refactor(sessionresolver): minor changes in files and types naming (#810)

Part of https://github.com/ooni/probe/issues/2135

internal/engine/internal/sessionresolver/lookup.go

@@ -0,0 +1,41 @@
+package sessionresolver
+
+//
+// Actual lookup code
+//
+
+import (
+	"context"
+	"time"
+
+	"github.com/ooni/probe-cli/v3/internal/model"
+)
+
+// defaultTimeLimitedLookupTimeout is the default timeout the code should
+// pass to the timeLimitedLookup function.
+//
+// This algorithm is similar to Firefox using TRR2 mode. See:
+// https://wiki.mozilla.org/Trusted_Recursive_Resolver#DNS-over-HTTPS_Prefs_in_Firefox
+//
+// We use a higher timeout than Firefox's timeout (1.5s) to be on the safe side
+// and therefore see to use DoH more often.
+const defaultTimeLimitedLookupTimeout = 4 * time.Second
+
+// timeLimitedLookup performs a time-limited lookup using the given re.
+func timeLimitedLookup(ctx context.Context, re model.Resolver, hostname string) ([]string, error) {
+	return timeLimitedLookupWithTimeout(ctx, re, hostname, defaultTimeLimitedLookupTimeout)
+}
+
+// timeLimitedLookupWithTimeout is like timeLimitedLookup but with explicit timeout.
+func timeLimitedLookupWithTimeout(ctx context.Context, re model.Resolver,
+	hostname string, timeout time.Duration) ([]string, error) {
+	// In https://github.com/ooni/probe-cli/pull/807, I modified this code to
+	// run in a background goroutine and this resulted in a data race, see
+	// https://github.com/ooni/probe/issues/2135#issuecomment-1149840579. While
+	// I could not reproduce the data race in a simple way, the race itself
+	// seems to happen inside the http3 package. For now, I am going to revert
+	// the change causing the race and I'll investigate later.
+	ctx, cancel := context.WithTimeout(ctx, timeout)
+	defer cancel()
+	return re.LookupHost(ctx, hostname)
+}