Add nomad monitor export command (#26178)

* Add MonitorExport command and handlers
* Implement autocomplete
* Require nomad in serviceName
* Fix race in StreamReader.Read
* Add and use framer.Flush() to coordinate function exit
* Add LogFile to client/Server config and read NomadLogPath in rpcHandler instead of HTTPServer
* Parameterize StreamFixed stream size

api/agent.go

@@ -302,8 +302,20 @@ func (a *Agent) Host(serverID, nodeID string, q *QueryOptions) (*HostDataRespons
 // Monitor returns a channel which will receive streaming logs from the agent
 // Providing a non-nil stopCh can be used to close the connection and stop log streaming
 func (a *Agent) Monitor(stopCh <-chan struct{}, q *QueryOptions) (<-chan *StreamFrame, <-chan error) {
+	frames, errCh := a.monitorHelper(stopCh, q, "/v1/agent/monitor")
+	return frames, errCh
+}
+
+// MonitorExport returns a channel which will receive streaming logs from the agent
+// Providing a non-nil stopCh can be used to close the connection and stop log streaming
+func (a *Agent) MonitorExport(stopCh <-chan struct{}, q *QueryOptions) (<-chan *StreamFrame, <-chan error) {
+	frames, errCh := a.monitorHelper(stopCh, q, "/v1/agent/monitor/export")
+	return frames, errCh
+}
+
+func (a *Agent) monitorHelper(stopCh <-chan struct{}, q *QueryOptions, path string) (chan *StreamFrame, chan error) {
 	errCh := make(chan error, 1)
-	r, err := a.client.newRequest("GET", "/v1/agent/monitor")
+	r, err := a.client.newRequest("GET", path)
 	if err != nil {
 		errCh <- err
 		return nil, errCh

api/fs.go

@@ -389,12 +389,23 @@ func (f *FrameReader) Read(p []byte) (n int, err error) {
 		case <-unblock:
 			return 0, nil
 		case err := <-f.errCh:
-			return 0, err
+			// check for race with f.frames before returning error
+			select {
+			case frame, ok := <-f.frames:
+				if !ok {
+					return 0, io.EOF
+				}
+				f.frame = frame
+
+				// Store the total offset into the file
+				f.byteOffset = int(f.frame.Offset)
+			default:
+				return 0, err
+			}
 		case <-f.cancelCh:
 			return 0, io.EOF
 		}
 	}
-
 	// Copy the data out of the frame and update our offset
 	n = copy(p, f.frame.Data[f.frameOffset:])
 	f.frameOffset += n