- Bug report #2709004 (http://curl.haxx.se/bug/view.cgi?id=2709004) by Tim

Chen pointed out how curl couldn't upload with resume when reading from a
  pipe.

  This ended up with the introduction of a new return code for the
  CURLOPT_SEEKFUNCTION callback that basically says that the seek failed but
  that libcurl may try to resolve the situation anyway. In our case this means
  libcurl will attempt to instead read that much data from the stream instead
  of seeking and that way curl can now upload with resume when data is read
  from a stream!

docs/libcurl/curl_easy_setopt.3

@@ -189,8 +189,11 @@ uploaded bytes with the normal read function/callback). It is also called to
 rewind a stream when doing a HTTP PUT or POST with a multi-pass authentication
 method. The function shall work like "fseek" or "lseek" and accepted SEEK_SET,
 SEEK_CUR and SEEK_END as argument for origin, although (in 7.18.0) libcurl
-only passes SEEK_SET. The callback must return 0 on success as returning
-something else will cause the upload operation to fail.
+only passes SEEK_SET. The callback must return 0 (CURL_SEEKFUNC_OK) on
+success, 1 (CURL_SEEKFUNC_FAIL) to cause the upload operation to fail or 2
+(CURL_SEEKFUNC_CANTSEEK) to indicate that while the seek failed, libcurl is
+free to work around the problem if possible. The latter can sometimes be done
+by instead reading from the input or similar.
 
 If you forward the input arguments directly to "fseek" or "lseek", note that
 the data type for \fIoffset\fP is not the same as defined for curl_off_t on