOME must be an absolute
+		// path.
+		for _, want := range []string{
+			"scan and dwim store", databaseEnv, rootDatabasePath,
+			stateHomeEnv, "~/.local/state", "not an absolute path is an error",
+		} {
+			if !strings.Contains(stderr.String(), want) {
+				t.Errorf("run(%v) stderr = %q, want it to name %q",
+					args, stderr.String(), want)
+			}
+		}
 	}
 
 	// The scan help lists --exclude with its argument.
@@ -758,28 +776,24 @@ func TestRunScanUsageLine(t *testing.T) {
 	}
 }
 
-func TestRunHelpDescribesDatabaseAndOutput(t *testing.T) {
+func TestRunHelpDescribesOutput(t *testing.T) {
 	t.Parallel()
 
-	// README §Settings: the help names the database, and says what
-	// report, trees, info and dwim print. The help of report and trees
-	// also explains their PATH operands.
-	database := []string{
-		"scan and dwim store", databaseEnv, defaultDatabasePath, "needs root",
-	}
+	// README §Settings: the help of report, trees, info and dwim says
+	// what each prints. The help of report and trees also explains
+	// their PATH operands. TestRunHelp checks the database paragraph
+	// of the root and scan help.
 	escapes := []string{`\\`, `\t`, `\n`, `\r`}
 	operands := []string{
 		"With PATH operands", "at or below them", "working directory",
 		"No PATH is looked up", "not followed",
 	}
 
-	// The subcommand whose help is printed; none for the root.
+	// The subcommand whose help is printed.
 	cases := []struct {
 		subcommand []string
 		want       []string
 	}{
-		{nil, database},
-		{[]string{cmdScan}, database},
 		{
 			[]string{cmdReport},
 			slices.Concat([]string{
@@ -1612,6 +1626,100 @@ func TestRunDatabasePathUsedAsGiven(t *testing.T) {
 	})
 }
 
+func TestRunNonRootUsesStateDirectory(t *testing.T) {
+	// README §Location: without SFDUPES_DATABASE, a user other than
+	// root keeps the database in the XDG state directory. scan creates
+	// the missing directories with mode 0700, and report and trees read
+	// the same file.
+	if os.Geteuid() == 0 {
+		t.Skip("root uses " + rootDatabasePath)
+	}
+
+	t.Cleanup(xdg.Reload)
+
+	state := filepath.Join(t.TempDir(), "state")
+	t.Setenv(stateHomeEnv, state)
+	t.Setenv(databaseEnv, "")
+	xdg.Reload()
+
+	assertRunsUseDatabase(t, filepath.Join(state, "sfdupes"), "db.sqlite")
+
+	for _, dir := range []string{state, filepath.Join(state, "sfdupes")} {
+		fi, err := os.Stat(dir)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		if got := fi.Mode().Perm(); got != stateDirPerm {
+			t.Errorf("%s mode = %#o, want %#o", dir, got, stateDirPerm)
+		}
+	}
+}
+
+func TestRunNonRootDatabaseEnvDirectoryMode(t *testing.T) {
+	// README §Location: for SFDUPES_DATABASE, scan creates each missing
+	// directory with mode 755, for a user other than root too.
+	if os.Geteuid() == 0 {
+		t.Skip("needs a user other than root")
+	}
+
+	top := filepath.Join(t.TempDir(), "missing")
+	sub := filepath.Join(top, "sub")
+	t.Setenv(databaseEnv, filepath.Join(sub, "db.sqlite"))
+
+	scanOK(t, t.TempDir())
+
+	for _, dir := range []string{top, sub} {
+		fi, err := os.Stat(dir)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		if got := fi.Mode().Perm(); got != dbDirPerm {
+			t.Errorf("%s mode = %#o, want %#o", dir, got, dbDirPerm)
+		}
+	}
+}
+
+func TestRunRelativeStateHomeFails(t *testing.T) {
+	// README §Location: without SFDUPES_DATABASE, a user other than
+	// root whose XDG_STATE_HOME is not an absolute path gets an error
+	// naming it from every subcommand, which exits 1 before it does
+	// anything else.
+	if os.Geteuid() == 0 {
+		t.Skip("root does not use " + stateHomeEnv)
+	}
+
+	t.Setenv(stateHomeEnv, "relative/state")
+	t.Setenv(databaseEnv, "")
+
+	want := "sfdupes: " + stateHomeEnv +
+		" must be an absolute path, got \"relative/state\"\n"
+
+	// A scan that got past the check would stop at this missing
+	// operand, before it creates anything.
+	missing := filepath.Join(t.TempDir(), "missing")
+
+	for _, args := range [][]string{
+		{cmdScan, missing}, {cmdReport}, {cmdTrees}, {cmdInfo},
+	} {
+		var stdout, stderr bytes.Buffer
+
+		code := run(args, &stdout, &stderr)
+		if code != exitFatal || stderr.String() != want {
+			t.Errorf("run(%v) = %d with stderr %q, want %d and %q",
+				args, code, stderr.String(), exitFatal, want)
+		}
+	}
+
+	// dwim scans /, so runDwim gets the missing operand instead.
+	err := runDwim(t.Context(), missing, scanOptions{workers: 1}, io.Discard)
+	if !errors.Is(err, errStateHomeNotAbsolute) {
+		t.Errorf("runDwim: error = %v, want %v", err,
+			errStateHomeNotAbsolute)
+	}
+}
+
 // 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.

internal/sfdupes/db.go

@@ -15,28 +15,40 @@ import (
 	"strings"
 	"time"
 
+	"github.com/adrg/xdg"
 	"golang.org/x/sys/unix"
 	// The pure-Go SQLite driver, registered as "sqlite"; keeps cgo
 	// disabled.
 	_ "modernc.org/sqlite"
 )
 
-// defaultDatabasePath is where the persistent scan database lives when
-// SFDUPES_DATABASE is not set.
-const defaultDatabasePath = "/var/lib/sfdupes/db.sqlite"
+// rootDatabasePath is the database root uses when SFDUPES_DATABASE is
+// not set.
+const rootDatabasePath = "/var/lib/sfdupes/db.sqlite"
 
 // databaseEnv is the environment variable that overrides the database
 // path.
 const databaseEnv = "SFDUPES_DATABASE"
 
+// stateHomeEnv is the environment variable that names the XDG state
+// directory.
+const stateHomeEnv = "XDG_STATE_HOME"
+
 // schemaVersion is the database schema version this build reads and
 // writes, stored in PRAGMA user_version.
 const schemaVersion = 1
 
-// dbDirPerm is the mode for a database parent directory created by
-// scan.
+// dbDirPerm is the mode for a directory scan creates for root's
+// default database or for SFDUPES_DATABASE.
 const dbDirPerm = 0o755
 
+// stateDirPerm is the mode for a directory scan creates for the
+// database at a user's default path in the XDG state directory. It is
+// the mode the XDG Base Directory specification asks for, and it keeps
+// other users from reading the database, which lists every scanned
+// path.
+const stateDirPerm = 0o700
+
 // dbFilePerm is the mode for a database file created by scan, the mode
 // SQLite gives a database file it creates.
 const dbFilePerm = 0o644
@@ -144,15 +156,48 @@ var errScanRunning = errors.New("another scan is running")
 // 32 bytes of a SHA-256.
 var errHashSize = errors.New("hash is not 32 bytes")
 
+// errStateHomeNotAbsolute reports an XDG_STATE_HOME that is set to a
+// path that is not absolute.
+var errStateHomeNotAbsolute = errors.New(
+	stateHomeEnv + " must be an absolute path")
+
+// defaultDatabasePath is the database used when SFDUPES_DATABASE is not
+// set: rootDatabasePath for root (euid 0), and db.sqlite in the
+// directory sfdupes under the XDG state directory for any other user.
+// xdg reads XDG_STATE_HOME and HOME once, when the program starts. For
+// any other user, an XDG_STATE_HOME that is set and not an absolute
+// path is an error, where xdg would use its default instead. The path
+// is joined as text, not cleaned, for the reason absolutePath gives.
+func defaultDatabasePath(euid int) (string, error) {
+	if euid == 0 {
+		return rootDatabasePath, nil
+	}
+
+	stateHome := os.Getenv(stateHomeEnv)
+	if stateHome != "" && !filepath.IsAbs(stateHome) {
+		return "", fmt.Errorf("%w, got %q", errStateHomeNotAbsolute,
+			stateHome)
+	}
+
+	stateDir := strings.TrimSuffix(xdg.StateHome, "/")
+
+	return stateDir + "/sfdupes/db.sqlite", nil
+}
+
 // databasePath resolves the database location: SFDUPES_DATABASE when
-// set and non-empty, the compiled-in default otherwise. It returns the
+// set and non-empty, defaultDatabasePath otherwise. It returns the
 // path, made absolute by absolutePath, and where the path came from,
 // "default" or "from SFDUPES_DATABASE", for the line each subcommand
 // prints first on stderr.
 func databasePath() (string, string, error) {
-	path, source := defaultDatabasePath, "default"
-	if p := os.Getenv(databaseEnv); p != "" {
-		path, source = p, "from "+databaseEnv
+	path, source := os.Getenv(databaseEnv), "from "+databaseEnv
+	if path == "" {
+		def, err := defaultDatabasePath(os.Geteuid())
+		if err != nil {
+			return "", "", err
+		}
+
+		path, source = def, "default"
 	}
 
 	abs, err := absolutePath(path)
@@ -188,15 +233,23 @@ func absolutePath(path string) (string, error) {
 
 // createDatabaseDirectory creates the directory the database at path
 // goes in, and any directory above it, if missing. path is absolute, as
-// databasePath returns it. The directory is path with its last element
-// cut off as text, by filepath.Split. filepath.Dir would clean it,
-// dropping each ".." together with the directory name before it; when
-// that name is a symlink, the cleaned path names another directory than
-// the one the system reaches when it opens path.
+// databasePath returns it. Each directory gets mode dbDirPerm, or
+// stateDirPerm when databasePath uses a user's default path in the XDG
+// state directory: the user is not root and SFDUPES_DATABASE is unset
+// or empty. The directory is path with its last element cut off as
+// text, by filepath.Split. filepath.Dir would clean it, dropping each
+// ".." together with the directory name before it; when that name is a
+// symlink, the cleaned path names another directory than the one the
+// system reaches when it opens path.
 func createDatabaseDirectory(path string) error {
 	dir, _ := filepath.Split(path)
 
-	err := os.MkdirAll(dir, dbDirPerm)
+	perm := os.FileMode(dbDirPerm)
+	if os.Geteuid() != 0 && os.Getenv(databaseEnv) == "" {
+		perm = stateDirPerm
+	}
+
+	err := os.MkdirAll(dir, perm)
 	if err != nil {
 		return fmt.Errorf("create database directory: %w", err)
 	}

internal/sfdupes/db_test.go

@@ -9,10 +9,13 @@ import (
 	"io/fs"
 	"os"
 	"path/filepath"
+	"runtime"
 	"slices"
 	"strings"
 	"testing"
 	"time"
+
+	"github.com/adrg/xdg"
 )
 
 // testDBPath returns a database path inside a fresh temp dir, with
@@ -56,11 +59,16 @@ func TestDatabasePath(t *testing.T) {
 
 	t.Chdir(dir)
 
+	defaultPath, err := defaultDatabasePath(os.Geteuid())
+	if err != nil {
+		t.Fatal(err)
+	}
+
 	fromEnv := "from " + databaseEnv
 	cases := []struct {
 		env, path, source string
 	}{
-		{"", defaultDatabasePath, "default"},
+		{"", defaultPath, "default"},
 		{"/custom/place.sqlite", "/custom/place.sqlite", fromEnv},
 		{"/custom/link/../place.sqlite", "/custom/link/../place.sqlite",
 			fromEnv},
@@ -89,6 +97,48 @@ func TestDatabasePath(t *testing.T) {
 	}
 }
 
+func TestDefaultDatabasePath(t *testing.T) {
+	// README §Location: root's database is /var/lib/sfdupes/db.sqlite.
+	// Any other user's is db.sqlite in sfdupes under XDG_STATE_HOME, or
+	// under ~/.local/state when XDG_STATE_HOME is unset or empty. For
+	// that user, an XDG_STATE_HOME that is not an absolute path is an
+	// error.
+	if runtime.GOOS != goosLinux {
+		t.Skip("macOS has another default state directory")
+	}
+
+	t.Cleanup(xdg.Reload)
+	t.Setenv("HOME", "/home/someone")
+
+	cases := []struct {
+		euid      int
+		stateHome string
+		want      string
+		wantErr   error
+	}{
+		{0, "", rootDatabasePath, nil},
+		{0, "/elsewhere/state", rootDatabasePath, nil},
+		{0, "relative/state", rootDatabasePath, nil},
+		{1000, "", "/home/someone/.local/state/sfdupes/db.sqlite", nil},
+		{1000, "relative/state", "", errStateHomeNotAbsolute},
+		{1000, "~/state", "", errStateHomeNotAbsolute},
+		{1000, "/elsewhere/state", "/elsewhere/state/sfdupes/db.sqlite", nil},
+		{1000, "/elsewhere/state/", "/elsewhere/state/sfdupes/db.sqlite", nil},
+	}
+
+	for _, tc := range cases {
+		t.Setenv(stateHomeEnv, tc.stateHome)
+		xdg.Reload()
+
+		got, err := defaultDatabasePath(tc.euid)
+		if got != tc.want || !errors.Is(err, tc.wantErr) {
+			t.Errorf("defaultDatabasePath(%d) with %s=%q = %q, %v; want %q, %v",
+				tc.euid, stateHomeEnv, tc.stateHome, got, err, tc.want,
+				tc.wantErr)
+		}
+	}
+}
+
 func TestOpenScanDatabaseCreates(t *testing.T) {
 	t.Parallel()
 

internal/sfdupes/progress.go

@@ -9,8 +9,10 @@ import (
 	"strings"
 	"sync"
 	"time"
+	"unicode"
 	"unicode/utf8"
 
+	"github.com/rivo/uniseg"
 	"github.com/schollz/progressbar/v3"
 	"golang.org/x/term"
 )
@@ -116,7 +118,8 @@ type drawnCount struct {
 // terminal's width. A phase with an exact total ends the line with its
 // percent and bar, at the terminal's right edge, when the terminal has
 // room for them. It is drawn again every redrawInterval, so its elapsed
-// time keeps moving while the phase waits for its next item. When
+// time keeps moving while the phase waits for its next item. Each draw
+// of the walk shows the path it reached last on the line below. When
 // stderr is not a TTY it emits no ANSI redraws: it prints line as the
 // phase starts, every plainInterval while it runs, and as it ends.
 //
@@ -141,6 +144,7 @@ type progress struct {
 	estimate   bool
 	waitingFor string
 	stalledOn  string
+	path       string // the path the walk reached last; "" for none
 	count      int64
 	start      time.Time // when the phase started, for the elapsed time
 	rateStart  time.Time // when the count started, for the time remaining
@@ -239,6 +243,82 @@ func (p *progress) draw() {
 	// less the terminal's last column, which spinnerMargin keeps clear. A
 	// bar's cells take one column each but more than one byte.
 	p.widest = max(p.widest, utf8.RuneCountInString(text)+spinnerMargin-1)
+
+	if p.path != "" {
+		// The library draws the path on the line below and returns to
+		// the phase's line. The path is escaped as warnings are, and its
+		// other control characters are written out, so that the terminal
+		// shows them instead of acting on them. It stays clear of the
+		// last column, so that no terminal wraps it.
+		shown := escapeControlChars(escapePath(p.path))
+		_ = p.bar.AddDetail(cutToColumns(shown, width-1))
+	}
+}
+
+// escapeControlChars returns s with each control character written as
+// \x and its code in two hex digits, as \x1b for the escape character.
+// The control characters are U+0000 to U+001F, U+007F and U+0080 to
+// U+009F, as unicode.IsControl reports them. A byte from 0x80 to 0x9F
+// that is not part of a valid UTF-8 character is written the same way,
+// as \x9b, since a terminal not in UTF-8 mode acts on it. Every other
+// byte is kept as it is.
+func escapeControlChars(s string) string {
+	var b strings.Builder
+
+	for rest := s; rest != ""; {
+		char, size := utf8.DecodeRuneInString(rest)
+		if char == utf8.RuneError && size == 1 {
+			// Not part of a valid UTF-8 character. A terminal not in
+			// UTF-8 mode reads the byte as the character with its code,
+			// so that 0x9b is a control character there and 0xe9 is not.
+			char = rune(rest[0])
+		}
+
+		if unicode.IsControl(char) {
+			fmt.Fprintf(&b, `\x%02x`, char)
+		} else {
+			b.WriteString(rest[:size])
+		}
+
+		rest = rest[size:]
+	}
+
+	return b.String()
+}
+
+// cutToColumns returns s cut to at most columns terminal columns, at a
+// character boundary. A wide character, as in Chinese, takes two
+// columns.
+func cutToColumns(s string, columns int) string {
+	used, end, state := 0, 0, -1
+
+	for rest := s; rest != ""; {
+		char, next, width, nextState := uniseg.FirstGraphemeClusterInString(
+			rest, state)
+		if used+width > columns {
+			break
+		}
+
+		used += width
+		end += len(char)
+		rest, state = next, nextState
+	}
+
+	return s[:end]
+}
+
+// clear erases the display from the terminal: its line, and the path on
+// the line below when one is shown. p.mu must be held.
+func (p *progress) clear() {
+	if p.bar == nil {
+		return
+	}
+
+	if p.path != "" {
+		_ = p.bar.AddDetail("")
+	}
+
+	_ = p.bar.Clear()
 }
 
 // newBar has the library make what draw draws: its spinner, followed by
@@ -263,6 +343,8 @@ func (p *progress) newBar(width int) {
 		// The text carries the elapsed time and the time remaining.
 		progressbar.OptionSetElapsedTime(false),
 		progressbar.OptionSetPredictTime(false),
+		// The line below the phase's line, for the walk's path.
+		progressbar.OptionSetMaxDetailRow(1),
 	)
 	p.widest = 0
 }
@@ -314,6 +396,19 @@ func (p *progress) increment() {
 	p.lastItem = now
 }
 
+// setPath records the path the walk reached last, which a display on a
+// terminal shows at its next draw.
+func (p *progress) setPath(path string) {
+	if p == nil {
+		return
+	}
+
+	p.mu.Lock()
+	defer p.mu.Unlock()
+
+	p.path = path
+}
+
 // warnf prints a one-line warning to stderr without corrupting the
 // display. The whole message is escaped like a report's path columns,
 // so a path holding a newline cannot split the warning.
@@ -327,11 +422,9 @@ func (p *progress) warnf(format string, args ...any) {
 	p.mu.Lock()
 	defer p.mu.Unlock()
 
-	if p.bar != nil {
-		// The warning takes the display's line; the next draw shows
-		// the display again, on the line below.
-		_ = p.bar.Clear()
-	}
+	// The warning takes the display's line; the next draw shows the
+	// display again, on the line below.
+	p.clear()
 
 	fmt.Fprintln(os.Stderr, msg)
 }
@@ -392,9 +485,7 @@ func (w *progressWriter) Flush() error {
 		w.p.mu.Lock()
 		defer w.p.mu.Unlock()
 
-		if w.p.bar != nil {
-			_ = w.p.bar.Clear()
-		}
+		w.p.clear()
 	}
 
 	_, err := w.out.Write(w.buf)
@@ -414,6 +505,13 @@ func (p *progress) finish() {
 	close(p.stop)
 	<-p.done
 
+	// The walk's path is only for someone watching the walk. Its line is
+	// erased, and the next phase is drawn on it.
+	p.mu.Lock()
+	p.clear()
+	p.path = ""
+	p.mu.Unlock()
+
 	p.draw()
 
 	if p.width != nil {

internal/sfdupes/progress_test.go

@@ -6,6 +6,7 @@ import (
 	"math"
 	"os"
 	"path/filepath"
+	"regexp"
 	"slices"
 	"strings"
 	"sync"
@@ -1138,3 +1139,164 @@ func TestWarningAfterLastItemKeepsBar(t *testing.T) {
 		t.Errorf("terminal shows %q, want a count of 2 of 2", shown)
 	}
 }
+
+func TestCutToColumns(t *testing.T) {
+	t.Parallel()
+
+	cases := []struct {
+		s       string
+		columns int
+		want    string
+	}{
+		{"/srv/a", 10, "/srv/a"},
+		{"/srv/photos", 8, "/srv/pho"},
+		// Each of these two fullwidth letters takes two columns.
+		{"/ＡＢ/a", 4, "/Ａ"},
+		{"/ＡＢ/a", 5, "/ＡＢ"},
+		// An accent that combines with the letter before stays with it.
+		{"/café/a", 5, "/café"},
+		{"/srv", 0, ""},
+	}
+
+	for _, tc := range cases {
+		if got := cutToColumns(tc.s, tc.columns); got != tc.want {
+			t.Errorf("cutToColumns(%q, %d) = %q, want %q",
+				tc.s, tc.columns, got, tc.want)
+		}
+	}
+}
+
+func TestEscapeControlChars(t *testing.T) {
+	t.Parallel()
+
+	cases := map[string]string{
+		"/srv/music":    "/srv/music",
+		"/a\x1b[2Jb":    `/a\x1b[2Jb`,
+		"/a\x00\a\x7fb": `/a\x00\x07\x7fb`,
+		// U+009B takes two bytes in UTF-8.
+		"/a\u009b2Jb": `/a\x9b2Jb`,
+		// Byte 0x9b alone is not valid UTF-8. A terminal not in UTF-8
+		// mode acts on it, as on every byte from 0x80 to 0x9f.
+		"/a\x9b2Jb":       `/a\x9b2Jb`,
+		"/a\x80\x9f\xa0b": `/a\x80\x9f` + "\xa0b",
+		// In ě, which is c4 9b in UTF-8, byte 0x9b is part of a valid
+		// character.
+		"/aě2Jb":          "/aě2Jb",
+		"/café/ＡＢ":        "/café/ＡＢ",
+		"/latin1-caf\xe9": "/latin1-caf\xe9",
+	}
+	for in, want := range cases {
+		if got := escapeControlChars(in); got != want {
+			t.Errorf("escapeControlChars(%q) = %q, want %q", in, got, want)
+		}
+	}
+}
+
+// pathLines returns the text of each line the library wrote below a
+// display's line. It writes one as an erase to the end of the line, a
+// carriage return, the text and a newline.
+func pathLines(out string) []string {
+	matches := regexp.MustCompile("\x1b\\[K\r([^\n]*)\n").
+		FindAllStringSubmatch(out, -1)
+	lines := make([]string, 0, len(matches))
+
+	for _, m := range matches {
+		lines = append(lines, m[1])
+	}
+
+	return lines
+}
+
+// TestTerminalWalkShowsPathBelow checks that the walk's display on a
+// terminal 30 columns wide shows the path it reached last on the line
+// below, cut to 29 columns. That line is erased before a warning is
+// printed, and again when the walk ends, so the next phase is drawn on
+// an empty line.
+//
+//nolint:paralleltest // captureStderr replaces the process-wide os.Stderr
+func TestTerminalWalkShowsPathBelow(t *testing.T) {
+	stderr := captureStderr(t)
+	p := startProgress(progressParams{
+		label: scanPhase("walk"), unit: scanUnit, total: -1,
+	}, func() int { return 30 }, testRedraw, time.Now)
+
+	const shown = "/srv/photos/2026/holiday/beac"
+
+	p.setPath("/srv/photos/2026/holiday/beach.jpg")
+	p.increment()
+	awaitOutput(t, stderr, shown, 1)
+
+	p.warnf("warning")
+
+	before, _, _ := strings.Cut(stderr(), "warning")
+	if lines := pathLines(before); lines[len(lines)-1] != "" {
+		t.Errorf("line below shows %q as the warning is printed, want it "+
+			"empty", lines[len(lines)-1])
+	}
+
+	// Drawn again below the warning.
+	awaitOutput(t, stderr, shown, strings.Count(before, shown)+1)
+	p.finish()
+
+	lines := pathLines(stderr())
+	if !slices.Contains(lines, shown) {
+		t.Errorf("lines below the display = %q, want %q among them",
+			lines, shown)
+	}
+
+	if last := lines[len(lines)-1]; last != "" {
+		t.Errorf("line below the display shows %q after the walk, want it "+
+			"empty", last)
+	}
+}
+
+// TestTerminalWalkEscapesControlChars checks that a walked path holding
+// escape characters, which would clear the screen and set the window
+// title, puts none on the line below the walk's display. The line shows
+// each control character written out, as \x1b.
+//
+//nolint:paralleltest // captureStderr replaces the process-wide os.Stderr
+func TestTerminalWalkEscapesControlChars(t *testing.T) {
+	stderr := captureStderr(t)
+	p := startProgress(progressParams{
+		label: scanPhase("walk"), unit: scanUnit, total: -1,
+	}, func() int { return 80 }, testRedraw, time.Now)
+
+	p.setPath("/srv/\x1b[2J\x1b]0;title\a/beach.jpg")
+	p.increment()
+	awaitOutput(t, stderr, "beach.jpg", 1)
+	p.finish()
+
+	lines := pathLines(stderr())
+	for _, line := range lines {
+		if strings.Contains(line, "\x1b") {
+			t.Errorf("line below the display = %q, want no escape "+
+				"character", line)
+		}
+	}
+
+	const shown = `/srv/\x1b[2J\x1b]0;title\x07/beach.jpg`
+	if !slices.Contains(lines, shown) {
+		t.Errorf("lines below the display = %q, want %q among them",
+			lines, shown)
+	}
+}
+
+// TestPlainWalkShowsNoPath checks that the walk's display prints no path
+// when stderr is not a terminal.
+//
+//nolint:paralleltest // captureStderr replaces the process-wide os.Stderr
+func TestPlainWalkShowsNoPath(t *testing.T) {
+	stderr := captureStderr(t)
+	p := startProgress(progressParams{
+		label: scanPhase("walk"), unit: scanUnit, total: -1,
+	}, nil, testRedraw, time.Now)
+
+	p.setPath("/srv/a")
+	p.increment()
+	p.finish()
+
+	if strings.Contains(stderr(), "/srv/a") {
+		t.Errorf("stderr = %q, want no path", stderr())
+	}
+}

internal/sfdupes/scan.go

@@ -562,6 +562,7 @@ func (s *scanState) walkPhase(events <-chan walkEvent, roots []string) {
 
 		s.st.walked++
 
+		prog.setPath(ev.rec.path)
 		prog.increment()
 
 		old, ok := s.existing[ev.rec.path]

internal/sfdupes/trees.go

@@ -108,7 +108,7 @@ func runTrees(ctx context.Context, args []string, stdout io.Writer) error {
 		label: treesPhase("write"), unit: "groups", total: int64(len(dupes)),
 		stalledOn: "out",
 	})
-	dupeTrees, reclaimable, err := writeTreeGroups(stdout, dupes, prog)
+	dupeTrees, err := writeTreeGroups(stdout, dupes, prog)
 
 	prog.finish()
 
@@ -119,7 +119,7 @@ func runTrees(ctx context.Context, args []string, stdout io.Writer) error {
 	fmt.Fprintf(os.Stderr,
 		"trees: %d records read, %d duplicate tree groups, %d dupe trees, "+
 			"%s reclaimable\n",
-		records, len(dupes), dupeTrees, humanBytes(reclaimable))
+		records, len(dupes), dupeTrees, humanBytes(reclaimableBytes(dupes)))
 
 	return nil
 }
@@ -177,22 +177,19 @@ func readTree(ctx context.Context, db *sql.DB, operands []string,
 // a row for every member of each group but its first, counting each
 // group on prog once its rows are written. The disk_size column is the
 // dupe's own. stdout may be the terminal prog is drawn on, so the lines
-// go through prog's writer. It returns the number of rows (dupe trees)
-// and the bytes they could reclaim.
+// go through prog's writer. It returns the number of rows (dupe trees).
 func writeTreeGroups(stdout io.Writer, dupes [][]*treeNode,
 	prog *progress,
-) (int, int64, error) {
+) (int, error) {
 	out := prog.writer(stdout)
 
 	_, err := fmt.Fprintln(out, "first\tdupe\tfiles\tsize\tdisk_size")
 	if err != nil {
-		return 0, 0, fmt.Errorf("write stdout: %w", err)
+		return 0, fmt.Errorf("write stdout: %w", err)
 	}
 
 	dupeTrees := 0
 
-	var reclaimable int64
-
 	for _, g := range dupes {
 		first := g[0]
 		for _, n := range g[1:] {
@@ -200,11 +197,10 @@ func writeTreeGroups(stdout io.Writer, dupes [][]*treeNode,
 				escapePath(first.path), escapePath(n.path),
 				first.fileCount, first.totalSize, n.diskSize)
 			if err != nil {
-				return 0, 0, fmt.Errorf("write stdout: %w", err)
+				return 0, fmt.Errorf("write stdout: %w", err)
 			}
 
 			dupeTrees++
-			reclaimable += first.totalSize
 		}
 
 		prog.increment()
@@ -212,10 +208,44 @@ func writeTreeGroups(stdout io.Writer, dupes [][]*treeNode,
 
 	err = out.Flush()
 	if err != nil {
-		return 0, 0, fmt.Errorf("write stdout: %w", err)
+		return 0, fmt.Errorf("write stdout: %w", err)
 	}
 
-	return dupeTrees, reclaimable, nil
+	return dupeTrees, nil
+}
+
+// reclaimableBytes returns the sum of the size column over the rows
+// that writeTreeGroups writes for dupes, leaving out a row whose dupe
+// lies inside another row's dupe.
+func reclaimableBytes(dupes [][]*treeNode) int64 {
+	isDupe := make(map[*treeNode]bool)
+
+	for _, g := range dupes {
+		for _, n := range g[1:] {
+			isDupe[n] = true
+		}
+	}
+
+	var total int64
+
+	for n := range isDupe {
+		if !insideDupe(n, isDupe) {
+			total += n.totalSize
+		}
+	}
+
+	return total
+}
+
+// insideDupe reports whether a directory above n is in isDupe.
+func insideDupe(n *treeNode, isDupe map[*treeNode]bool) bool {
+	for p := n.parent; p != nil; p = p.parent {
+		if isDupe[p] {
+			return true
+		}
+	}
+
+	return false
 }
 
 // treeBuilder reconstructs the directory hierarchy from records added

internal/sfdupes/trees_test.go

@@ -364,7 +364,7 @@ func TestWriteTreeGroupsOnTerminal(t *testing.T) {
 		label: treesPhase("write"), unit: "groups", total: int64(len(dupes)),
 	}, terminalWidth, time.Hour, time.Now)
 
-	_, _, err := writeTreeGroups(os.Stderr, dupes, p)
+	_, err := writeTreeGroups(os.Stderr, dupes, p)
 
 	p.finish()
 
@@ -638,3 +638,64 @@ func TestCollectTreeGroupsKeepsGroupLeftWithTwoCopies(t *testing.T) {
 		t.Errorf("groups = %v, want %v", got, want)
 	}
 }
+
+// TestReclaimableBytesCountsNestedDupeOnce checks that a dupe tree
+// inside another dupe tree adds nothing to the reclaimable bytes. /B/x
+// lies in /B, so the total adds the sizes of /B and /C/x, 1500 + 1000.
+func TestReclaimableBytesCountsNestedDupeOnce(t *testing.T) {
+	t.Parallel()
+
+	fh, ft, fc := testHash("fh"), testHash("ft"), testHash("fc")
+	gh, gt, gc := testHash("gh"), testHash("gt"), testHash("gc")
+	recs := []scanRec{
+		{size: 1000, head: fh, tail: ft, content: fc, path: "/A/x/f"},
+		{size: 500, head: gh, tail: gt, content: gc, path: "/A/y/g"},
+		{size: 1000, head: fh, tail: ft, content: fc, path: "/B/x/f"},
+		{size: 500, head: gh, tail: gt, content: gc, path: "/B/y/g"},
+		{size: 1000, head: fh, tail: ft, content: fc, path: "/C/x/f"},
+	}
+
+	super, dirs := treeOf(t, recs)
+	dupes := collectTreeGroups(dirs, super, everyPathExists, nil)
+
+	wantGroups := [][]string{{"/A", "/B"}, {"/A/x", "/B/x", "/C/x"}}
+	if got := groupPaths(dupes); !slices.EqualFunc(got, wantGroups,
+		slices.Equal) {
+		t.Fatalf("groups = %v, want %v", got, wantGroups)
+	}
+
+	if got := reclaimableBytes(dupes); got != 2500 {
+		t.Errorf("reclaimableBytes = %d, want 2500", got)
+	}
+}
+
+// TestReclaimableBytesCountsDupeInsideGrandparentDupeOnce checks that a
+// dupe tree adds nothing when a dupe tree above its parent holds it.
+// /B/m/x lies in /B, and /B/m is no dupe, since the group of /A and /B
+// implies that of /A/m and /B/m. The total adds the sizes of /B and
+// /C/x, 1300 + 1000.
+func TestReclaimableBytesCountsDupeInsideGrandparentDupeOnce(t *testing.T) {
+	t.Parallel()
+
+	f, w := testHash("f"), testHash("w")
+	recs := []scanRec{
+		{size: 1000, head: f, tail: f, content: f, path: "/A/m/x/f"},
+		{size: 300, head: w, tail: w, content: w, path: "/A/m/w"},
+		{size: 1000, head: f, tail: f, content: f, path: "/B/m/x/f"},
+		{size: 300, head: w, tail: w, content: w, path: "/B/m/w"},
+		{size: 1000, head: f, tail: f, content: f, path: "/C/x/f"},
+	}
+
+	super, dirs := treeOf(t, recs)
+	dupes := collectTreeGroups(dirs, super, everyPathExists, nil)
+
+	wantGroups := [][]string{{"/A", "/B"}, {"/A/m/x", "/B/m/x", "/C/x"}}
+	if got := groupPaths(dupes); !slices.EqualFunc(got, wantGroups,
+		slices.Equal) {
+		t.Fatalf("groups = %v, want %v", got, wantGroups)
+	}
+
+	if got := reclaimableBytes(dupes); got != 2300 {
+		t.Errorf("reclaimableBytes = %d, want 2300", got)
+	}
+}