scan: create the database's directory where its path leads (closes #119)

scan created a missing database directory from filepath.Dir of the
path, which cleans it as text and drops each ".." together with the
name before it. After a symlinked directory that is another directory
than the one the system reaches when it opens the file, so scan left an
empty directory beside the symlink and then failed to create the
database. lockScanDatabase and openScanDatabase now both call
createDatabaseDirectory, beside databasePath, which cuts the last
element off the path as text and creates what is missing along it. A
comment in databasePath now says why the working directory has its
symlinks resolved.

Unverified: a missing directory before a ".." in the path is created along the way; no test covers it.

Model: opus-5-5

TODO.md

@@ -29,6 +29,14 @@
 
 # Completed Steps
 
+- `scan` creates the database's directory where the database path leads: the
+  path with its last element cut off as text, never cleaned, so a `..` after a
+  symlinked directory is followed through the symlink, as when the file is
+  opened. With `SFDUPES_DATABASE=/srv/link/../new/db.sqlite` and `/srv/link` a
+  symlink to `/data/x`, it creates `/data/new` and the database in it, where it
+  used to create `/srv/new` and then fail (2026-10-17,
+  https://git.eeqj.de/sneak/sfdupes/issues/119)
+
 - `scan` never records its own database: once the database and its `.lock` file
   are open, it notes the device and inode of the database file, its `-wal` and
   `-shm` files and its `.lock` file, and the walk passes over a file with one of

db.go

@@ -109,10 +109,8 @@ func databasePath() (string, string, error) {
 	}
 
 	// The working directory as the kernel reports it, with no symlink
-	// in it. os.Getwd may return PWD instead, which can name it through
-	// a symlink; a ".." after that symlink then leads somewhere else
-	// once the path is cleaned, as scan cleans it to create the
-	// database's directory.
+	// in it, as README "Database" says. os.Getwd may return PWD
+	// instead, which can name it through a symlink.
 	wd, err := unix.Getwd()
 	if err != nil {
 		return "", "", fmt.Errorf("resolve database path %s: %w", path, err)
@@ -125,6 +123,24 @@ func databasePath() (string, string, error) {
 	return strings.TrimSuffix(wd, "/") + "/" + path, source, nil
 }
 
+// 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.
+func createDatabaseDirectory(path string) error {
+	dir, _ := filepath.Split(path)
+
+	err := os.MkdirAll(dir, dbDirPerm)
+	if err != nil {
+		return fmt.Errorf("create database directory: %w", err)
+	}
+
+	return nil
+}
+
 // scanParams are the connection parameters for scan: read-write, with
 // WAL journaling and a busy timeout, so a report can run while a cron
 // scan is in progress. closeScanDatabase leaves WAL mode again.
@@ -179,9 +195,9 @@ func openDB(path, params string) (*sql.DB, error) {
 // that deleted it would let the next scan lock a new file while
 // another still holds the old one.
 func lockScanDatabase(path string) (*os.File, error) {
-	err := os.MkdirAll(filepath.Dir(path), dbDirPerm)
+	err := createDatabaseDirectory(path)
 	if err != nil {
-		return nil, fmt.Errorf("create database directory: %w", err)
+		return nil, err
 	}
 
 	// Like SQLite, this follows a symlink at path and creates its
@@ -225,9 +241,9 @@ func lockScanDatabase(path string) (*os.File, error) {
 // openScanDatabase opens the database for the scan subcommand, creating
 // the file, its parent directory, and the schema as needed.
 func openScanDatabase(ctx context.Context, path string) (*sql.DB, error) {
-	err := os.MkdirAll(filepath.Dir(path), dbDirPerm)
+	err := createDatabaseDirectory(path)
 	if err != nil {
-		return nil, fmt.Errorf("create database directory: %w", err)
+		return nil, err
 	}
 
 	db, err := openDB(path, scanParams)

db_test.go

@@ -5,6 +5,7 @@ import (
 	"database/sql"
 	"errors"
 	"fmt"
+	"io/fs"
 	"os"
 	"path/filepath"
 	"slices"
@@ -118,6 +119,78 @@ func TestOpenScanDatabaseCreates(t *testing.T) {
 	}
 }
 
+// TestScanCreatesDatabaseDirectoryAfterSymlink has a database path go
+// through a symlink to a directory, then "..", to a missing directory.
+// With srv/link a symlink to data/x, srv/link/../new/db.sqlite leads to
+// data/new/db.sqlite. lockScanDatabase and openScanDatabase, the two
+// places scan creates the database's directory, must each create the
+// database there, and nothing at srv/new, where the path leads once
+// cleaned as text.
+func TestScanCreatesDatabaseDirectoryAfterSymlink(t *testing.T) {
+	t.Parallel()
+
+	creates := map[string]func(ctx context.Context, path string) error{
+		"lockScanDatabase": func(_ context.Context, path string) error {
+			lock, err := lockScanDatabase(path)
+			if err == nil {
+				_ = lock.Close()
+			}
+
+			return err
+		},
+		"openScanDatabase": func(ctx context.Context, path string) error {
+			db, err := openScanDatabase(ctx, path)
+			if err == nil {
+				_ = db.Close()
+			}
+
+			return err
+		},
+	}
+
+	for name, create := range creates {
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+
+			dir, err := filepath.EvalSymlinks(t.TempDir())
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			err = os.MkdirAll(filepath.Join(dir, "data", "x"), 0o750)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			err = os.Mkdir(filepath.Join(dir, "srv"), 0o750)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			err = os.Symlink(filepath.Join(dir, "data", "x"),
+				filepath.Join(dir, "srv", "link"))
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			err = create(t.Context(), dir+"/srv/link/../new/db.sqlite")
+			if err != nil {
+				t.Fatalf("%s: %v", name, err)
+			}
+
+			_, err = os.Stat(filepath.Join(dir, "data", "new", "db.sqlite"))
+			if err != nil {
+				t.Errorf("database the path leads to: %v", err)
+			}
+
+			_, err = os.Lstat(filepath.Join(dir, "srv", "new"))
+			if !errors.Is(err, fs.ErrNotExist) {
+				t.Errorf("srv/new: stat error %v, want it missing", err)
+			}
+		})
+	}
+}
+
 // TestDatabaseFiles opens a database through a symlink to its file:
 // databaseFiles finds the database file and its -wal and -shm files
 // beside the symlink's target, where SQLite keeps them, and leaves out