Socket read freezes on Linux despite timeout

Question

We're using Linux 2.6 multiprocessor machines and Sun JDK 6.
Servers connect to each other over HTTP, setting socket timeout of 60 seconds.
Despite that sometimes sockets freeze forever in read() operation.
The code is like this:
URL url = new URL(endpoint);
URLConnection c = url.openConnection();
c.setConnectTimeout( 60000 );
c.setReadTimeout( 60000 );
InputStream in = c.getInputStream();
byte[] buffer = new byte[256 * 1024];
int readBytes = -1;
while ((readBytes = inputStream.read(buffer)) != -1) {
// use remote data
And the frozen stack trace is:
java.net.SocketInputStream.socketRead0(Native Method)
java.net.SocketInputStream.read(SocketInputStream.java:129)
java.io.BufferedInputStream.read1(BufferedInputStream.java:256)
java.io.BufferedInputStream.read(BufferedInputStream.java:317)
sun.net.www.MeteredStream.read(MeteredStream.java:116)
java.io.FilterInputStream.read(FilterInputStream.java:116)
sun.net.www.protocol.http.HttpURLConnection$HttpInputStream.read(HttpURLConnection.java:2391)
sun.net.www.protocol.http.HttpURLConnection$HttpInputStream.read(HttpURLConnection.java:2386)
com.pmstation.shared.admin.core.MoveFile.pipeStream(MoveFile.java:189)
com.pmstation.shared.admin.core.MoveFile.transferFile(MoveFile.java:173)
Timeout not always working?
So to be sure HTTP read from remote server is never blocking I'm forced to do IO on a separate thread? Or there are better ideas?

Answer 1

Well then, it's important to note that a read timeout only specifies the amount of time to block while waiting for data. Once data has started to arrive, even one bit of data, the socket won't ever timeout because of a read timeout.
I think I have a solution, however. Don't use the built-in timeout function, write a polling method that times out after so many seconds without receiving data.
Try this ;-)
URL url = new URL(endpoint);
URLConnection c = url.openConnection();
c.setConnectTimeout( 60000 );
c.setReadTimeout( 60000 );
InputStream in = c.getInputStream();
byte[] buffer = new byte[256 * 1024];
int readBytes = -1;
/* Define our polling rates */
int max_wait  = 60000; // 60 seconds
int poll_rate = 2000;  // 2 seconds
int last_amount;
/* Poll for max_wait until we receive a full buffer */
for (int i = max_wait; i > 0; i -= poll_rate) {
    /* Get the number of available bytes */
    int available_bytes = inputStream.available();
    /* Reset the poll if we are actively receiving bytes */
    if (available_bytes > last_amount)
     i = max_wait;
    /* Update the last amount variable */
    last_amount = available_bytes;
    /* Don't read until we have a full buffer or are about to timeout */
    if ((available_bytes < buffer.length) && (i >= poll_rate))  {
     Thread.currentThread.sleep(poll_rate);
    else if (available_bytes > 0) {
        /* Must have enough bytes or be last poll before the timeout */
        if ((readBytes = inputStream.read(buffer,0,available_bytes)) != -1) {
            /* Reset the last_amount variable */
            last_amount = 0;
            // use remote data
}