ExternalProject: Re-implement download logic as a dedicated script

Move the content to a `ExternalProject-download.cmake.in` file and use
`configure_file` to generate the final script.

Retry logic was not working before because similar script trigger
FATAL_ERROR if 'file(DOWNLOAD ...)' exits with nonzero 'status_code'.
FATAL_ERROR makes the whole chain of commands stop and
'_ep_write_verifyfile_script' retry logic was not used in fact.

Default retry number set to 5 with pauses 0, 5, 5, 15, 60 seconds. Some
space left for future improvements if needed (90, 300, 1200=20min). Can
be controlled by user.

2 files changed, 180 insertions, 42 deletions

diff --git a/Modules/ExternalProject-download.cmake.in b/Modules/ExternalProject-download.cmake.in
new file mode 100644
index 0000000..5b73cd8
--- /dev/null
+++ b/Modules/ExternalProject-download.cmake.in
@@ -0,0 +1,161 @@
+#=============================================================================
+# Copyright 2008-2013 Kitware, Inc.
+# Copyright 2016 Ruslan Baratov
+#
+# Distributed under the OSI-approved BSD License (the "License");
+# see accompanying file Copyright.txt for details.
+#
+# This software is distributed WITHOUT ANY WARRANTY; without even the
+# implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+# See the License for more information.
+#=============================================================================
+# (To distribute this file outside of CMake, substitute the full
+#  License text for the above reference.)
+
+cmake_minimum_required(VERSION 3.5)
+
+function(check_file_hash has_hash hash_is_good)
+  if("${has_hash}" STREQUAL "")
+    message(FATAL_ERROR "has_hash Can't be empty")
+  endif()
+
+  if("${hash_is_good}" STREQUAL "")
+    message(FATAL_ERROR "hash_is_good Can't be empty")
+  endif()
+
+  if("@ALGO@" STREQUAL "")
+    # No check
+    set("${has_hash}" FALSE PARENT_SCOPE)
+    set("${hash_is_good}" FALSE PARENT_SCOPE)
+    return()
+  endif()
+
+  set("${has_hash}" TRUE PARENT_SCOPE)
+
+  message(STATUS "verifying file...
+       file='@LOCAL@'")
+
+  file("@ALGO@" "@LOCAL@" actual_value)
+
+  if(NOT "${actual_value}" STREQUAL "@EXPECT_VALUE@")
+    set("${hash_is_good}" FALSE PARENT_SCOPE)
+    message(STATUS "@ALGO@ hash of
+    @LOCAL@
+  does not match expected value
+    expected: '@EXPECT_VALUE@'
+      actual: '${actual_value}'")
+  else()
+    set("${hash_is_good}" TRUE PARENT_SCOPE)
+  endif()
+endfunction()
+
+function(sleep_before_download attempt)
+  if(attempt EQUAL 0)
+    return()
+  endif()
+
+  if(attempt EQUAL 1)
+    message(STATUS "Retrying...")
+    return()
+  endif()
+
+  set(sleep_seconds 0)
+
+  if(attempt EQUAL 2)
+    set(sleep_seconds 5)
+  elseif(attempt EQUAL 3)
+    set(sleep_seconds 5)
+  elseif(attempt EQUAL 4)
+    set(sleep_seconds 15)
+  elseif(attempt EQUAL 5)
+    set(sleep_seconds 60)
+  elseif(attempt EQUAL 6)
+    set(sleep_seconds 90)
+  elseif(attempt EQUAL 7)
+    set(sleep_seconds 300)
+  else()
+    set(sleep_seconds 1200)
+  endif()
+
+  message(STATUS "Retry after ${sleep_seconds} seconds (attempt #${attempt}) ...")
+
+  execute_process(COMMAND "${CMAKE_COMMAND}" -E sleep "${sleep_seconds}")
+endfunction()
+
+if("@LOCAL@" STREQUAL "")
+  message(FATAL_ERROR "LOCAL can't be empty")
+endif()
+
+if("@REMOTE@" STREQUAL "")
+  message(FATAL_ERROR "REMOTE can't be empty")
+endif()
+
+if(EXISTS "@LOCAL@")
+  check_file_hash(has_hash hash_is_good)
+  if(has_hash)
+    if(hash_is_good)
+      message(STATUS "File already exists and hash match (skip download):
+  file='@LOCAL@'
+  @ALGO@='@EXPECT_VALUE@'"
+      )
+      return()
+    else()
+      message(STATUS "File already exists but hash mismatch. Removing...")
+      file(REMOVE "@LOCAL@")
+    endif()
+  else()
+    message(STATUS "File already exists but no hash specified (use URL_HASH):
+  file='@LOCAL@'
+Old file will be removed and new file downloaded from URL."
+    )
+    file(REMOVE "@LOCAL@")
+  endif()
+endif()
+
+set(retry_number 5)
+
+foreach(i RANGE ${retry_number})
+  sleep_before_download(${i})
+
+  message(STATUS "downloading...
+       src='@REMOTE@'
+       dst='@LOCAL@'
+       timeout='@TIMEOUT_MSG@'")
+
+  @TLS_VERIFY_CODE@
+  @TLS_CAINFO_CODE@
+
+  file(
+      DOWNLOAD
+      "@REMOTE@" "@LOCAL@"
+      @SHOW_PROGRESS@
+      @TIMEOUT_ARGS@
+      STATUS status
+      LOG log
+  )
+
+  list(GET status 0 status_code)
+  list(GET status 1 status_string)
+
+  if(status_code EQUAL 0)
+    check_file_hash(has_hash hash_is_good)
+    if(has_hash AND NOT hash_is_good)
+      message(STATUS "Hash mismatch, removing...")
+      file(REMOVE "@LOCAL@")
+    else()
+      message(STATUS "Downloading... done")
+      return()
+    endif()
+  else()
+    message("error: downloading '@REMOTE@' failed
+  status_code: ${status_code}
+  status_string: ${status_string}
+  log:
+  --- LOG BEGIN ---
+  ${log}
+  --- LOG END ---"
+    )
+  endif()
+endforeach()
+
+message(FATAL_ERROR "Downloading failed")
diff --git a/Modules/ExternalProject.cmake b/Modules/ExternalProject.cmake
index 2249501..ad6de18 100644
--- a/Modules/ExternalProject.cmake
+++ b/Modules/ExternalProject.cmake
@@ -867,16 +867,11 @@ function(_ep_write_downloadfile_script script_filename REMOTE LOCAL timeout no_p
   endif()
 
   if("${hash}" MATCHES "${_ep_hash_regex}")
-    string(CONCAT hash_check
-      "if(EXISTS \"${LOCAL}\")\n"
-      "  file(\"${CMAKE_MATCH_1}\" \"${LOCAL}\" hash_value)\n"
-      "  if(\"x\${hash_value}\" STREQUAL \"x${CMAKE_MATCH_2}\")\n"
-      "    return()\n"
-      "  endif()\n"
-      "endif()\n"
-      )
+    set(ALGO "${CMAKE_MATCH_1}")
+    set(EXPECT_VALUE "${CMAKE_MATCH_2}")
   else()
-    set(hash_check "")
+    set(ALGO "")
+    set(EXPECT_VALUE "")
   endif()
 
   set(TLS_VERIFY_CODE "")
@@ -904,41 +899,23 @@ function(_ep_write_downloadfile_script script_filename REMOTE LOCAL timeout no_p
     set(TLS_CAINFO_CODE "set(CMAKE_TLS_CAINFO \"${tls_cainfo}\")")
   endif()
 
-  file(WRITE ${script_filename}
-"${hash_check}message(STATUS \"downloading...
-     src='${REMOTE}'
-     dst='${LOCAL}'
-     timeout='${TIMEOUT_MSG}'\")
-
-${TLS_VERIFY_CODE}
-${TLS_CAINFO_CODE}
-
-file(DOWNLOAD
-  \"${REMOTE}\"
-  \"${LOCAL}\"
-  ${SHOW_PROGRESS}
-  ${TIMEOUT_ARGS}
-  STATUS status
-  LOG log)
-
-list(GET status 0 status_code)
-list(GET status 1 status_string)
-
-if(NOT status_code EQUAL 0)
-  message(FATAL_ERROR \"error: downloading '${REMOTE}' failed
-  status_code: \${status_code}
-  status_string: \${status_string}
-  log: \${log}
-\")
-endif()
-
-message(STATUS \"downloading... done\")
-"
-)
-
+  # Used variables:
+  # * TLS_VERIFY_CODE
+  # * TLS_CAINFO_CODE
+  # * ALGO
+  # * EXPECT_VALUE
+  # * REMOTE
+  # * LOCAL
+  # * SHOW_PROGRESS
+  # * TIMEOUT_ARGS
+  # * TIMEOUT_MSG
+  configure_file(
+      "${_ExternalProject_SELF_DIR}/ExternalProject-download.cmake.in"
+      "${script_filename}"
+      @ONLY
+  )
 endfunction()
 
-
 function(_ep_write_verifyfile_script script_filename LOCAL hash)
   if("${hash}" MATCHES "${_ep_hash_regex}")
     set(ALGO "${CMAKE_MATCH_1}")