Add "fully set" flag to AgentToServer message

This is a draft PR to demonstrate how open-telemetry/opamp-spec#109
can be resolved.

The new StatusIsFullSet is set by the agent the status is fully set the message.
The server checks the flag to know if it needs to request the full status.

This is a backward compatible change. New servers that work with old agents may
make an extra request for the full status because old agents don't set this flag,
but this was already a problem in the past even before this change.

client/clientimpl_test.go

@@ -696,6 +696,7 @@ func TestReportAgentDescription(t *testing.T) {
 			assert.EqualValues(t, 0, msg.SequenceNum)
 			// The first status report after Start must have full AgentDescription.
 			assert.True(t, proto.Equal(client.AgentDescription(), msg.AgentDescription))
+			assert.True(t, msg.Flags&protobufs.AgentToServer_StatusIsFullySet != 0)
 			return &protobufs.ServerToAgent{InstanceUid: msg.InstanceUid}
 		})
 
@@ -709,6 +710,7 @@ func TestReportAgentDescription(t *testing.T) {
 			assert.Nil(t, msg.AgentDescription)
 
 			assert.EqualValues(t, 1, msg.SequenceNum)
+			assert.True(t, msg.Flags&protobufs.AgentToServer_StatusIsFullySet == 0)
 
 			// Ask client for full AgentDescription.
 			return &protobufs.ServerToAgent{
@@ -725,6 +727,7 @@ func TestReportAgentDescription(t *testing.T) {
 			// The status report must again have full AgentDescription
 			// because the Server asked for it.
 			assert.True(t, proto.Equal(client.AgentDescription(), msg.AgentDescription))
+			assert.True(t, msg.Flags&protobufs.AgentToServer_StatusIsFullySet != 0)
 			return &protobufs.ServerToAgent{InstanceUid: msg.InstanceUid}
 		})
 

client/internal/clientcommon.go

@@ -180,6 +180,7 @@ func (c *ClientCommon) PrepareFirstMessage(ctx context.Context) error {
 			msg.EffectiveConfig = cfg
 			msg.RemoteConfigStatus = c.ClientSyncedState.RemoteConfigStatus()
 			msg.PackageStatuses = c.ClientSyncedState.PackageStatuses()
+			msg.Flags |= protobufs.AgentToServer_StatusIsFullySet
 
 			if c.PackagesStateProvider != nil {
 				// We have a state provider, so package related capabilities can work.

client/internal/receivedprocessor.go

@@ -121,6 +121,7 @@ func (r *receivedProcessor) rcvFlags(
 				msg.Health = r.clientSyncedState.Health()
 				msg.RemoteConfigStatus = r.clientSyncedState.RemoteConfigStatus()
 				msg.PackageStatuses = r.clientSyncedState.PackageStatuses()
+				msg.Flags |= protobufs.AgentToServer_StatusIsFullySet
 
 				// The logic for EffectiveConfig is similar to the previous 4 sub-messages however
 				// the EffectiveConfig is fetched using GetEffectiveConfig instead of

internal/examples/server/data/agent.go

@@ -198,8 +198,17 @@ func (agent *Agent) processStatusUpdate(
 	newStatus *protobufs.AgentToServer,
 	response *protobufs.ServerToAgent,
 ) {
-	if agent.Status != nil && agent.Status.SequenceNum+1 != newStatus.SequenceNum {
-		// We lost the previous status update. Request full status update from the agent.
+	// The status is not fully set in the message that we received. Agent compressed the status
+	// by omitting some fields.
+	receivedFullStatus := newStatus.Flags&protobufs.AgentToServer_StatusIsFullySet != 0
+
+	// We don't have any status for this Agent, or we lost the previous status update from the Agent, so our
+	// current status is not up-to-date.
+	lostPreviousUpdate := (agent.Status == nil) || (agent.Status != nil && agent.Status.SequenceNum+1 != newStatus.SequenceNum)
+
+	if !receivedFullStatus && lostPreviousUpdate {
+		// The status message is not fully set in the message that we received, but we lost the previous
+		// status update. Request full status update from the agent.
 		response.Flags |= protobufs.ServerToAgent_ReportFullState
 	}
 
@@ -219,8 +228,8 @@ func (agent *Agent) processStatusUpdate(
 	// If remote config is changed and different from what the Agent has then
 	// send the new remote config to the Agent.
 	if configChanged ||
-		(newStatus.RemoteConfigStatus != nil &&
-			bytes.Compare(newStatus.RemoteConfigStatus.LastRemoteConfigHash, agent.remoteConfig.ConfigHash) != 0) {
+		(agent.Status.RemoteConfigStatus != nil &&
+			bytes.Compare(agent.Status.RemoteConfigStatus.LastRemoteConfigHash, agent.remoteConfig.ConfigHash) != 0) {
 		// The new status resulted in a change in the config of the Agent or the Agent
 		// does not have this config (hash is different). Send the new config the Agent.
 		response.RemoteConfig = agent.remoteConfig

internal/proto/opamp.proto

@@ -80,6 +80,12 @@ message AgentToServer {
         // The Agent requests Server go generate a new instance_uid, which will
         // be sent back in ServerToAgent message
         RequestInstanceUid     = 0x00000001;
+
+        // Indicates that the Agent included full status in this message, that none
+        // of the the status fields is omitted. This tells the Server that it is
+        // that this message is full and there is no need to request full status.
+        // See https://github.com/open-telemetry/opamp-spec/blob/main/specification.md#agent-status-compression
+        StatusIsFullySet       = 0x00000002;
     }
     // Bit flags as defined by AgentToServerFlags bit masks.
     AgentToServerFlags flags = 10;