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

internal/platform/platform.go

@@ -0,0 +1,51 @@
+// Package platform returns the platform name. The name returned here
+// is compatible with the names returned by Measurement Kit.
+package platform
+
+import "runtime"
+
+// Name returns the platform name. The returned value is one of:
+//
+// 1. "android"
+//
+// 2. "ios"
+//
+// 3. "linux"
+//
+// 5. "macos"
+//
+// 4. "windows"
+//
+// 5. "unknown"
+//
+// The android, ios, linux, macos, windows, and unknown strings are
+// also returned by Measurement Kit. As a known bug, the detection of
+// darwin-based systems relies on the architecture, when CGO support
+// has been disabled. In such case, the code will return "ios" when
+// using arm{,64} and "macos" when using x86{,_64}.
+func Name() string {
+	if name := cgoname(); name != "unknown" {
+		return name
+	}
+	return puregoname(runtime.GOOS, runtime.GOARCH)
+}
+
+func puregoname(goos, goarch string) string {
+	switch goos {
+	case "android", "linux", "windows":
+		return goos
+	case "darwin":
+		return detectDarwin(goarch)
+	}
+	return "unknown"
+}
+
+func detectDarwin(goarch string) string {
+	switch goarch {
+	case "386", "amd64":
+		return "macos"
+	case "arm", "arm64":
+		return "ios"
+	}
+	return "unknown"
+}

internal/platform/platform_cgo.go

@@ -0,0 +1,31 @@
+// +build cgo
+
+package platform
+
+//
+// /* Guess the platform in which we are.
+//
+//     See: <https://sourceforge.net/p/predef/wiki/OperatingSystems/>
+//          <http://stackoverflow.com/a/18729350> */
+//
+//#if defined __ANDROID__
+//#  define OONI_PLATFORM "android"
+//#elif defined __linux__
+//#  define OONI_PLATFORM "linux"
+//#elif defined _WIN32
+//#  define OONI_PLATFORM "windows"
+//#elif defined __APPLE__
+//#  include <TargetConditionals.h>
+//#  if TARGET_OS_IPHONE
+//#    define OONI_PLATFORM "ios"
+//#  else
+//#    define OONI_PLATFORM "macos"
+//#  endif
+//#else
+//#  define OONI_PLATFORM "unknown"
+//#endif
+import "C"
+
+func cgoname() string {
+	return C.OONI_PLATFORM
+}

internal/platform/platform_otherwise.go

@@ -0,0 +1,7 @@
+// +build !cgo
+
+package platform
+
+func cgoname() string {
+	return "unknown"
+}

internal/platform/platform_test.go

@@ -0,0 +1,68 @@
+package platform
+
+import (
+	"fmt"
+	"testing"
+)
+
+func TestGood(t *testing.T) {
+	var expected bool
+	switch Name() {
+	case "android", "ios", "linux", "macos", "windows":
+		expected = true
+	}
+	if !expected {
+		t.Fatal("unexpected platform name")
+	}
+}
+
+func TestPuregoname(t *testing.T) {
+	var runtimevariables = []struct {
+		expected string
+		goarch   string
+		goos     string
+	}{{
+		expected: "android",
+		goarch:   "*",
+		goos:     "android",
+	}, {
+		expected: "ios",
+		goarch:   "arm64",
+		goos:     "darwin",
+	}, {
+		expected: "ios",
+		goarch:   "arm",
+		goos:     "darwin",
+	}, {
+		expected: "linux",
+		goarch:   "*",
+		goos:     "linux",
+	}, {
+		expected: "macos",
+		goarch:   "amd64",
+		goos:     "darwin",
+	}, {
+		expected: "macos",
+		goarch:   "386",
+		goos:     "darwin",
+	}, {
+		expected: "unknown",
+		goarch:   "*",
+		goos:     "solaris",
+	}, {
+		expected: "unknown",
+		goarch:   "mips",
+		goos:     "darwin",
+	}, {
+		expected: "windows",
+		goarch:   "*",
+		goos:     "windows",
+	}}
+	for _, v := range runtimevariables {
+		t.Run(fmt.Sprintf("with %s/%s", v.goos, v.goarch), func(t *testing.T) {
+			if puregoname(v.goos, v.goarch) != v.expected {
+				t.Fatal("unexpected results")
+			}
+		})
+	}
+}