listDirectoryContent() gets stuck with message "about to open data connection"

[aziztitu]

Describe the bug
The listDirectoryContent() method gets stuck indefinitely. If the debug is set to true, we can see this log:

I/flutter ( 4570): [17.01.2021 15:14:43.636] Connecting...
I/flutter ( 4570): [17.01.2021 15:14:43.840] < 220 Service ready for new user.
I/flutter ( 4570): [17.01.2021 15:14:43.841] > USER anonymous
I/flutter ( 4570): [17.01.2021 15:14:44.144] < 331 Guest login okay, send your complete e-mail address as password.
I/flutter ( 4570): [17.01.2021 15:14:44.144] > PASS
I/flutter ( 4570): [17.01.2021 15:14:44.246] < 230 User logged in, proceed.
I/flutter ( 4570): [17.01.2021 15:14:44.246] Connected!
I/flutter ( 4570): Connected successfully
I/flutter ( 4570): [17.01.2021 15:14:44.250] > PWD
I/flutter ( 4570): [17.01.2021 15:14:44.351] < 257 "/" is current directory.
I/flutter ( 4570): /
I/flutter ( 4570): [17.01.2021 15:14:44.354] > TYPE A
I/flutter ( 4570): [17.01.2021 15:14:44.455] < 200 Command TYPE okay.
I/flutter ( 4570): [17.01.2021 15:14:44.455] > PASV
I/flutter ( 4570): [17.01.2021 15:14:44.556] < 227 Entering Passive Mode (192,168,1,8,167,251)
I/flutter ( 4570): [17.01.2021 15:14:44.558] > MLSD
I/flutter ( 4570): [17.01.2021 15:14:44.569] < 150 File status okay; about to open data connection.

And it gets stuck at this point. The listDirectoryContent() method never returns.

Code Snippet:

FTPClient ftpClient = FTPClient(host.text,
            port: int.parse(port.text),
            user: username.text.isEmpty ? "anonymous" : username.text,
            pass: password.text,
            debug: true);
ftpClient.connect();     

debugPrint("Connected successfully");
debugPrint(ftpClient.currentDirectory());

var dirs = ftpClient.listDirectoryContent();         // <-- It gets stuck here
ftpClient.disconnect();

for (var dir in dirs) {
  debugPrint(dir.name);
}

Expected behavior
listDirectoryContent() returns the directory content, or throws a proper error.

Platform:

• Flutter

Smartphone (when using Flutter):

• Device: OnePlus 8 Pro
• OS: Oxygen OS 11.0.2.2 [Android Version 11]

FTP Server Software

• Dragon Touch Picture Frame FTP

[aziztitu]

Upon further inspection at the source code, I think I've figured out the issue.

This bug occurs when the current directory in the FTP Server is empty. This following loop in directory.dart never exits if there is no file/directory in the current directory.

dart_ftpclient/lib/src/commands/directory.dart

Lines 79 to 95 in 7124f81

	int iToRead = 0;
	List<int> lstDirectoryListing = List<int>();
	do {
	if (iToRead > 0) {
	List<int> buffer = List<int>(iToRead);
	dataSocket.readIntoSync(buffer);
	buffer.forEach(lstDirectoryListing.add);
	}
	iToRead = dataSocket.available();
	if (iToRead == 0 || lstDirectoryListing.isEmpty) {
	sleep(Duration(milliseconds: 500));
	iToRead = dataSocket.available();
	}
	} while (iToRead > 0 || lstDirectoryListing.isEmpty);

A simple change in logic here would fix the issue. I can create a PR later for this if you need.