lib: CURL_WRITEFUNC_ABORT is a new write callback error

docs/libcurl/opts/CURLOPT_WRITEFUNCTION.3

@@ -63,8 +63,11 @@ that amount differs from the amount passed to your callback function, it
 signals an error condition to the library. This causes the transfer to get
 aborted and the libcurl function used returns \fICURLE_WRITE_ERROR\fP.
 
-You can also abort the transfer by returning CURL_WRITEFUNC_ERROR (added in
-7.87.0), which makes \fICURLE_WRITE_ERROR\fP get returned.
+You can stop the transfer by returning CURL_WRITEFUNC_ERROR (added in 7.87.0),
+which makes \fICURLE_WRITE_ERROR\fP get returned.
+
+You can abort the transfer by returning CURL_WRITEFUNC_ABORT (added in 8.5.0),
+which makes \fICURLE_ABORTED_BY_CALLBACK\fP get returned.
 
 If the callback function returns CURL_WRITEFUNC_PAUSE it pauses this
 transfer. See \fIcurl_easy_pause(3)\fP for further details.

docs/libcurl/symbols-in-versions

@@ -191,6 +191,7 @@ CURL_WAIT_POLLIN                7.28.0
 CURL_WAIT_POLLOUT               7.28.0
 CURL_WAIT_POLLPRI               7.28.0
 CURL_WIN32                      7.69.0
+CURL_WRITEFUNC_ABORT            8.5.0
 CURL_WRITEFUNC_ERROR            7.87.0
 CURL_WRITEFUNC_PAUSE            7.18.0
 CURL_ZERO_TERMINATED            7.56.0

include/curl/curl.h

@@ -275,9 +275,15 @@ typedef int (*curl_xferinfo_callback)(void *clientp,
 #define CURL_WRITEFUNC_PAUSE 0x10000001
 
 /* This is a magic return code for the write callback that, when returned,
-   will signal an error from the callback. */
+   will signal an error from the callback. Makes libcurl return
+   CURLE_WRITE_ERROR as a result for the transfer. */
 #define CURL_WRITEFUNC_ERROR 0xFFFFFFFF
 
+/* This is a magic return code for the write callback that, when returned,
+   will signal an error from the callback. Makes libcurl return
+   CURLE_ABORTED_BY_CALLBACK as a result for the transfer. */
+#define CURL_WRITEFUNC_ABORT 0xFFFFFFFE
+
 typedef size_t (*curl_write_callback)(char *buffer,
                                       size_t size,
                                       size_t nitems,

lib/sendf.c

@@ -331,6 +331,10 @@ static CURLcode chop_write(struct Curl_easy *data,
         }
         return pausewrite(data, type, TRUE, ptr, len);
       }
+      else if(CURL_WRITEFUNC_ABORT == wrote) {
+        failf(data, "write callback aborted");
+        return CURLE_ABORTED_BY_CALLBACK;
+      }
       if(wrote != chunklen) {
         failf(data, "Failure writing output to destination");
         return CURLE_WRITE_ERROR;