net.http: enable HTTP/2 by default for https requests (#27384)

* net.http: enable HTTP/2 by default for https requests

Flip the `enable_http2` default on Request and FetchConfig from false to true,
so https requests made with http.fetch / http.get negotiate HTTP/2 when the
server offers it (ALPN `h2`), and transparently fall back to HTTP/1.1 otherwise.
Set `enable_http2: false` to force HTTP/1.1.

This is opt-out rather than opt-in now that the HTTP/2 client supports
redirects, streaming response callbacks, and flow control, and has been
exercised against real servers. Plain http:// is unaffected (no ALPN), and the
Windows SChannel backend keeps using HTTP/1.1 until it gains ALPN (#27383).

Updates the gated real-server test to assert the new default (a plain get()
negotiates HTTP/2) and that `enable_http2: false` still forces HTTP/1.1.

Notes for review:
- No latency regression: the client sends the preface, SETTINGS, and the
  request together without waiting for the server's SETTINGS, so there is no
  extra round trip; HPACK also shrinks request headers.
- Each fetch is still one connection / one request (no pooling yet), so the
  multiplexing win does not yet apply; this change is about defaulting to the
  newer protocol, with pooling/multiplexing as a follow-up.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

* net.http: gate default-h2 network test on Windows; refresh ALPN comment

Address review feedback on enabling HTTP/2 by default:

- test_http2_fetch_real_server now skips on the default Windows/SChannel
  configuration (`$if windows && !no_vschannel ?`), since SChannel has no ALPN
  yet (#27383) and a default get() there stays on HTTP/1.1. The path remains
  covered with `-d no_vschannel` (mbedtls client).
- Refresh the now-stale comment in backend.c.v: with enable_http2 defaulting to
  true, ordinary get()/fetch() calls advertise ALPN and use HTTP/2 when the
  server selects it (opt out with enable_http2: false).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

---------

Co-authored-by: Richard Wheeler <quaesitor.scientiam@gmail.com>
Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

Author: 3 people

vlib/net/http/backend.c.v

@@ -21,10 +21,12 @@ fn net_ssl_do(req &Request, port int, method Method, host_name string, path stri
 		eprint(req_headers)
 		eprintln('')
 	}
-	// Advertise ALPN `h2` (with an `http/1.1` fallback) only when HTTP/2 is
-	// requested, so existing callers see no change on the wire. The HTTP/2 read
-	// path now feeds the same streaming callbacks and honors the stop limits,
-	// so they no longer force HTTP/1.1.
+	// Advertise ALPN `h2` (with an `http/1.1` fallback) when HTTP/2 is enabled.
+	// This is the default for https requests, so ordinary get()/fetch() calls
+	// advertise ALPN and use HTTP/2 when the server selects it; callers can opt
+	// out with `enable_http2: false`. The HTTP/2 read path feeds the same
+	// streaming callbacks and honors the stop limits, so they do not force
+	// HTTP/1.1.
 	alpn := if req.enable_http2 { ['h2', 'http/1.1'] } else { []string{} }
 	for {
 		mut ssl_conn := ssl.new_ssl_conn(

vlib/net/http/h2_client_test.v

@@ -79,18 +79,26 @@ fn test_h2_authority_omits_default_port() {
 	assert h2_authority('example.com', 8443) == 'example.com:8443'
 }
 
-// Opt-in end-to-end test against a real HTTP/2 server. Run with `-d network`,
+// End-to-end test against a real HTTP/2 server. Run with `-d network`,
 // e.g. `v -d network test vlib/net/http/h2_client_test.v`.
 fn test_http2_fetch_real_server() {
 	$if !network ? {
 		return
 	}
-	resp := fetch(url: 'https://www.google.com/', enable_http2: true)!
+	$if windows && !no_vschannel ? {
+		// On Windows the default HTTPS client is SChannel, which has no ALPN
+		// yet (vlang/v#27383), so it cannot negotiate HTTP/2. Covered with
+		// `-d no_vschannel` (mbedtls client).
+		eprintln('skipping: SChannel client has no ALPN/HTTP2 support yet')
+		return
+	}
+	// HTTP/2 is negotiated by default for https requests.
+	resp := get('https://www.google.com/')!
 	assert resp.version() == .v2_0
 	assert resp.status_code == 200
 	assert resp.body.len > 0
-	// Without opting in, the same server is reached over HTTP/1.1.
-	plain := get('https://www.google.com/')!
+	// Opting out forces HTTP/1.1 against the same server.
+	plain := fetch(url: 'https://www.google.com/', enable_http2: false)!
 	assert plain.version() == .v1_1
 }
 

vlib/net/http/http.v

@@ -36,7 +36,7 @@ pub mut:
 	in_memory_verification bool   // if true, verify, cert, and cert_key are read from memory, not from a file
 	allow_redirect         bool = true // whether to allow redirect
 	max_retries            int  = 5    // maximum number of retries required when an underlying socket error occurs
-	enable_http2           bool // opt in to HTTP/2 over TLS: advertise ALPN `h2` and, if the server selects it, speak HTTP/2 instead of HTTP/1.1. on_progress / on_progress_body / stop_copying_limit / stop_receiving_limit are honored on the HTTP/2 path; on_progress fires per DATA frame payload rather than per raw network read.
+	enable_http2           bool = true // when true (the default) and the URL is https, advertise ALPN `h2, http/1.1` and use HTTP/2 if the server selects it; set to false to force HTTP/1.1. Ignored for plain http://, and for the Windows SChannel backend which has no ALPN yet (see vlang/v#27383). on_progress / on_progress_body / stop_copying_limit / stop_receiving_limit are honored on the HTTP/2 path; on_progress fires per DATA frame payload rather than per raw network read.
 	// callbacks to allow custom reporting code to run, while the request is running, and to implement streaming
 	on_redirect      RequestRedirectFn     = unsafe { nil }
 	on_progress      RequestProgressFn     = unsafe { nil }

vlib/net/http/request.v

@@ -46,7 +46,7 @@ pub mut:
 	in_memory_verification bool // if true, verify, cert, and cert_key are read from memory, not from a file
 	allow_redirect         bool = true // whether to allow redirect
 	max_retries            int  = 5    // maximum number of retries required when an underlying socket error occurs
-	enable_http2           bool // opt in to HTTP/2 over TLS: advertise ALPN `h2` and, if the server selects it, speak HTTP/2 instead of HTTP/1.1. on_progress / on_progress_body / stop_copying_limit / stop_receiving_limit are honored on the HTTP/2 path; on_progress fires per DATA frame payload rather than per raw network read.
+	enable_http2           bool = true // when true (the default) and the URL is https, advertise ALPN `h2, http/1.1` and use HTTP/2 if the server selects it; set to false to force HTTP/1.1. Ignored for plain http://, and for the Windows SChannel backend which has no ALPN yet (see vlang/v#27383). on_progress / on_progress_body / stop_copying_limit / stop_receiving_limit are honored on the HTTP/2 path; on_progress fires per DATA frame payload rather than per raw network read.
 	// callbacks to allow custom reporting code to run, while the request is running, and to implement streaming
 	on_redirect      RequestRedirectFn     = unsafe { nil }
 	on_progress      RequestProgressFn     = unsafe { nil }