Fix a deadlock in SFTP upload.

I tried to do an SFTP upload through connection sharing the other day
and found that pscp sent some data and then hung. Now I debug it, what
seems to have happened was that we were looping in sftp_recv() waiting
for an SFTP packet from the remote, but we didn't have any outstanding
SFTP requests that the remote was going to reply to. Checking further,
xfer_upload_ready() reported true, so we _could_ have sent something -
but the logic in the upload loop had a hole through which we managed
to get into 'waiting for a packet' state.

I think what must have happened is that xfer_upload_ready() reported
false so that we entered sftp_recv(), but then the event loop inside
sftp_recv() ran a toplevel callback that made xfer_upload_ready()
return true. So, the fix: sftp_recv() is our last-ditch fallback, and
we always try emptying our callback queue and rechecking upload_ready
before we resort to waiting for a remote packet.

This not only fixes the hang I observed: it also hugely improves the
upload speed. My guess is that the bug must have been preventing us
from filling our outgoing request pipeline a _lot_ - but I didn't
notice it until the one time the queue accidentally ended up empty,
rather than just sparse enough to make transfers slow.

Annoyingly, I actually considered this fix back when I was trying to
fix the proftpd issue mentioned in commit cd97b7e7e. I decided fixing
ssh_sendbuffer() was a better idea. In fact it would have been an even
better idea to do both! Oh well, better late than never.

(cherry picked from commit 3a633bed35)

pscp.c

@@ -812,6 +812,15 @@ int scp_send_filedata(char *data, int len)
         }
 
         while (!xfer_upload_ready(scp_sftp_xfer)) {
+            if (toplevel_callback_pending()) {
+                /* If we have pending callbacks, they might make
+                 * xfer_upload_ready start to return true. So we should
+                 * run them and then re-check xfer_upload_ready, before
+                 * we go as far as waiting for an entire packet to
+                 * arrive. */
+                run_toplevel_callbacks();
+                continue;
+            }
             pktin = sftp_recv();
             ret = xfer_upload_gotpkt(scp_sftp_xfer, pktin);
             if (ret <= 0) {

psftp.c

@@ -724,6 +724,16 @@ bool sftp_put_file(char *fname, char *outfname, bool recurse, bool restart)
             }
         }
 
+        if (toplevel_callback_pending() && !err && !eof) {
+            /* If we have pending callbacks, they might make
+             * xfer_upload_ready start to return true. So we should
+             * run them and then re-check xfer_upload_ready, before
+             * we go as far as waiting for an entire packet to
+             * arrive. */
+            run_toplevel_callbacks();
+            continue;
+        }
+
         if (!xfer_done(xfer)) {
             pktin = sftp_recv();
             ret = xfer_upload_gotpkt(xfer, pktin);