This CMake module will download files during do_configure.  This is bad as it
means we can't do offline builds.

Add an option to disallow downloads by emitting a fatal error.

Upstream-Status: Pending
Signed-off-by: Ross Burton <ross.burton@intel.com>

diff --git a/cmake/OpenCVDownload.cmake b/cmake/OpenCVDownload.cmake
index cdc47ad2cb..74573f45a2 100644
--- a/cmake/OpenCVDownload.cmake
+++ b/cmake/OpenCVDownload.cmake
@@ -14,6 +14,7 @@
 #    RELATIVE_URL - if set, then URL is treated as a base, and FILENAME will be appended to it
 #  Note: uses OPENCV_DOWNLOAD_PATH folder as cache, default is <opencv>/.cache
 
+set(OPENCV_ALLOW_DOWNLOADS ON CACHE BOOL "Allow downloads")
 set(HELP_OPENCV_DOWNLOAD_PATH "Cache directory for downloaded files")
 if(DEFINED ENV{OPENCV_DOWNLOAD_PATH})
   set(OPENCV_DOWNLOAD_PATH "$ENV{OPENCV_DOWNLOAD_PATH}" CACHE PATH "${HELP_OPENCV_DOWNLOAD_PATH}")
@@ -153,6 +154,11 @@ function(ocv_download)
 
   # Download
   if(NOT EXISTS "${CACHE_CANDIDATE}")
+    if(NOT OPENCV_ALLOW_DOWNLOADS)
+      message(FATAL_ERROR "Not going to download ${DL_FILENAME}")
+      return()
+    endif()
+
     ocv_download_log("#cmake_download \"${CACHE_CANDIDATE}\" \"${DL_URL}\"")
     file(DOWNLOAD "${DL_URL}" "${CACHE_CANDIDATE}"
          INACTIVITY_TIMEOUT 60