lib/vmselectapi: suppress "broken pipe" error logs on vmstorage side

The "broken pipe" error is emitted when the connection has been interrupted abruptly.
It could happen due to unexpected network glitch or because connection was
interrupted by remote client. In both cases, remote client will notice
connection breach and handle it on its own. No need in logging this error
on both: server and client side.

This change should reduce the amount of log noise on vmstorage side. In the same time,
it is not expected to lose any information, since important logs should be still
emitted by the vmselect.

To conduct an experiment for testing this change see the following instructions:
1. Setup vmcluster with at least 2 storage nodes, 1 vminsert and 1 vmselect
2. Run vmselect with complexity limit checked on the client side: `-search.maxSamplesPerQuery=1`
3. Ingest some data and query it back: `count({__name__!=""})`
4. Observe the logs on vmselect and vmstorage side

Before the change, vmselect will log message about complexity limits exceeded. When this happens,
vmselect closes network connections to vmstorage nodes signalizing that it doesn't expect any data back.
Both vmstorage processes will try to push data to the connection and will fail with "broken pipe" error,
means that vmselect closed the connection.

After the change, vmstorages should remain silent. And vmselect will continue emittin the error message
about complexity limits exceeded.

Signed-off-by: hagen1778 <roman@victoriametrics.com>

lib/vmselectapi/server.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"io"
 	"net"
+	"strings"
 	"sync"
 	"sync/atomic"
 	"time"
@@ -256,6 +257,13 @@ func (s *Server) processConn(bc *handshake.BufferedConn) error {
 				// Remote client gracefully closed the connection.
 				return nil
 			}
+			if err == net.ErrClosed || strings.Contains(err.Error(), "broken pipe") {
+				// The connection has been interrupted abruptly.
+				// It could happen due to unexpected network glitch or because connection was
+				// interrupted by remote client. In both cases, remote client will notice
+				// connection breach and handle it on its own. No need in mirroring the error here.
+				return nil
+			}
 			if errors.Is(err, storage.ErrDeadlineExceeded) {
 				return fmt.Errorf("cannot process vmselect request in %d seconds: %w", ctx.timeout, err)
 			}