TLS: TLSv1.3 earlydata support for curl

Based on #14135, implement TLSv1.3 earlydata support for the curl
command line, libcurl and its implementation in GnuTLS.

If a known TLS session announces early data support, and the feature is
enabled *and* it is not a "connect-only" transfer, delay the TLS
handshake until the first request is being sent.

- Add --tls-earldata as new boolean command line option for curl.
- Add CURLSSLOPT_EARLYDATA to libcurl to enable use of the feature.
- Add CURLINFO_EARLYDATA_SENT_T to libcurl, reporting the amount of
  bytes sent and accepted/rejected by the server.

Implementation details:
- store the ALPN protocol selected at the SSL session.
- When reusing the session and enabling earlydata, use exactly
  that ALPN protocol for negoptiation with the server. When the
  sessions ALPN does not match the connections ALPN, earlydata
  will not be enabled.
- Check that the server selected the correct ALPN protocol for
  an earlydata connect. If the server does not confirm or reports
  something different, the connect fails.
- HTTP/2: delay sending the initial SETTINGS frames during connect,
  if not connect-only.

Verification:
- add test_02_32 to verify earlydata GET with nghttpx.
- add test_07_70 to verify earlydata PUT with nghttpx.
- add support in 'hx-download', 'hx-upload' clients for the feature

Assisted-by: ad-chaos on github
Closes #15211

docs/libcurl/curl_easy_getinfo.md

@@ -112,6 +112,11 @@ curl_easy_header(3) instead. See CURLINFO_CONTENT_TYPE(3)
 
 List of all known cookies. See CURLINFO_COOKIELIST(3)
 
+## CURLINFO_EARLYDATA_SENT_T
+
+Amount of TLS early data sent (in number of bytes) when
+CURLSSLOPT_EARLYDATA is enabled.
+
 ## CURLINFO_EFFECTIVE_METHOD
 
 Last used HTTP method. See CURLINFO_EFFECTIVE_METHOD(3)

docs/libcurl/opts/CURLINFO_EARLYDATA_SENT_T.md

@@ -0,0 +1,75 @@
+---
+c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+SPDX-License-Identifier: curl
+Title: CURLINFO_EARLYDATA_SENT_T
+Section: 3
+Source: libcurl
+See-also:
+  - curl_easy_getinfo (3)
+  - curl_easy_setopt (3)
+Protocol:
+  - TLS
+TLS-backend:
+  - GnuTLS
+Added-in: 8.11.0
+---
+
+# NAME
+
+CURLINFO_EARLYDATA_SENT_T - get the number of bytes sent as TLS early data
+
+# SYNOPSIS
+
+~~~c
+#include <curl/curl.h>
+
+CURLcode curl_easy_getinfo(CURL *handle, CURLINFO_EARLYDATA_SENT_T,
+                           curl_off_t *amount);
+~~~
+
+# DESCRIPTION
+
+Pass a pointer to an *curl_off_t* to receive the total amount of bytes that
+were sent to the server as TLSv1.3 early data. When no TLS early
+data is used, this reports 0.
+
+TLS early data is only attempted when CURLSSLOPT_EARLYDATA is set for the
+transfer. In addition, it is only used by libcurl when a TLS session exists
+that announces support.
+
+The amount is **negative** when the sent data was rejected
+by the server. TLS allows a server that announces support for early data to
+reject any attempt to use it at its own discretion. When for example 127
+bytes had been sent, but were rejected, it reports -127 as the amount "sent".
+
+# %PROTOCOLS%
+
+# EXAMPLE
+
+~~~c
+int main(void)
+{
+  CURL *curl = curl_easy_init();
+  if(curl) {
+    CURLcode res;
+    curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
+
+    /* Perform the request */
+    res = curl_easy_perform(curl);
+
+    if(!res) {
+      curl_off_t amount;
+      res = curl_easy_getinfo(curl, CURLINFO_EARLYDATA_SENT_T, &amount);
+      if(!res) {
+        printf("TLS earlydata: %" CURL_FORMAT_CURL_OFF_T " bytes\n", amount);
+      }
+    }
+  }
+}
+~~~
+
+# %AVAILABILITY%
+
+# RETURN VALUE
+
+Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.

docs/libcurl/opts/CURLOPT_SSL_OPTIONS.md

@@ -86,6 +86,15 @@ certificate that supports client authentication in the OS certificate store it
 could be a privacy violation and unexpected.
 (Added in 7.77.0)
 
+## CURLSSLOPT_EARLYDATA
+
+Tell libcurl to try sending application data as TLS1.3 early data. This option
+is only supported for GnuTLS. This option works on a best effort basis,
+in cases when it wasn't possible to send early data the request is resent
+normally post-handshake.
+This option does not work when using QUIC.
+(Added in 8.11.0)
+
 # DEFAULT
 
 0

docs/libcurl/opts/Makefile.inc

@@ -167,6 +167,7 @@ man_MANS =                                      \
   CURLOPT_DOH_SSL_VERIFYPEER.3                  \
   CURLOPT_DOH_SSL_VERIFYSTATUS.3                \
   CURLOPT_DOH_URL.3                             \
+  CURLINFO_EARLYDATA_SENT_T.3                   \
   CURLOPT_ECH.3                                 \
   CURLOPT_EGDSOCKET.3                           \
   CURLOPT_ERRORBUFFER.3                         \

docs/libcurl/symbols-in-versions

@@ -435,6 +435,7 @@ CURLINFO_COOKIELIST             7.14.1
 CURLINFO_DATA_IN                7.9.6
 CURLINFO_DATA_OUT               7.9.6
 CURLINFO_DOUBLE                 7.4.1
+CURLINFO_EARLYDATA_SENT_T       8.11.0
 CURLINFO_EFFECTIVE_METHOD       7.72.0
 CURLINFO_EFFECTIVE_URL          7.4
 CURLINFO_END                    7.9.6
@@ -1054,6 +1055,7 @@ CURLSSLOPT_NATIVE_CA            7.71.0
 CURLSSLOPT_NO_PARTIALCHAIN      7.68.0
 CURLSSLOPT_NO_REVOKE            7.44.0
 CURLSSLOPT_REVOKE_BEST_EFFORT   7.70.0
+CURLSSLOPT_EARLYDATA            8.11.0
 CURLSSLSET_NO_BACKENDS          7.56.0
 CURLSSLSET_OK                   7.56.0
 CURLSSLSET_TOO_LATE             7.56.0