Correctly track the original length when generating a stream frame

txp_generate_stream_frames() plans chunks of data to send via the
function txp_plan_stream_chunk(). That function may clamp the amount in
the chunk due to flow control, even though there is more available to send.

We should take this into account when deciding whether or not to try
serializing the next chunk.

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from #22601)

(cherry picked from commit e718b24)

ssl/quic/quic_txp.c

@@ -2073,6 +2073,7 @@ static int txp_generate_crypto_frames(OSSL_QUIC_TX_PACKETISER *txp,
 
 struct chunk_info {
     OSSL_QUIC_FRAME_STREAM shdr;
+    uint64_t orig_len;
     OSSL_QTX_IOVEC iov[2];
     size_t num_stream_iovec;
     int valid;
@@ -2099,6 +2100,8 @@ static int txp_plan_stream_chunk(OSSL_QUIC_TX_PACKETISER *txp,
         /* Should only have 0-length chunk if FIN */
         return 0;
 
+    chunk->orig_len = chunk->shdr.len;
+
     /* Clamp according to connection and stream-level TXFC. */
     fc_credit   = ossl_quic_txfc_get_credit(stream_txfc);
     fc_swm      = ossl_quic_txfc_get_swm(stream_txfc);
@@ -2199,7 +2202,7 @@ static int txp_generate_stream_frames(OSSL_QUIC_TX_PACKETISER *txp,
             goto err;
 
         shdr = &chunks[i % 2].shdr;
-        orig_len = shdr->len;
+        orig_len = chunks[i % 2].orig_len;
         if (i > 0)
             /* Load next chunk for lookahead. */
             if (!txp_plan_stream_chunk(txp, h, sstream, stream_txfc, i + 1,
@@ -2331,8 +2334,7 @@ static int txp_generate_stream_frames(OSSL_QUIC_TX_PACKETISER *txp,
         if (shdr->len < orig_len) {
             /*
              * If we did not serialize all of this chunk we definitely do not
-             * want to try the next chunk (and we must not mark the stream
-             * as drained).
+             * want to try the next chunk
              */
             rc = 1;
             goto err;