feat: clearly indicate which resolver we're using (#885)

See what we documented at https://github.com/ooni/spec/pull/257

Reference issue: https://github.com/ooni/probe/issues/2238

See also the related ooni/spec PR: https://github.com/ooni/spec/pull/257

See also https://github.com/ooni/probe/issues/2237

While there, bump webconnectivity@v0.5 version because this change
has an impact onto the generated data format.

The drop in coverage is unavoidable because we've written some
tests for `measurex` to ensure we deal with DNS resolvers and transport
names correctly depending on the splitting policy we use.

(However, `measurex` is only used for the `tor` experiment and, per
the step-by-step design document, new experiments should use
`measurexlite` instead, so this is hopefully fine(TM).)

While there, fix a broken integration test that does not run in `-short` mode.

internal/netxlite/dnsovergetaddrinfo.go

@@ -24,6 +24,11 @@ type dnsOverGetaddrinfoTransport struct {
 	testableLookupANY func(ctx context.Context, domain string) ([]string, string, error)
 }
 
+// NewDNSOverGetaddrinfoTransport creates a new dns-over-getaddrinfo transport.
+func NewDNSOverGetaddrinfoTransport() model.DNSTransport {
+	return &dnsOverGetaddrinfoTransport{}
+}
+
 var _ model.DNSTransport = &dnsOverGetaddrinfoTransport{}
 
 func (txp *dnsOverGetaddrinfoTransport) RoundTrip(

internal/netxlite/doc.go

@@ -47,7 +47,7 @@
 //
 // 1. establishing a TCP connection;
 //
-// 2. performing a domain name resolution with the "system" resolver
+// 2. performing a domain name resolution with the "stdlib" resolver
 // (i.e., getaddrinfo on Unix) or custom DNS transports (e.g., DoT, DoH);
 //
 // 3. performing the TLS handshake;

internal/netxlite/getaddrinfo.go

@@ -2,6 +2,23 @@ package netxlite
 
 import "errors"
 
+// Name of the resolver we use when we link with libc and use getaddrinfo directly.
+//
+// See https://github.com/ooni/spec/pull/257 for more info.
+const StdlibResolverGetaddrinfo = "getaddrinfo"
+
+// Name of the resolver we use when we don't link with libc and use net.Resolver.
+//
+// See https://github.com/ooni/spec/pull/257 for more info.
+const StdlibResolverGolangNetResolver = "golang_net_resolver"
+
+// Legacy name of the resolver we use when we're don't know whether we're using
+// getaddrinfo, but we're using net.Resolver, and we're splitting the answer
+// in two A and AAAA queries. Eventually will become deprecated.
+//
+// See https://github.com/ooni/spec/pull/257 for more info.
+const StdlibResolverSystem = "system"
+
 // ErrGetaddrinfo represents a getaddrinfo failure.
 type ErrGetaddrinfo struct {
 	// Err is the error proper.

internal/netxlite/getaddrinfo_cgo.go

@@ -28,10 +28,13 @@ import (
 // been used to implement the getaddrinfo resolver.
 //
 // This is the CGO_ENABLED=1 implementation of this function, which
-// always returns the string "system", because in this scenario
+// always returns the string [StdlibResolverGetaddrinfo], because in this scenario
 // we are actually calling the getaddrinfo libc function.
+//
+// See https://github.com/ooni/spec/pull/257 for more information on how
+// we evolved our naming of the "stdlib" resolver over time.
 func getaddrinfoResolverNetwork() string {
-	return "system"
+	return StdlibResolverGetaddrinfo
 }
 
 // getaddrinfoLookupANY attempts to perform an ANY lookup using getaddrinfo.

internal/netxlite/getaddrinfo_otherwise.go

@@ -11,13 +11,16 @@ import (
 // been used to implement the getaddrinfo resolver.
 //
 // This is the CGO_ENABLED=0 implementation of this function, which
-// always returns the string "go", because in this scenario we are actually
-// using whatever resolver is used under the hood by the stdlib.
+// always returns the string [StdlibResolverGolangNetResolver], because in this scenario
+// we are actually using whatever resolver is used under the hood by the stdlib.
 //
 // See https://github.com/ooni/probe/issues/2029#issuecomment-1140805266
-// for an explanation of why calling this resolver "netgo" is wrong.
+// for an explanation of why calling this resolver "netgo" was wrong.
+//
+// See https://github.com/ooni/spec/pull/257 for additional documentation
+// regarding using "golang_net_resolver" instead of "go".
 func getaddrinfoResolverNetwork() string {
-	return "go"
+	return StdlibResolverGolangNetResolver
 }
 
 // getaddrinfoLookupANY attempts to perform an ANY lookup using getaddrinfo.

internal/netxlite/resolvercore.go

@@ -20,11 +20,11 @@ import (
 
 // ErrNoDNSTransport is the error returned when you attempt to perform
 // a DNS operation that requires a custom DNSTransport (e.g., DNSOverHTTPSTransport)
-// but you are using the "system" resolver instead.
+// but you are using the "stdlib" resolver instead.
 var ErrNoDNSTransport = errors.New("operation requires a DNS transport")
 
 // NewStdlibResolver creates a new Resolver by combining WrapResolver
-// with an internal "system" resolver type. The list of optional wrappers
+// with an internal "stdlib" resolver type. The list of optional wrappers
 // allow to wrap the underlying getaddrinfo transport. Any nil wrapper
 // will be silently ignored by the code that performs the wrapping.
 func NewStdlibResolver(logger model.DebugLogger, wrappers ...model.DNSTransportWrapper) model.Resolver {
@@ -45,7 +45,7 @@ func NewParallelDNSOverHTTPSResolver(logger model.DebugLogger, URL string) model
 // implies, this function returns an unwrapped resolver.
 func NewUnwrappedStdlibResolver(wrappers ...model.DNSTransportWrapper) model.Resolver {
 	return &resolverSystem{
-		t: WrapDNSTransport(&dnsOverGetaddrinfoTransport{}, wrappers...),
+		t: WrapDNSTransport(NewDNSOverGetaddrinfoTransport(), wrappers...),
 	}
 }
 

internal/netxlite/resolvercore_test.go

@@ -347,7 +347,7 @@ func TestResolverLogger(t *testing.T) {
 						return expected, nil
 					},
 					MockNetwork: func() string {
-						return "system"
+						return StdlibResolverGetaddrinfo
 					},
 					MockAddress: func() string {
 						return ""
@@ -381,7 +381,7 @@ func TestResolverLogger(t *testing.T) {
 						return nil, expected
 					},
 					MockNetwork: func() string {
-						return "system"
+						return StdlibResolverGetaddrinfo
 					},
 					MockAddress: func() string {
 						return ""
@@ -420,7 +420,7 @@ func TestResolverLogger(t *testing.T) {
 						return expected, nil
 					},
 					MockNetwork: func() string {
-						return "system"
+						return StdlibResolverGetaddrinfo
 					},
 					MockAddress: func() string {
 						return ""
@@ -454,7 +454,7 @@ func TestResolverLogger(t *testing.T) {
 						return nil, expected
 					},
 					MockNetwork: func() string {
-						return "system"
+						return StdlibResolverGetaddrinfo
 					},
 					MockAddress: func() string {
 						return ""
@@ -509,7 +509,7 @@ func TestResolverLogger(t *testing.T) {
 						return expected, nil
 					},
 					MockNetwork: func() string {
-						return "system"
+						return StdlibResolverGetaddrinfo
 					},
 					MockAddress: func() string {
 						return ""
@@ -543,7 +543,7 @@ func TestResolverLogger(t *testing.T) {
 						return nil, expected
 					},
 					MockNetwork: func() string {
-						return "system"
+						return StdlibResolverGetaddrinfo
 					},
 					MockAddress: func() string {
 						return ""