refactor: flatten and separate (#353)

* refactor(atomicx): move outside the engine package

After merging probe-engine into probe-cli, my impression is that we have
too much unnecessary nesting of packages in this repository.

The idea of this commit and of a bunch of following commits will instead
be to reduce the nesting and simplify the structure.

While there, improve the documentation.

* fix: always use the atomicx package

For consistency, never use sync/atomic and always use ./internal/atomicx
so we can just grep and make sure we're not risking to crash if we make
a subtle mistake on a 32 bit platform.

While there, mention in the contributing guidelines that we want to
always prefer the ./internal/atomicx package over sync/atomic.

* fix(atomicx): remove unnecessary constructor

We don't need a constructor here. The default constructed `&Int64{}`
instance is already usable and the constructor does not add anything to
what we are doing, rather it just creates extra confusion.

* cleanup(atomicx): we are not using Float64

Because atomicx.Float64 is unused, we can safely zap it.

* cleanup(atomicx): simplify impl and improve tests

We can simplify the implementation by using defer and by letting
the Load() method call Add(0).

We can improve tests by making many goroutines updated the
atomic int64 value concurrently.

* refactor(fsx): can live in the ./internal pkg

Let us reduce the amount of nesting. While there, ensure that the
package only exports the bare minimum, and improve the documentation
of the tests, to ease reading the code.

* refactor: move runtimex to ./internal

* refactor: move shellx into the ./internal package

While there, remove unnecessary dependency between packages.

While there, specify in the contributing guidelines that
one should use x/sys/execabs instead of os/exec.

* refactor: move ooapi into the ./internal pkg

* refactor(humanize): move to ./internal and better docs

* refactor: move platform to ./internal

* refactor(randx): move to ./internal

* refactor(multierror): move into the ./internal pkg

* refactor(kvstore): all kvstores in ./internal

Rather than having part of the kvstore inside ./internal/engine/kvstore
and part in ./internal/engine/kvstore.go, let us put every piece of code
that is kvstore related into the ./internal/kvstore package.

* fix(kvstore): always return ErrNoSuchKey on Get() error

It should help to use the kvstore everywhere removing all the
copies that are lingering around the tree.

* sessionresolver: make KVStore mandatory

Simplifies implementation. While there, use the ./internal/kvstore
package rather than having our private implementation.

* fix(ooapi): use the ./internal/kvstore package

* fix(platform): better documentation

cmd/ooniprobe/internal/autorun/autorun_darwin.go

@@ -13,7 +13,7 @@ import (
 
 	"github.com/apex/log"
 	"github.com/ooni/probe-cli/v3/cmd/ooniprobe/internal/utils"
-	"github.com/ooni/probe-cli/v3/internal/engine/shellx"
+	"github.com/ooni/probe-cli/v3/internal/shellx"
 	"golang.org/x/sys/execabs"
 	"golang.org/x/sys/unix"
 )

cmd/ooniprobe/internal/nettests/dnscheck.go

@@ -5,7 +5,7 @@ import (
 
 	"github.com/ooni/probe-cli/v3/internal/engine/experiment/dnscheck"
 	"github.com/ooni/probe-cli/v3/internal/engine/experiment/run"
-	"github.com/ooni/probe-cli/v3/internal/engine/runtimex"
+	"github.com/ooni/probe-cli/v3/internal/runtimex"
 )
 
 // DNSCheck nettest implementation.

cmd/ooniprobe/internal/ooni/ooni.go

@@ -6,7 +6,6 @@ import (
 	"io/ioutil"
 	"os"
 	"os/signal"
-	"sync/atomic"
 	"syscall"
 
 	"github.com/apex/log"
@@ -14,8 +13,10 @@ import (
 	"github.com/ooni/probe-cli/v3/cmd/ooniprobe/internal/database"
 	"github.com/ooni/probe-cli/v3/cmd/ooniprobe/internal/enginex"
 	"github.com/ooni/probe-cli/v3/cmd/ooniprobe/internal/utils"
-	engine "github.com/ooni/probe-cli/v3/internal/engine"
+	"github.com/ooni/probe-cli/v3/internal/atomicx"
+	"github.com/ooni/probe-cli/v3/internal/engine"
 	"github.com/ooni/probe-cli/v3/internal/engine/legacy/assetsdir"
+	"github.com/ooni/probe-cli/v3/internal/kvstore"
 	"github.com/pkg/errors"
 	"upper.io/db.v3/lib/sqlbuilder"
 )
@@ -53,11 +54,7 @@ type Probe struct {
 	dbPath     string
 	configPath string
 
-	// We need to use a int32 in order to use the atomic.AddInt32/LoadInt32
-	// operations to ensure consistent reads of the variables. We do not use
-	// a 64 bit integer here because that may lead to crashes with 32 bit
-	// OSes as documented in https://golang.org/pkg/sync/atomic/#pkg-note-BUG.
-	isTerminatedAtomicInt int32
+	isTerminated *atomicx.Int64
 
 	softwareName    string
 	softwareVersion string
@@ -96,13 +93,12 @@ func (p *Probe) TempDir() string {
 // IsTerminated checks to see if the isTerminatedAtomicInt is set to a non zero
 // value and therefore we have received the signal to shutdown the running test
 func (p *Probe) IsTerminated() bool {
-	i := atomic.LoadInt32(&p.isTerminatedAtomicInt)
-	return i != 0
+	return p.isTerminated.Load() != 0
 }
 
 // Terminate interrupts the running context
 func (p *Probe) Terminate() {
-	atomic.AddInt32(&p.isTerminatedAtomicInt, 1)
+	p.isTerminated.Add(1)
 }
 
 // ListenForSignals will listen for SIGINT and SIGTERM. When it receives those
@@ -203,7 +199,7 @@ func (p *Probe) Init(softwareName, softwareVersion string) error {
 // current configuration inside the context. The caller must close
 // the session when done using it, by calling sess.Close().
 func (p *Probe) NewSession(ctx context.Context) (*engine.Session, error) {
-	kvstore, err := engine.NewFileSystemKVStore(
+	kvstore, err := kvstore.NewFS(
 		utils.EngineDir(p.home),
 	)
 	if err != nil {
@@ -234,10 +230,10 @@ func (p *Probe) NewProbeEngine(ctx context.Context) (ProbeEngine, error) {
 // NewProbe creates a new probe instance.
 func NewProbe(configPath string, homePath string) *Probe {
 	return &Probe{
-		home:                  homePath,
-		config:                &config.Config{},
-		configPath:            configPath,
-		isTerminatedAtomicInt: 0,
+		home:         homePath,
+		config:       &config.Config{},
+		configPath:   configPath,
+		isTerminated: &atomicx.Int64{},
 	}
 }