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

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)