- 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!

include/curl/curl.h

@@ -186,15 +186,21 @@ typedef size_t (*curl_write_callback)(char *buffer,
                                       size_t nitems,
                                       void *outstream);
 
+/* this is the return codes for the seek callbacks */
+#define CURL_SEEKFUNC_OK       0
+#define CURL_SEEKFUNC_FAIL     1 /* fail the entire transfer */
+#define CURL_SEEKFUNC_CANTSEEK 2 /* tell libcurl seeking can't be done, so
+                                    libcurl might try other means instead */
+typedef int (*curl_seek_callback)(void *instream,
+                                  curl_off_t offset,
+                                  int origin); /* 'whence' */
+
 /* This is a return code for the read callback that, when returned, will
    signal libcurl to immediately abort the current transfer. */
 #define CURL_READFUNC_ABORT 0x10000000
 /* This is a return code for the read callback that, when returned, will
    signal libcurl to pause sending data on the current transfer. */
 #define CURL_READFUNC_PAUSE 0x10000001
-typedef int (*curl_seek_callback)(void *instream,
-                                  curl_off_t offset,
-                                  int origin); /* 'whence' */
 
 typedef size_t (*curl_read_callback)(char *buffer,
                                       size_t size,