From 419f6404d46d8f5e404988149a0881d015113bb9 Mon Sep 17 00:00:00 2001
From: hunterinvariants <217359725+hunterinvariants@users.noreply.github.com>
Date: Tue, 11 Aug 2026 06:43:06 +0200
Subject: [PATCH] schannel: reuse the send buffer

- Reuse a buffer for schannel_send instead of allocating and freeing a
  buffer for every send.

Closes https://github.com/curl/curl/pull/22540
---
 lib/vtls/schannel.c     | 19 +++++++++++++------
 lib/vtls/schannel_int.h |  2 ++
 2 files changed, 15 insertions(+), 6 deletions(-)

diff --git a/lib/vtls/schannel.c b/lib/vtls/schannel.c
index 88d7ccec88..aca109bfdc 100644
--- a/lib/vtls/schannel.c
+++ b/lib/vtls/schannel.c
@@ -1996,13 +1996,18 @@ static CURLcode schannel_send(struct Curl_cfilter *cf, struct Curl_easy *data,
     len = backend->stream_sizes.cbMaximumMessage;
   }
 
-  /* calculate the complete message length and allocate a buffer for it */
+  /* calculate the complete message length and prepare the send buffer */
   data_len = backend->stream_sizes.cbHeader + len +
     backend->stream_sizes.cbTrailer;
-  ptr = curlx_malloc(data_len);
-  if(!ptr) {
-    return CURLE_OUT_OF_MEMORY;
+  if(data_len > backend->send_buffer_len) {
+    ptr = curlx_realloc(backend->send_buffer, data_len);
+    if(!ptr)
+      return CURLE_OUT_OF_MEMORY;
+    backend->send_buffer = ptr;
+    backend->send_buffer_len = data_len;
   }
+  else
+    ptr = backend->send_buffer;
 
   /* setup output buffers (header, data, trailer, empty) */
   InitSecBuffer(&outbuf[0], SECBUFFER_STREAM_HEADER,
@@ -2091,8 +2096,6 @@ static CURLcode schannel_send(struct Curl_cfilter *cf, struct Curl_easy *data,
     result = CURLE_SEND_ERROR;
   }
 
-  curlx_safefree(ptr);
-
   if(len == *pnwritten)
     /* Encrypted message including header, data and trailer entirely sent.
        The return value is the number of unencrypted bytes that were sent. */
@@ -2567,6 +2570,10 @@ static void schannel_close(struct Curl_cfilter *cf, struct Curl_easy *data)
     backend->cred = NULL;
   }
 
+  /* free the buffer used to encrypt outgoing data */
+  curlx_safefree(backend->send_buffer);
+  backend->send_buffer_len = 0;
+
   /* free internal buffer for received encrypted data */
   if(backend->encdata.buffer) {
     curlx_safefree(backend->encdata.buffer);
diff --git a/lib/vtls/schannel_int.h b/lib/vtls/schannel_int.h
index 242f6a0f44..ccbb63d7e2 100644
--- a/lib/vtls/schannel_int.h
+++ b/lib/vtls/schannel_int.h
@@ -112,6 +112,8 @@ struct sbuffer {
 };
 
 struct schannel_ssl_backend_data {
+  unsigned char *send_buffer;
+  size_t send_buffer_len;
   struct sbuffer encdata;
   struct sbuffer decdata;
   struct Curl_schannel_cred *cred;