fix(all): introduce and use iox.ReadAllContext (#379)

* fix(all): introduce and use iox.ReadAllContext

This improvement over the ioutil.ReadAll utility returns early
if the context expires. This enables us to unblock stuck code in
case there's censorship confounding the TCP stack.

See https://github.com/ooni/probe/issues/1417.

Compared to the functionality postulated in the above mentioned
issue, I choose to be more generic and separate limiting the
maximum body size (not implemented here) from using the context
to return early when reading a body (or any other reader).

After implementing iox.ReadAllContext, I made sure we always
use it everywhere in the tree instead of ioutil.ReadAll.

This includes many parts of the codebase where in theory we don't
need iox.ReadAllContext. Though, changing all the places makes
checking whether we're not using ioutil.ReadAll where we should
not be using it easy: `git grep` should return no lines.

* Update internal/iox/iox_test.go

* fix(ndt7): treat context errors as non-errors

The rationale is explained by the comment documenting reduceErr.

* Update internal/engine/experiment/ndt7/download.go

internal/iox/example_test.go

@@ -0,0 +1,21 @@
+package iox_test
+
+import (
+	"context"
+	"fmt"
+	"log"
+	"strings"
+
+	"github.com/ooni/probe-cli/v3/internal/iox"
+)
+
+func ExampleReadAllContext() {
+	r := strings.NewReader("deadbeef")
+	ctx := context.Background()
+	out, err := iox.ReadAllContext(ctx, r)
+	if err != nil {
+		log.Fatal(err)
+	}
+	fmt.Printf("%d\n", len(out))
+	// Output: 8
+}

internal/iox/iox.go

@@ -0,0 +1,47 @@
+// Package iox contains io extensions.
+package iox
+
+import (
+	"context"
+	"io"
+)
+
+// ReadAllContext reads the whole reader r in a
+// background goroutine. This function will return
+// earlier if the context is cancelled. In which case
+// we will continue reading from r in the background
+// goroutine, and we will discard the result. To stop
+// the long-running goroutine, you need to close the
+// connection bound to the r reader, if possible.
+func ReadAllContext(ctx context.Context, r io.Reader) ([]byte, error) {
+	datach, errch := make(chan []byte, 1), make(chan error, 1) // buffers
+	go func() {
+		data, err := io.ReadAll(r)
+		if err != nil {
+			errch <- err
+			return
+		}
+		datach <- data
+	}()
+	select {
+	case data := <-datach:
+		return data, nil
+	case <-ctx.Done():
+		return nil, ctx.Err()
+	case err := <-errch:
+		return nil, err
+	}
+}
+
+// MockableReader allows to mock any io.Reader.
+type MockableReader struct {
+	MockRead func(b []byte) (int, error)
+}
+
+// MockableReader implements an io.Reader.
+var _ io.Reader = &MockableReader{}
+
+// Read implements io.Reader.Read.
+func (r *MockableReader) Read(b []byte) (int, error) {
+	return r.MockRead(b)
+}

internal/iox/iox_test.go

@@ -0,0 +1,70 @@
+package iox
+
+import (
+	"context"
+	"errors"
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestReadAllContextCommonCase(t *testing.T) {
+	r := strings.NewReader("deadbeef")
+	ctx := context.Background()
+	out, err := ReadAllContext(ctx, r)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(out) != 8 {
+		t.Fatal("not the expected number of bytes")
+	}
+}
+
+func TestReadAllContextWithError(t *testing.T) {
+	expected := errors.New("mocked error")
+	r := &MockableReader{
+		MockRead: func(b []byte) (int, error) {
+			return 0, expected
+		},
+	}
+	ctx := context.Background()
+	out, err := ReadAllContext(ctx, r)
+	if !errors.Is(err, expected) {
+		t.Fatal("not the error we expected", err)
+	}
+	if len(out) != 0 {
+		t.Fatal("not the expected number of bytes")
+	}
+}
+
+func TestReadAllContextWithCancelledContext(t *testing.T) {
+	r := strings.NewReader("deadbeef")
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel() // fail immediately
+	out, err := ReadAllContext(ctx, r)
+	if !errors.Is(err, context.Canceled) {
+		t.Fatal("not the error we expected", err)
+	}
+	if len(out) != 0 {
+		t.Fatal("not the expected number of bytes")
+	}
+}
+
+func TestReadAllContextWithErrorAndCancelledContext(t *testing.T) {
+	expected := errors.New("mocked error")
+	r := &MockableReader{
+		MockRead: func(b []byte) (int, error) {
+			time.Sleep(time.Millisecond)
+			return 0, expected
+		},
+	}
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel() // fail immediately
+	out, err := ReadAllContext(ctx, r)
+	if !errors.Is(err, context.Canceled) {
+		t.Fatal("not the error we expected", err)
+	}
+	if len(out) != 0 {
+		t.Fatal("not the expected number of bytes")
+	}
+}