osslq: use SSL_poll to determine writeability of QUIC streams

This discussion:
https://github.com/openssl/openssl/discussions/23339#discussion-6094341

Specifically item number 2 (Send Blocking) was raised by the curl team,
noting that SSL_want_write returning false was not a good indicator of
when a stream is writeable. The suggestion in that discussion was to use
SSL_poll with an SSL_POLL_EVENT_W flag instead, as that is a proper
indication of when an SSL_object will allow writing without blocking.

While ssl_want_write updates its state based on the last error
encountered (implying a need to retry an operation to update the
last_error state again), SSL_poll checks stream buffer status during the
call, giving it more up to date information on request. This is the
method used by our guide demos (quic-hq-interop specifically), and it
works well.

This change has been run through the curl test suite, and shown to pass
all tests. However, given the initial problem description I'm not sure
if there is a test case that explicitly checks for blocking and
unblocking of streams. As such some additional testing may be warranted.

Closes #15909

lib/vquic/curl_osslq.c

@@ -292,6 +292,9 @@ struct cf_osslq_ctx {
   struct Curl_hash streams;          /* hash `data->mid` to `h3_stream_ctx` */
   size_t max_stream_window;          /* max flow window for one stream */
   uint64_t max_idle_ms;              /* max idle time for QUIC connection */
+  SSL_POLL_ITEM *poll_items;         /* Array for polling on writable state */
+  struct Curl_easy **curl_items;     /* Array of easy objs */
+  size_t item_count;                 /* count of elements in poll/curl_items */
   BIT(initialized);
   BIT(got_first_byte);               /* if first byte was received */
   BIT(x509_store_setup);             /* if x509 store has been set up */
@@ -308,6 +311,9 @@ static void cf_osslq_ctx_init(struct cf_osslq_ctx *ctx)
   Curl_bufcp_init(&ctx->stream_bufcp, H3_STREAM_CHUNK_SIZE,
                   H3_STREAM_POOL_SPARES);
   Curl_hash_offt_init(&ctx->streams, 63, h3_stream_hash_free);
+  ctx->poll_items = NULL;
+  ctx->curl_items = NULL;
+  ctx->item_count = 0;
   ctx->initialized = TRUE;
 }
 
@@ -318,6 +324,8 @@ static void cf_osslq_ctx_free(struct cf_osslq_ctx *ctx)
     Curl_hash_clean(&ctx->streams);
     Curl_hash_destroy(&ctx->streams);
     Curl_ssl_peer_cleanup(&ctx->peer);
+    free(ctx->poll_items);
+    free(ctx->curl_items);
   }
   free(ctx);
 }
@@ -1461,24 +1469,80 @@ static CURLcode cf_osslq_check_and_unblock(struct Curl_cfilter *cf,
 {
   struct cf_osslq_ctx *ctx = cf->ctx;
   struct h3_stream_ctx *stream;
+  size_t poll_count = 0;
+  size_t result_count = 0;
+  size_t idx_count = 0;
+  CURLcode res = CURLE_OK;
+  struct timeval timeout;
+  void *tmpptr;
 
   if(ctx->h3.conn) {
     struct Curl_llist_node *e;
+
+    res = CURLE_OUT_OF_MEMORY;
+
+    if(ctx->item_count < Curl_llist_count(&data->multi->process)) {
+      ctx->item_count = 0;
+      tmpptr = realloc(ctx->poll_items,
+                       Curl_llist_count(&data->multi->process) *
+                       sizeof(SSL_POLL_ITEM));
+      if(!tmpptr) {
+        free(ctx->poll_items);
+        ctx->poll_items = NULL;
+        goto out;
+      }
+      ctx->poll_items = tmpptr;
+
+      tmpptr = realloc(ctx->curl_items,
+                       Curl_llist_count(&data->multi->process) *
+                       sizeof(struct Curl_easy *));
+      if(!tmpptr) {
+        free(ctx->curl_items);
+        ctx->curl_items = NULL;
+        goto out;
+      }
+      ctx->curl_items = tmpptr;
+
+      ctx->item_count = Curl_llist_count(&data->multi->process);
+    }
+
     for(e = Curl_llist_head(&data->multi->process); e; e = Curl_node_next(e)) {
       struct Curl_easy *sdata = Curl_node_elem(e);
       if(sdata->conn == data->conn) {
         stream = H3_STREAM_CTX(ctx, sdata);
-        if(stream && stream->s.ssl && stream->s.send_blocked &&
-           !SSL_want_write(stream->s.ssl)) {
-          nghttp3_conn_unblock_stream(ctx->h3.conn, stream->s.id);
-          stream->s.send_blocked = FALSE;
-          h3_drain_stream(cf, sdata);
-          CURL_TRC_CF(sdata, cf, "unblocked");
+        if(stream && stream->s.ssl && stream->s.send_blocked) {
+          ctx->poll_items[poll_count].desc =
+            SSL_as_poll_descriptor(stream->s.ssl);
+          ctx->poll_items[poll_count].events = SSL_POLL_EVENT_W;
+          ctx->curl_items[poll_count] = sdata;
+          poll_count++;
         }
       }
     }
+
+    memset(&timeout, 0, sizeof(struct timeval));
+    res = CURLE_UNRECOVERABLE_POLL;
+    if(!SSL_poll(ctx->poll_items, poll_count, sizeof(SSL_POLL_ITEM), &timeout,
+                 0, &result_count))
+        goto out;
+
+    res = CURLE_OK;
+
+    for(idx_count = 0; idx_count < poll_count && result_count > 0;
+        idx_count++) {
+      if(ctx->poll_items[idx_count].revents & SSL_POLL_EVENT_W) {
+        stream = H3_STREAM_CTX(ctx, ctx->curl_items[idx_count]);
+        nghttp3_conn_unblock_stream(ctx->h3.conn, stream->s.id);
+        stream->s.send_blocked = FALSE;
+        h3_drain_stream(cf, ctx->curl_items[idx_count]);
+        CURL_TRC_CF(ctx->curl_items[idx_count], cf, "unblocked");
+        result_count--;
+      }
+    }
   }
-  return CURLE_OK;
+
+out:
+  return res;
 }
 
 static CURLcode h3_send_streams(struct Curl_cfilter *cf,