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libusb backend: attempt to fix DRAIN_OUTPUT race condition. Fixes #1461 #1500

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Merged
michaelrsweet merged 3 commits into from
Mar 4, 2026
Merged

libusb backend: attempt to fix DRAIN_OUTPUT race condition. Fixes #1461 #1500

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3715ea2
libusb backend: attempt to fix DRAIN_OUTPUT race condition. Fixes #1461
ValdikSS Feb 26, 2026
6c12bbf
libusb backend: use pipe to wake up select()'ing main thread
ValdikSS Feb 26, 2026
510ebb3
libusb backend: send DRAIN response after drain_output is handled
ValdikSS Feb 26, 2026
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47 changes: 45 additions & 2 deletions backend/usb-libusb.c
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Original file line number Diff line number Diff line change
Expand Up @@ -78,6 +78,8 @@ typedef struct usb_globals_s /* Global USB printer information */
pthread_cond_t sidechannel_thread_cond;
int sidechannel_thread_stop;
int sidechannel_thread_done;

int wakeup_pipe[2]; /* Pipe for waking up select() in the main thread */
} usb_globals_t;

/*
Expand Down Expand Up @@ -188,12 +190,14 @@ print_device(const char *uri, /* I - Device URI */
*print_ptr; /* Pointer into print data buffer */
fd_set input_set; /* Input set for select() */
int nfds; /* Number of file descriptors */
int nfd_max; /* Maximum file descriptor number for select() */
struct timeval *timeout, /* Timeout pointer */
tv; /* Time value */
struct timespec cond_timeout; /* pthread condition timeout */
int num_opts; /* Number of options */
cups_option_t *opts; /* Options */
const char *val; /* Option value */
char dummy; /* select() wakeup bogus data */


load_quirks();
Expand All @@ -205,6 +209,17 @@ print_device(const char *uri, /* I - Device URI */
have_sidechannel = !fstat(CUPS_SC_FD, &sidechannel_info) &&
S_ISSOCK(sidechannel_info.st_mode);

/*
* Create a pipe for waking up the main thread from select()...
*/

if (pipe(g.wakeup_pipe) < 0)
{
fprintf(stderr, "DEBUG: Unable to create wakeup pipe - %s\n",
strerror(errno));
return (CUPS_BACKEND_STOP);
}

g.wait_eof = WAIT_EOF;

/*
Expand Down Expand Up @@ -346,12 +361,15 @@ print_device(const char *uri, /* I - Device URI */
lseek(print_fd, 0, SEEK_SET);
}

nfd_max = (print_fd > g.wakeup_pipe[0] ? print_fd : g.wakeup_pipe[0]) + 1;

while (status == CUPS_BACKEND_OK)
{
FD_ZERO(&input_set);

if (!g.print_bytes)
FD_SET(print_fd, &input_set);
FD_SET(g.wakeup_pipe[0], &input_set);

/*
* Calculate select timeout...
Expand Down Expand Up @@ -384,7 +402,7 @@ print_device(const char *uri, /* I - Device URI */
pthread_cond_signal(&g.readwrite_lock_cond);
pthread_mutex_unlock(&g.readwrite_lock_mutex);

nfds = select(print_fd + 1, &input_set, NULL, NULL, timeout);
nfds = select(nfd_max, &input_set, NULL, NULL, timeout);

/*
* Reacquire the lock...
Expand Down Expand Up @@ -415,15 +433,22 @@ print_device(const char *uri, /* I - Device URI */
}
}

/*
* Check if we were woken up by the sidechannel thread...
*/

if (FD_ISSET(g.wakeup_pipe[0], &input_set))
read(g.wakeup_pipe[0], &dummy, 1);

/*
* If drain output has finished send a response...
*/

if (g.drain_output && !nfds && !g.print_bytes)
{
/* Send a response... */
cupsSideChannelWrite(CUPS_SC_CMD_DRAIN_OUTPUT, CUPS_SC_STATUS_OK, NULL, 0, 1.0);
g.drain_output = 0;
cupsSideChannelWrite(CUPS_SC_CMD_DRAIN_OUTPUT, CUPS_SC_STATUS_OK, NULL, 0, 1.0);
}

/*
Expand Down Expand Up @@ -641,6 +666,13 @@ print_device(const char *uri, /* I - Device URI */

close_device(g.printer);

/*
* Close the wakeup pipe...
*/

close(g.wakeup_pipe[0]);
close(g.wakeup_pipe[1]);

/*
* Clean up ....
*/
Expand Down Expand Up @@ -1885,6 +1917,17 @@ sidechannel_thread(void *reference)
stderr);

g.drain_output = 1;
pthread_mutex_lock(&g.readwrite_lock_mutex);
if (!g.readwrite_lock)
{
/*
* If main thread is inside select(), assume it has infinite
* timeout and needs to be woken up.
*/
if (write(g.wakeup_pipe[1], &data, 1) < 0)
fputs("DEBUG: Error writing to wakeup pipe\n", stderr);
}
pthread_mutex_unlock(&g.readwrite_lock_mutex);
break;

case CUPS_SC_CMD_GET_BIDI: /* Is the connection bidirectional? */
Expand Down
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