Convert more of the SSL write path to size_t and Spans.

We still have our <= 0 return values because anything with BIOs tries to
preserve BIO_write's error returns. (Maybe we can stop doing this?
BIO_read's error return is a little subtle with EOF vs error, but
BIO_write's is uninteresting.) But the rest of the logic is size_t-clean
and hopefully a little clearer. We still have to support SSL_write's
rather goofy calling convention, however.

I haven't pushed Spans down into the low-level record construction logic
yet. We should probably do that, but there are enough offsets tossed
around there that they warrant their own CL.

Bug: 507
Change-Id: Ia0c702d1a2d3713e71b0bbfa8d65649d3b20da9b
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/47544
Commit-Queue: Bob Beck <bbe@google.com>
Reviewed-by: Bob Beck <bbe@google.com>

ssl/ssl_lib.cc

diff --git a/ssl/ssl_lib.cc b/ssl/ssl_lib.cc
index 7035748..4d3ad44 100644
--- a/ssl/ssl_lib.cc
+++ b/ssl/ssl_lib.cc
@@ -1058,6 +1058,7 @@
   }
 
   int ret = 0;
+  size_t bytes_written = 0;
   bool needs_handshake = false;
   do {
     // If necessary, complete the handshake implicitly.
@@ -1072,10 +1073,16 @@
       }
     }
 
-    ret = ssl->method->write_app_data(ssl, &needs_handshake,
-                                      (const uint8_t *)buf, num);
+    if (num < 0) {
+      OPENSSL_PUT_ERROR(SSL, SSL_R_BAD_LENGTH);
+      return -1;
+    }
+    ret = ssl->method->write_app_data(
+        ssl, &needs_handshake, &bytes_written,
+        MakeConstSpan(static_cast<const uint8_t *>(buf),
+                      static_cast<size_t>(num)));
   } while (needs_handshake);
-  return ret;
+  return ret <= 0 ? ret : static_cast<int>(bytes_written);
 }
 
 int SSL_key_update(SSL *ssl, int request_type) {
@@ -1239,8 +1246,7 @@
   // Discard any unfinished writes from the perspective of |SSL_write|'s
   // retry. The handshake will transparently flush out the pending record
   // (discarded by the server) to keep the framing correct.
-  ssl->s3->wpend_buf = nullptr;
-  ssl->s3->wpend_tot = 0;
+  ssl->s3->pending_write = {};
 }
 
 enum ssl_early_data_reason_t SSL_get_early_data_reason(const SSL *ssl) {