- Bryan Henderson introduces two things:

1) the progress callback gets called more frequently (at times)
  2) libcurl *might* call the callback when it receives a signal

lib/transfer.c

@@ -1603,8 +1603,6 @@ CURLcode Curl_readwrite(struct connectdata *conn,
       failf(data, "transfer closed with outstanding read data remaining");
       return CURLE_PARTIAL_FILE;
     }
-    if(Curl_pgrsUpdate(conn))
-      return CURLE_ABORTED_BY_CALLBACK;
   }
 
   /* Now update the "done" boolean we return */
@@ -1754,6 +1752,18 @@ int Curl_single_getsock(struct connectdata *conn,
 }
 
 
+
+static bool
+errnoIsInterruption(int errnoarg)
+{
+#ifdef EINTR
+  return (errnoarg == EINTR);
+#else
+  return FALSE;
+#endif
+}
+
+
 /*
  * Transfer()
  *
@@ -1775,6 +1785,12 @@ Transfer(struct connectdata *conn)
   struct SessionHandle *data = conn->data;
   struct Curl_transfer_keeper *k = &data->reqdata.keep;
   bool done=FALSE;
+  sigset_t callersigmask;
+  sigset_t allsignals;
+  int pgrsrc;
+  int selectrc;
+
+  sigfillset(&allsignals);
 
   if(!(conn->protocol & PROT_FILE)) {
     /* Only do this if we are not transferring FILE:, since the file: treatment
@@ -1828,28 +1844,33 @@ Transfer(struct connectdata *conn)
        the timeout case and if we limit transfer speed we must make sure that
        this function doesn't transfer anything while in HOLD status. */
 
-    switch (Curl_select(fd_read, fd_write, 1000)) {
-    case -1: /* select() error, stop reading */
-#ifdef EINTR
-      /* The EINTR is not serious, and it seems you might get this more
-         ofen when using the lib in a multi-threaded environment! */
-      if(SOCKERRNO == EINTR)
-        ;
-      else
-#endif
-        done = TRUE; /* no more read or write */
+    sigprocmask(SIG_SETMASK, &allsignals, &callersigmask);
+
+    pgrsrc = Curl_pgrsUpdate(conn);
+
+    if(!pgrsrc)
+      selectrc = Curl_pselect(fd_read, fd_write, 3000, &callersigmask);
+
+    sigprocmask(SIG_SETMASK, &callersigmask, NULL);
+
+    if(pgrsrc)
+      return CURLE_ABORTED_BY_CALLBACK;
+
+    if (selectrc == -1 && !errnoIsInterruption(SOCKERRNO)) {
+      done = TRUE; /* no more read or write */
       continue;
-    case 0:  /* timeout */
-    default: /* readable descriptors */
+    } else {
+      /* ready files, timeout, or signal received */
 
       result = Curl_readwrite(conn, &done);
-      break;
-    }
-    if(result)
-      return result;
 
-    /* "done" signals to us if the transfer(s) are ready */
+      /* "done" signals to us if the transfer(s) are ready */
+
+      if(result)
+        return result;
+    }
   }
+  Curl_pgrsUpdate(conn);
 
   return CURLE_OK;
 }