[libvirt] [PATCH v2] stream: Check for stream EOF

[Michal Privoznik]

If client stream does not have any data to sink and neither received
EOF, a dummy packet is sent to the daemon signalising client is ready to
sink some data. However, after we added event loop to client a race may
occur:

Thread 1 calls virNetClientStreamRecvPacket and since no data are cached
nor stream has EOF, it decides to send dummy packet to server which will
sent some data in turn. However, during this decision and actual message
exchange with server -

Thread 2 receives last stream data from server. Therefore an EOF is set
on stream and if there is a call waiting (which is not yet) it is woken
up. However, Thread 1 haven't sent anything so far, so there is no call
to be woken up. So this thread sent dummy packet to daemon, which
ignores that as no stream is associated with such packet and therefore
no reply will ever come.

This race causes client to hang indefinitely.
---
diff to v1:
-moved client locking one level higher
-checking for stream EOF one level higher too

 src/rpc/virnetclient.c       |   51 +++++++++++++++++++++++++++++++++++++-----
 src/rpc/virnetclient.h       |    3 ++
 src/rpc/virnetclientstream.c |    7 +++++-
 src/rpc/virnetclientstream.h |    2 +
 4 files changed, 56 insertions(+), 7 deletions(-)

diff --git a/src/rpc/virnetclient.c b/src/rpc/virnetclient.c
index 469c6a5..ffe067c 100644
--- a/src/rpc/virnetclient.c
+++ b/src/rpc/virnetclient.c
@@ -1703,8 +1703,6 @@ static int virNetClientSendInternal(virNetClientPtr client,
         return -1;
     }
 
-    virNetClientLock(client);
-
     if (!client->sock || client->wantClose) {
         virNetError(VIR_ERR_INTERNAL_ERROR, "%s",
                     _("client socket is closed"));
@@ -1739,7 +1737,6 @@ static int virNetClientSendInternal(virNetClientPtr client,
 cleanup:
     if (ret != 1)
         VIR_FREE(call);
-    virNetClientUnlock(client);
     return ret;
 }
 
@@ -1757,7 +1754,10 @@ cleanup:
 int virNetClientSendWithReply(virNetClientPtr client,
                               virNetMessagePtr msg)
 {
-    int ret = virNetClientSendInternal(client, msg, true, false);
+    int ret;
+    virNetClientLock(client);
+    ret = virNetClientSendInternal(client, msg, true, false);
+    virNetClientUnlock(client);
     if (ret < 0)
         return -1;
     return 0;
@@ -1777,7 +1777,10 @@ int virNetClientSendWithReply(virNetClientPtr client,
 int virNetClientSendNoReply(virNetClientPtr client,
                             virNetMessagePtr msg)
 {
-    int ret = virNetClientSendInternal(client, msg, false, false);
+    int ret;
+    virNetClientLock(client);
+    ret = virNetClientSendInternal(client, msg, false, false);
+    virNetClientUnlock(client);
     if (ret < 0)
         return -1;
     return 0;
@@ -1796,5 +1799,41 @@ int virNetClientSendNoReply(virNetClientPtr client,
 int virNetClientSendNonBlock(virNetClientPtr client,
                              virNetMessagePtr msg)
 {
-    return virNetClientSendInternal(client, msg, false, true);
+    int ret;
+    virNetClientLock(client);
+    ret = virNetClientSendInternal(client, msg, false, true);
+    virNetClientUnlock(client);
+    return ret;
+}
+
+/*
+ * @msg: a message allocated on heap or stack
+ *
+ * Send a message synchronously, and wait for the reply synchronously
+ *
+ * The caller is responsible for free'ing @msg if it was allocated
+ * on the heap
+ *
+ * Returns 0 on success, -1 on failure
+ */
+int virNetClientSendWithReplyStream(virNetClientPtr client,
+                                    virNetMessagePtr msg,
+                                    virNetClientStreamPtr st)
+{
+    int ret;
+    virNetClientLock(client);
+    /* Other thread might have already received
+     * stream EOF so we don't want sent anything.
+     * Server won't respond anyway.
+     */
+    if (virNetClientStreamEOF(st)) {
+        virNetClientUnlock(client);
+        return 0;
+    }
+
+    ret = virNetClientSendInternal(client, msg, true, false);
+    virNetClientUnlock(client);
+    if (ret < 0)
+        return -1;
+    return 0;
 }
diff --git a/src/rpc/virnetclient.h b/src/rpc/virnetclient.h
index 61d51e1..7c30d2b 100644
--- a/src/rpc/virnetclient.h
+++ b/src/rpc/virnetclient.h
@@ -76,6 +76,9 @@ int virNetClientSendNoReply(virNetClientPtr client,
 int virNetClientSendNonBlock(virNetClientPtr client,
                              virNetMessagePtr msg);
 
+int virNetClientSendWithReplyStream(virNetClientPtr client,
+                                    virNetMessagePtr msg,
+                                    virNetClientStreamPtr st);
 
 # ifdef HAVE_SASL
 void virNetClientSetSASLSession(virNetClientPtr client,
diff --git a/src/rpc/virnetclientstream.c b/src/rpc/virnetclientstream.c
index a4292e7..be06c66 100644
--- a/src/rpc/virnetclientstream.c
+++ b/src/rpc/virnetclientstream.c
@@ -408,7 +408,7 @@ int virNetClientStreamRecvPacket(virNetClientStreamPtr st,
 
         VIR_DEBUG("Dummy packet to wait for stream data");
         virMutexUnlock(&st->lock);
-        ret = virNetClientSendWithReply(client, msg);
+        ret = virNetClientSendWithReplyStream(client, msg, st);
         virMutexLock(&st->lock);
         virNetMessageFree(msg);
 
@@ -530,3 +530,8 @@ cleanup:
     virMutexUnlock(&st->lock);
     return ret;
 }
+
+bool virNetClientStreamEOF(virNetClientStreamPtr st)
+{
+    return st->incomingEOF;
+}
diff --git a/src/rpc/virnetclientstream.h b/src/rpc/virnetclientstream.h
index 6c8d538..fd7a2ee 100644
--- a/src/rpc/virnetclientstream.h
+++ b/src/rpc/virnetclientstream.h
@@ -72,5 +72,7 @@ int virNetClientStreamEventUpdateCallback(virNetClientStreamPtr st,
                                           int events);
 int virNetClientStreamEventRemoveCallback(virNetClientStreamPtr st);
 
+bool virNetClientStreamEOF(virNetClientStreamPtr st)
+    ATTRIBUTE_NONNULL(1);
 
 #endif /* __VIR_NET_CLIENT_STREAM_H__ */
-- 
1.7.3.4