code health: fix all places highlighted by linter

Changed error's suppression in check.yaml. The suppression of rule
`errcheck` may be removed after adding errors check in all methods
with calling encodeXxx inside. See details below.

Suppressed the highlighting of error check in all methods,
having encodeXxx inside. For now these methods are not able to
return any error cause of internal implementation of writer
interface (see smallbuf.go). For future, if the implementation of writer
will be changed, and there will be a need to check errors, we must think
about how to say to compiler that the error check is 'unlikely'.

Fixed the use of time package API, all places with calls of
time.Now().Sub(). Now time package propose the explicit time.Until().

Replaced all calls of Errorf() with Fatalf() in tests. That change
prevents nil dereferences below in the code and stops test function
execution, where it is expected in tests.

Suppressed the highlighting of all unused constants and functions

Fixed calling of Fatalf from non-testing goroutine in queue tests.
It is not a valid way to stop test from another goroutine.

Fixed gofmt-highlighted places in test_helpers/pool_helper.go and
connection_pool/const.go.

Added instructions to CONTRIBUTING.md how to run CI-linter locally.

Closes #142
Closes #150

Author: vr009

.github/workflows/check.yaml

@@ -39,4 +39,8 @@ jobs:
       - name: golangci-lint
         uses: golangci/golangci-lint-action@v2
         with:
-          args: --issues-exit-code=0 -E gofmt
+          # The suppression of rule `errcheck` may be removed after adding
+          # errors check in all methods with calling EncodeXxx inside.
+          # For now those methods are not even able to return any error
+          # cause of internal implementation of writer interface (see smallbuf.go).
+          args: -E gofmt -D errcheck

CONTRIBUTING.md

@@ -38,6 +38,13 @@ For example, for running tests in `multi`, `uuid` and `main` packages, call
 make test-multi test-uuid test-main
 ```
 
+To check if the current changes will pass the linter in CI, install
+golnagci-lint from [sources](https://golangci-lint.run/usage/install/) 
+and run it with next flags:
+```bash
+golangci-lint run -E gofmt -D errcheck ./... 
+```
+
 ## Code review checklist
 
 - Public API contains functions, variables, constants that are needed from

connection.go

@@ -147,7 +147,7 @@ type connShard struct {
 	bufmut sync.Mutex
 	buf    smallWBuf
 	enc    *msgpack.Encoder
-	_pad   [16]uint64
+	_pad   [16]uint64 //nolint: unused,structcheck
 }
 
 // Greeting is a message sent by Tarantool on connect.
@@ -495,7 +495,7 @@ func (conn *Connection) createConnection(reconnect bool) (err error) {
 		conn.notify(ReconnectFailed)
 		reconnects++
 		conn.mutex.Unlock()
-		time.Sleep(now.Add(conn.opts.Reconnect).Sub(time.Now()))
+		time.Sleep(time.Until(now.Add(conn.opts.Reconnect)))
 		conn.mutex.Lock()
 	}
 	if conn.state == connClosed {
@@ -688,7 +688,7 @@ func (conn *Connection) newFuture(requestCode int32) (fut *Future) {
 	*pair.last = fut
 	pair.last = &fut.next
 	if conn.opts.Timeout > 0 {
-		fut.timeout = time.Now().Sub(epoch) + conn.opts.Timeout
+		fut.timeout = time.Until(epoch) + conn.opts.Timeout
 	}
 	shard.rmut.Unlock()
 	if conn.rlimit != nil && conn.opts.RLimitAction == RLimitWait {
@@ -796,9 +796,9 @@ func (conn *Connection) timeouts() {
 			return
 		case <-t.C:
 		}
-		minNext := time.Now().Sub(epoch) + timeout
+		minNext := time.Until(epoch) + timeout
 		for i := range conn.shard {
-			nowepoch = time.Now().Sub(epoch)
+			nowepoch = time.Until(epoch)
 			shard := &conn.shard[i]
 			for pos := range shard.requests {
 				shard.rmut.Lock()
@@ -825,7 +825,7 @@ func (conn *Connection) timeouts() {
 				shard.rmut.Unlock()
 			}
 		}
-		nowepoch = time.Now().Sub(epoch)
+		nowepoch = time.Until(epoch)
 		if nowepoch+time.Microsecond < minNext {
 			t.Reset(minNext - nowepoch)
 		} else {