feat(util:context) add error logging to context

This commit...

is a second attempt at: https://github.com/jbenet/go-ipfs/pull/68

partially addresses: https://github.com/jbenet/go-ipfs/issues/66

is the result of discussion at:

    https://gist.github.com/perfmode/f2951c1ed3a02c484d0b

util/context.go

+package util
+
+import (
+	context "github.com/jbenet/go-ipfs/Godeps/_workspace/src/code.google.com/p/go.net/context"
+)
+
+// privateChanType protects the channel. Since this is a package-private type,
+// only methods defined in this package can get the error value from the
+// context.
+type privateChanType chan error
+
+const errLogKey = "the key used to extract the error log from the context"
+
+// ContextWithErrorLog returns a copy of parent and an error channel that can
+// be used to receive errors sent with the LogError method.
+func ContextWithErrorLog(parent context.Context) (context.Context, <-chan error) {
+	errs := make(privateChanType)
+	ctx := context.WithValue(parent, errLogKey, errs)
+	return ctx, errs
+}
+
+// LogError logs the error to the owner of the context.
+//
+// If this context was created with ContextWithErrorLog, then this method
+// passes the error to context creator over an unbuffered channel.
+//
+// If this context was created by other means, this method is a no-op.
+func LogError(ctx context.Context, err error) {
+	v := ctx.Value(errLogKey)
+	errs, ok := v.(privateChanType)
+	if !ok {
+		return
+	}
+	errs <- err
+}

util/context_test.go

+package util
+
+import (
+	"errors"
+	"testing"
+
+	context "github.com/jbenet/go-ipfs/Godeps/_workspace/src/code.google.com/p/go.net/context"
+)
+
+func TestLogErrorDoesNotBlockWhenCtxIsNotSetUpForLogging(t *testing.T) {
+	ctx := context.Background()
+	LogError(ctx, errors.New("ignore me"))
+}
+
+func TestLogErrorReceivedByParent(t *testing.T) {
+
+	expected := errors.New("From child to parent")
+
+	ctx, errs := ContextWithErrorLog(context.Background())
+
+	go func() {
+		LogError(ctx, expected)
+	}()
+
+	if err := <-errs; err != expected {
+		t.Fatal("didn't receive the expected error")
+	}
+}