spice-qemu-char: Fix flow control in client -> guest direction
In the old spice-vmc device we used to have:
last_out = virtio_serial_write(&svc->port, p, MIN(len, VMC_MAX_HOST_WRITE));
if (last_out > 0)
   ...
Now in the chardev backend we have:
last_out = MIN(len, VMC_MAX_HOST_WRITE);
qemu_chr_read(scd->chr, p, last_out);
if (last_out > 0) {
   ...
Which causes us to no longer detect if the virtio port is not ready
to receive data from us. chardev actually has a mechanism to detect this,
but it requires a separate call to qemu_chr_can_read, before calling
qemu_chr_read (which return void).
This patch uses qemu_chr_can_read to fix the flow control from client to
guest.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
			
			
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					@ -36,14 +36,13 @@ static int vmc_write(SpiceCharDeviceInstance *sin, const uint8_t *buf, int len)
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    while (len > 0) {
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					    while (len > 0) {
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        last_out = MIN(len, VMC_MAX_HOST_WRITE);
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					        last_out = MIN(len, VMC_MAX_HOST_WRITE);
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        qemu_chr_read(scd->chr, p, last_out);
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					        if (qemu_chr_can_read(scd->chr) < last_out) {
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        if (last_out > 0) {
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            out += last_out;
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            len -= last_out;
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            p += last_out;
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        } else {
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            break;
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					            break;
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        }
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					        }
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					        qemu_chr_read(scd->chr, p, last_out);
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					        out += last_out;
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					        len -= last_out;
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					        p += last_out;
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    }
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					    }
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    dprintf(scd, 3, "%s: %lu/%zd\n", __func__, out, len + out);
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					    dprintf(scd, 3, "%s: %lu/%zd\n", __func__, out, len + out);
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