Channel cannot be closed or freed early while executing a long running command

[Jianhui Zhao]

Describe the bug
Channel cannot be closed or freed early while execute a command that will last a long time.

To Reproduce

This code execs a command sleep 100 and I cannot close the channel early.

#include <libssh2.h>

#include <sys/socket.h>
#include <unistd.h>
#include <netinet/in.h>
#include <arpa/inet.h>

#include <stdio.h>
#include <stdlib.h>

static const char *hostname = "127.0.0.1";
static const char *commandline = "sleep 100";
static const char *username = "root";
static const char *password = "1";

static int waitsocket(libssh2_socket_t socket_fd, LIBSSH2_SESSION *session)
{
    struct timeval timeout;
    int rc;
    fd_set fd;
    fd_set *writefd = NULL;
    fd_set *readfd = NULL;
    int dir;

    timeout.tv_sec = 10;
    timeout.tv_usec = 0;

    FD_ZERO(&fd);

    FD_SET(socket_fd, &fd);

    /* now make sure we wait in the correct direction */
    dir = libssh2_session_block_directions(session);

    if(dir & LIBSSH2_SESSION_BLOCK_INBOUND)
        readfd = &fd;

    if(dir & LIBSSH2_SESSION_BLOCK_OUTBOUND)
        writefd = &fd;

    rc = select((int)(socket_fd + 1), readfd, writefd, NULL, &timeout);

    return rc;
}

int main(int argc, char *argv[])
{
    uint32_t hostaddr;
    libssh2_socket_t sock;
    struct sockaddr_in sin;
    int rc;
    LIBSSH2_SESSION *session = NULL;
    LIBSSH2_CHANNEL *channel;
    int exitcode;

    rc = libssh2_init(0);
    if(rc) {
        fprintf(stderr, "libssh2 initialization failed (%d)\n", rc);
        return 1;
    }

    hostaddr = inet_addr(hostname);

    sock = socket(AF_INET, SOCK_STREAM, 0);
    if(sock == LIBSSH2_INVALID_SOCKET) {
        fprintf(stderr, "failed to create socket!\n");
        goto shutdown;
    }

    sin.sin_family = AF_INET;
    sin.sin_port = htons(22);
    sin.sin_addr.s_addr = hostaddr;
    if(connect(sock, (struct sockaddr*)(&sin), sizeof(struct sockaddr_in))) {
        fprintf(stderr, "failed to connect!\n");
        goto shutdown;
    }

    session = libssh2_session_init();
    if(!session) {
        fprintf(stderr, "Could not initialize SSH session!\n");
        goto shutdown;
    }

    libssh2_session_set_blocking(session, 0);

    while((rc = libssh2_session_handshake(session, sock)) ==
          LIBSSH2_ERROR_EAGAIN);
    if(rc) {
        fprintf(stderr, "Failure establishing SSH session: %d\n", rc);
        goto shutdown;
    }

    while((rc = libssh2_userauth_password(session, username, password)) ==
            LIBSSH2_ERROR_EAGAIN);
    if(rc) {
        fprintf(stderr, "Authentication by password failed!\n");
        goto shutdown;
    }

    do {
        channel = libssh2_channel_open_session(session);
        if(channel ||
           libssh2_session_last_error(session, NULL, NULL, 0) !=
           LIBSSH2_ERROR_EAGAIN)
            break;
        waitsocket(sock, session);
    } while(1);

    if(!channel) {
        fprintf(stderr, "Error\n");
        exit(1);
    }

    while((rc = libssh2_channel_exec(channel, commandline)) ==
          LIBSSH2_ERROR_EAGAIN) {
        waitsocket(sock, session);
    }
    if(rc) {
        fprintf(stderr, "exec error\n");
        exit(1);
    }

    exitcode = 127;
    while((rc = libssh2_channel_close(channel)) == LIBSSH2_ERROR_EAGAIN)
        waitsocket(sock, session);

    if(rc == 0)
        exitcode = libssh2_channel_get_exit_status(channel);

    fprintf(stderr, "\nEXIT: %d\n", exitcode);

    libssh2_channel_free(channel);

shutdown:

    if(session) {
        libssh2_session_disconnect(session, "Normal Shutdown");
        libssh2_session_free(session);
    }

    if(sock != LIBSSH2_INVALID_SOCKET) {
        close(sock);
    }

    fprintf(stderr, "all done\n");

    libssh2_exit();

    return 0;
}

Expected behavior
Channel can be freed early while execute a command that will last a long time.

Version (please complete the following information):

• OS and version: Ubuntu 22.04.2 LTS
• libssh2 version: 1.10.0
• crypto backend and version: OpenSSL 3.0.2

Additional context

[ren mingshuai]

In my opinion, it is normal to wait for the command to finish, regardless of how long the command takes.