io: Yield rather than wait when already in coroutine

The new qio_channel_{read,write}{,v}_all functions are documented
as yielding until data is available.  When used on a blocking
channel, this yield is done via qio_channel_wait() which spawns
a nested event loop under the hood (so it is that secondary loop
which yields as needed); but if we are already in a coroutine (at
which point QIO_CHANNEL_ERR_BLOCK is only possible if we are a
non-blocking channel), we want to yield the current coroutine
instead of spawning a nested event loop.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20170905191114.5959-2-eblake@redhat.com>
Acked-by: Daniel P. Berrange <berrange@redhat.com>
[commit message updated]
Signed-off-by: Eric Blake <eblake@redhat.com>

diff --git a/io/channel.c b/io/channel.c
index 5e8c2f0a91918ac05f99af3cd2478cbbb8bd9d69..9e62794cab278c625dcac242ef154a070f6dd4e6 100644 (file)
--- a/io/channel.c
+++ b/io/channel.c
@@ -105,7 +105,11 @@ int qio_channel_readv_all(QIOChannel *ioc,
         ssize_t len;
         len = qio_channel_readv(ioc, local_iov, nlocal_iov, errp);
         if (len == QIO_CHANNEL_ERR_BLOCK) {
-            qio_channel_wait(ioc, G_IO_IN);
+            if (qemu_in_coroutine()) {
+                qio_channel_yield(ioc, G_IO_IN);
+            } else {
+                qio_channel_wait(ioc, G_IO_IN);
+            }
             continue;
         } else if (len < 0) {
             goto cleanup;
@@ -143,7 +147,11 @@ int qio_channel_writev_all(QIOChannel *ioc,
         ssize_t len;
         len = qio_channel_writev(ioc, local_iov, nlocal_iov, errp);
         if (len == QIO_CHANNEL_ERR_BLOCK) {
-            qio_channel_wait(ioc, G_IO_OUT);
+            if (qemu_in_coroutine()) {
+                qio_channel_yield(ioc, G_IO_OUT);
+            } else {
+                qio_channel_wait(ioc, G_IO_OUT);
+            }
             continue;
         }
         if (len < 0) {