libstore: warn if http3 && !http2

curl does not support this configuration. we can't easily change the
config setup to mirror how curl works, but we *can* print a warning.

fixes #1223

Change-Id: Iaa1583275d9cc9e5eed5552210db183a8e0628c8

lix/libstore/file-transfer-settings/http2.md

@@ -5,3 +5,4 @@ type: bool
 default: true
 ---
 Whether to enable HTTP/2 support.
+HTTP/2 support cannot be disabled if HTTP/3 support is enabled, the `http2` setting will be ignored in this case.

lix/libstore/file-transfer-settings/http3.md

@@ -5,3 +5,4 @@ type: bool
 default: false
 ---
 Whether to enable HTTP/3 support.
+Enabling HTTP/3 support forcibly enables HTTP/2 as well; the `http2` will be ignored in this case.

lix/libstore/filetransfer.cc

@@ -23,6 +23,7 @@
 #include <kj/encoding.h>
 #include <kj/time.h>
 #include <memory>
+#include <mutex>
 
 #if ENABLE_DTRACE
 #include "trace-probes.gen.hh"
@@ -196,6 +197,12 @@ struct TransferItem
         );
         curl_easy_setopt(req.get(), CURLOPT_PIPEWAIT, 1);
         if (fileTransferSettings.enableHttp3) {
+            if (!fileTransferSettings.enableHttp2) {
+                static std::once_flag warningPrinted;
+                std::call_once(warningPrinted, [] {
+                    printTaggedWarning("http3 implies http2; ignoring explicit http2 setting.");
+                });
+            }
             curl_easy_setopt(req.get(), CURLOPT_HTTP_VERSION, CURL_HTTP_VERSION_3);
         } else if (fileTransferSettings.enableHttp2) {
             curl_easy_setopt(req.get(), CURLOPT_HTTP_VERSION, CURL_HTTP_VERSION_2TLS);