tool: fix memory use in parallel mode

The curl tool was creating a new transfer every time it checked if
it needed to add one to reach max parallelism. This led to eventually
all configured transfers to have easy handles created.

Limit the creation again to the ones needed for max parallelism.

scorecard.py: set --out-null only for curl versions that support it

Closes #22277

src/tool_operate.c

@@ -1552,7 +1552,13 @@ static CURLcode add_parallel_transfers(CURLM *multi, CURLSH *share,
     return CURLE_UNKNOWN_OPTION;
   }
 
-  if(nxfers < (curl_off_t)(global->parallel_max * 2)) {
+  if(all_added >= global->parallel_max) {
+    /* we are at max parallelism, no need to create more transfers */
+    *morep = TRUE; /* pretend there are, we have not checked */
+    return CURLE_OK;
+  }
+  /* if the list is empty, create one to kickstart the loop */
+  if(!transfers) {
     bool skipped = FALSE;
     do {
       result = create_transfer(share, addedp, &skipped);
@@ -1560,6 +1566,7 @@ static CURLcode add_parallel_transfers(CURLM *multi, CURLSH *share,
         return result;
     } while(skipped);
   }
+  /* add transfers until max parallelism is achieved again */
   for(per = transfers; per && (all_added < global->parallel_max);
       per = per->next) {
     if(per->added || per->skip)

tests/http/testenv/curl.py

@@ -729,7 +729,10 @@ class CurlClient:
         if extra_args is None:
             extra_args = []
         if no_save:
-            extra_args.extend(['--out-null'])
+            if self.env.curl_version_at_least('8.16.0'):
+                extra_args.extend(['--out-null'])
+            else:
+                extra_args.extend(['-o', '/dev/null'])
         else:
             extra_args.extend(['-o', 'download_#1.data'])
         if limit_rate:

tests/http/testenv/env.py

@@ -512,6 +512,13 @@ class Env:
     def curl_version_string() -> str:
         return Env.CONFIG.curl_props["version_string"]
 
+    @staticmethod
+    def curl_version_at_least(min_version) -> bool:
+        version = Env.curl_version()
+        return Env.CONFIG.versiontuple(min_version) <= Env.CONFIG.versiontuple(
+                version
+            )
+
     @staticmethod
     def curl_features_string() -> str:
         return Env.CONFIG.curl_props["features_string"]