refactor(measurex): allow to configure timeouts and max-snapshot-size (#645)

This diff lightly refactors the code in measurex to allow a user
to configure all possible timeouts and the max-snapshot-size.

There is currently a little bit of tension between setting timeouts
inside of measurex and the watchdog timeouts inside of netxlite.

This tension has been documented.

Let us repeat the issue also in this commit message. If you are
using a masurex.Measurer configured with very large timeouts and
the underlying netxlite implementation uses shorter whatchdog
timeouts, then you are going to see shorter than expected timeouts.

Ideally, we would like to have just a single timeout but there is
no way to ask the context "hey, can you tell me if you already have
a configured timeout?".

It may be that the right solution is to modify netxlite to have
some sort of root/library object with this configuration.

If that's the case, then a Measurer could be refactored as follows:

- create the underlying netxlite "library"

- initialize the timeouts desired by the Measurer

- create a Dialer, of whatever is needed

- use it

Now this is not possible because netxlite timeouts are internal
static settings rather than attributes of a structure.

Anyway, for now I'm happy with this just being documented.

(I suspect this issue will need to be addresses when we'll write
unit tests for measurex; at that time a proper solution should
come out naturally due to the unit tests constraints.)

I'm working on this refactoring, BTW, to facilitate rewriting `tor`
using measurex (see https://github.com/ooni/probe/issues/1688).

internal/measurex/http.go

@@ -37,21 +37,43 @@ import (
 // HTTP events into the WritableDB.
 func (mx *Measurer) WrapHTTPTransport(
 	db WritableDB, txp model.HTTPTransport) *HTTPTransportDB {
-	return WrapHTTPTransport(mx.Begin, db, txp)
+	return WrapHTTPTransport(mx.Begin, db, txp, mx.httpMaxBodySnapshotSize())
 }
 
-// We only read a small snapshot of the body to keep measurements
-// lean, since we're mostly interested in TLS interference nowadays
-// but we'll also allow for reading more bytes from the conn.
-const httpMaxBodySnapshot = 1 << 11
+// DefaultHTTPMaxBodySnapshotSize is the default size used when
+// saving HTTP body snapshots. We only save a small snapshot of the
+// body to keep measurements lean, since we're mostly interested
+// in TLS interference nowadays and much less in full bodies.
+const DefaultHTTPMaxBodySnapshotSize = 1 << 11
 
+// httpMaxBodySnapshotSize selects the maximum body snapshot size.
+func (mx *Measurer) httpMaxBodySnapshotSize() int64 {
+	if mx.HTTPMaxBodySnapshotSize > 0 {
+		return mx.HTTPMaxBodySnapshotSize
+	}
+	return DefaultHTTPMaxBodySnapshotSize
+}
+
+// WrapHTTPTransport creates a new model.HTTPTransport instance
+// using the following configuration:
+//
+// - begin is the conventional "zero time" indicating the
+// moment when the measurement begun;
+//
+// - db is the writable DB into which to write the measurement;
+//
+// - txp is the underlying transport to use;
+//
+// - maxBodySnapshotSize is the max size of the response body snapshot
+// to save: we'll truncate bodies larger than that.
 func WrapHTTPTransport(
-	begin time.Time, db WritableDB, txp model.HTTPTransport) *HTTPTransportDB {
+	begin time.Time, db WritableDB, txp model.HTTPTransport,
+	maxBodySnapshotSize int64) *HTTPTransportDB {
 	return &HTTPTransportDB{
 		HTTPTransport:       txp,
 		Begin:               begin,
 		DB:                  db,
-		MaxBodySnapshotSize: httpMaxBodySnapshot,
+		MaxBodySnapshotSize: maxBodySnapshotSize,
 	}
 }