Name the database each command uses on stderr (closes #111)

scan, report and trees now print one line on stderr before anything
else, naming the database and whether its path is the default or comes
from SFDUPES_DATABASE; scan also says when the file is new. report and
trees print it before opening the database, so a missing-database error
follows it. A relative path gets the working directory put in front of
it without cleaning: cleaning drops a ".." with the directory before
it, which names another file when that directory is a symlink. README
"Database" lists when scan writes each file.

Unverified: the line on a terminal has no test of its own.
Rule suppressed: TestDatabasePath carries nolint:paralleltest, as it
changes the working directory.

Model: opus-5-5

README.md

@@ -269,9 +269,10 @@ All three subcommands operate on a single SQLite database file:
   directory.
 - Each subcommand first prints one line on stderr naming the database it uses,
   before anything else it prints (progress displays included), whether or not
-  stderr is a terminal. The line gives the path made absolute against the
-  working directory, escaped like the paths in a report (see "Report output
-  format"), and says where the path came from:
+  stderr is a terminal. The line gives the path with the working directory put
+  in front of it when it is relative, and otherwise as set (a `..` in it stays,
+  so the line names the same file), escaped like the paths in a report (see
+  "Report output format"), and says where the path came from:
 
     ```
     scan: new database /var/lib/sfdupes/db.sqlite (default)

db.go

@@ -11,6 +11,7 @@ import (
 	"path/filepath"
 	"slices"
 	"strconv"
+	"strings"
 	"time"
 
 	"golang.org/x/sys/unix"
@@ -92,21 +93,30 @@ var errScanRunning = errors.New("another scan is running")
 
 // databasePath resolves the database location: SFDUPES_DATABASE when
 // set and non-empty, the compiled-in default otherwise. It returns the
-// path made absolute against the working directory, and where the path
-// came from, "default" or "from SFDUPES_DATABASE", for the line each
-// subcommand prints first on stderr.
+// path, with the working directory put in front of it when it is
+// relative, 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
 	}
 
-	abs, err := filepath.Abs(path)
+	if filepath.IsAbs(path) {
+		return path, source, nil
+	}
+
+	wd, err := os.Getwd()
 	if err != nil {
 		return "", "", fmt.Errorf("resolve database path %s: %w", path, err)
 	}
 
-	return abs, source, nil
+	// Not filepath.Abs or filepath.Join: they clean the path, which
+	// drops each ".." together with the directory name before it. When
+	// that directory is a symlink, the cleaned path names another file.
+	// The working directory ends in "/" only when it is the root.
+	return strings.TrimSuffix(wd, "/") + "/" + path, source, nil
 }
 
 // scanParams are the connection parameters for scan: read-write, with

db_test.go

@@ -45,7 +45,8 @@ func openTestDB(t *testing.T) *sql.DB {
 //nolint:paralleltest // changes the process-wide working directory
 func TestDatabasePath(t *testing.T) {
 	// README §Database: SFDUPES_DATABASE when set and non-empty, the
-	// default otherwise, made absolute against the working directory.
+	// default otherwise, with the working directory put in front of a
+	// relative path. Nothing is cleaned, so a ".." stays.
 	dir := t.TempDir()
 	t.Chdir(dir)
 
@@ -55,7 +56,10 @@ func TestDatabasePath(t *testing.T) {
 	}{
 		{"", defaultDatabasePath, "default"},
 		{"/custom/place.sqlite", "/custom/place.sqlite", fromEnv},
-		{"sub/place.sqlite", filepath.Join(dir, "sub/place.sqlite"), fromEnv},
+		{"/custom/link/../place.sqlite", "/custom/link/../place.sqlite",
+			fromEnv},
+		{"sub/place.sqlite", dir + "/sub/place.sqlite", fromEnv},
+		{"link/../place.sqlite", dir + "/link/../place.sqlite", fromEnv},
 	}
 
 	for _, tc := range cases {
@@ -67,6 +71,16 @@ func TestDatabasePath(t *testing.T) {
 				databaseEnv, tc.env, path, source, err, tc.path, tc.source)
 		}
 	}
+
+	// In the root directory the path gets one slash in front, not two.
+	t.Chdir("/")
+	t.Setenv(databaseEnv, "place.sqlite")
+
+	path, _, err := databasePath()
+	if err != nil || path != "/place.sqlite" {
+		t.Errorf("databasePath() in / = %q, %v; want %q",
+			path, err, "/place.sqlite")
+	}
 }
 
 func TestOpenScanDatabaseCreates(t *testing.T) {

main_test.go

@@ -352,6 +352,72 @@ func TestRunNamesRelativeDatabase(t *testing.T) {
 	}
 }
 
+func TestRunUsesDatabaseThroughSymlinkedDirectory(t *testing.T) {
+	// README §Database: the path names the file exactly. With link a
+	// symlink to the directory real/sub, link/../db.sqlite names
+	// real/db.sqlite, not db.sqlite beside link. Every subcommand must
+	// use that file, relative or absolute, and its first line must name
+	// the path as set, with the working directory put in front of a
+	// relative one.
+	dir := t.TempDir()
+	t.Chdir(dir)
+
+	err := os.MkdirAll(filepath.Join(dir, "real", "sub"), 0o750)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	err = os.Symlink(filepath.Join(dir, "real", "sub"),
+		filepath.Join(dir, "link"))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	named := dir + "/link/../db.sqlite"
+	from := " (from " + databaseEnv + ")\n"
+	tree := t.TempDir()
+
+	t.Setenv(databaseEnv, "link/../db.sqlite")
+
+	want := "scan: new database " + named + from
+	if got := scanOK(t, tree); !strings.HasPrefix(got, want) {
+		t.Errorf("relative scan stderr = %q, want it to start %q", got, want)
+	}
+
+	// The absolute path names the database the relative one created.
+	t.Setenv(databaseEnv, named)
+
+	want = "scan: database " + named + from
+	if got := scanOK(t, tree); !strings.HasPrefix(got, want) {
+		t.Errorf("absolute scan stderr = %q, want it to start %q", got, want)
+	}
+
+	for _, name := range []string{cmdReport, cmdTrees} {
+		stderr := captureStderr(t)
+
+		code := run([]string{name}, io.Discard, os.Stderr)
+
+		want = name + ": database " + named + from
+		if got := stderr(); code != exitOK || !strings.HasPrefix(got, want) {
+			t.Errorf("run(%s) = %d with stderr %q, want %d and it to "+
+				"start %q", name, code, got, exitOK, want)
+		}
+	}
+
+	_, err = os.Stat(filepath.Join(dir, "real", "db.sqlite"))
+	if err != nil {
+		t.Errorf("database the path names: %v", err)
+	}
+
+	// With no file at the cleaned path, report and trees succeeded on
+	// the database in real.
+	_, err = os.Stat(filepath.Join(dir, "db.sqlite"))
+	if !errors.Is(err, fs.ErrNotExist) {
+		t.Errorf("database at the cleaned path: stat error %v, want it "+
+			"missing", err)
+	}
+}
+
 func TestRunNamesDefaultDatabase(t *testing.T) {
 	// README §Database: with SFDUPES_DATABASE empty, each subcommand's
 	// first line on stderr names the default database. A database may