Split the Visual Studio 2017 AppVeyor jobs in two

The VS 2017 jobs hit AppVeyor's 60-minute limit per job while still
compiling the tests (77 of about 108 test targets after 58 minutes).
They pass /std:c++17 for everything anyway, so build only the C++17
variant of each test (JSON_TestStandards=17), and split the unit test
files across two jobs each with the new JSON_TestShard=<index>/<count>
option, which keeps every <count>-th test file starting at <index>.
The extra variants of single test files are built in shard 0 only.

CMAKE_OPTIONS is no longer quoted in appveyor.yml, so that it can hold
more than one option.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

.github/external_ci/appveyor.yml

@@ -16,6 +16,9 @@ only_commits:
 
 environment:
   matrix:
+    # The Visual Studio 2017 jobs compile everything with /std:c++17, so they
+    # only build the C++17 variant of each test, split into two jobs each to
+    # stay below AppVeyor's 60-minute limit per job.
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
       configuration: Debug
       platform: x86
@@ -34,7 +37,13 @@ environment:
       configuration: Release
       platform: x86
       CXX_FLAGS: "/permissive- /std:c++17 /utf-8 /W4 /WX"
-      CMAKE_OPTIONS: ""
+      CMAKE_OPTIONS: "-DJSON_TestStandards=17 -DJSON_TestShard=0/2"
+      GENERATOR: Visual Studio 15 2017
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
+      configuration: Release
+      platform: x86
+      CXX_FLAGS: "/permissive- /std:c++17 /utf-8 /W4 /WX"
+      CMAKE_OPTIONS: "-DJSON_TestStandards=17 -DJSON_TestShard=1/2"
       GENERATOR: Visual Studio 15 2017
 
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
@@ -55,7 +64,13 @@ environment:
       configuration: Release
       platform: x64
       CXX_FLAGS: "/permissive- /std:c++17 /Zc:__cplusplus /utf-8 /W4 /WX"
-      CMAKE_OPTIONS: ""
+      CMAKE_OPTIONS: "-DJSON_TestStandards=17 -DJSON_TestShard=0/2"
+      GENERATOR: Visual Studio 15 2017
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
+      configuration: Release
+      platform: x64
+      CXX_FLAGS: "/permissive- /std:c++17 /Zc:__cplusplus /utf-8 /W4 /WX"
+      CMAKE_OPTIONS: "-DJSON_TestStandards=17 -DJSON_TestShard=1/2"
       GENERATOR: Visual Studio 15 2017
 
 init:
@@ -66,7 +81,7 @@ install:
   - if "%platform%"=="x86"    set GENERATOR_PLATFORM=Win32
 
 before_build:
-  - cmake . -G "%GENERATOR%" -A "%GENERATOR_PLATFORM%" -DCMAKE_CXX_FLAGS="%CXX_FLAGS%" -DCMAKE_IGNORE_PATH="C:/Program Files/Git/usr/bin" -DJSON_BuildTests=On "%CMAKE_OPTIONS%"
+  - cmake . -G "%GENERATOR%" -A "%GENERATOR_PLATFORM%" -DCMAKE_CXX_FLAGS="%CXX_FLAGS%" -DCMAKE_IGNORE_PATH="C:/Program Files/Git/usr/bin" -DJSON_BuildTests=On %CMAKE_OPTIONS%
 
 build_script:
   - cmake --build . --config "%configuration%" --parallel 2

tests/CMakeLists.txt

@@ -8,6 +8,7 @@ set(JSON_SIMDUTF_VERSION 9.1.0 CACHE STRING "The simdutf version used by JSON_Te
 
 set(JSON_32bitTest     AUTO CACHE STRING "Enable the 32bit unit test (ON/OFF/AUTO/ONLY).")
 set(JSON_TestStandards "" CACHE STRING "The list of standards to test explicitly.")
+set(JSON_TestShard     "" CACHE STRING "Build only a part of the unit tests, given as <index>/<count> (e.g. 0/2), to split them across CI jobs with a time limit.")
 
 # using an env var, since this will also affect targets executing cmake (such as "ci_test_compiler_default")
 if (NOT "" STREQUAL "$ENV{JSON_FORCED_GLOBAL_COMPILE_OPTIONS}")
@@ -288,6 +289,33 @@ elseif(NOT json_32bit_test)
     list(FILTER files EXCLUDE REGEX src/unit-32bit.cpp)
 endif()
 
+# with JSON_TestShard=<index>/<count>, keep every <count>-th unit test file,
+# starting at <index> (the glob is sorted, so the split is stable)
+set(test_shard_index 0)
+if(NOT "${JSON_TestShard}" STREQUAL "")
+    if(NOT JSON_TestShard MATCHES "^([0-9]+)/([1-9][0-9]*)$")
+        message(FATAL_ERROR "JSON_TestShard must be <index>/<count>, e.g. 0/2, not '${JSON_TestShard}'.")
+    endif()
+    set(test_shard_index ${CMAKE_MATCH_1})
+    set(test_shard_count ${CMAKE_MATCH_2})
+    if(NOT test_shard_index LESS test_shard_count)
+        message(FATAL_ERROR "JSON_TestShard: the index must be less than the count, not '${JSON_TestShard}'.")
+    endif()
+    list(LENGTH files test_file_count)
+    set(shard_files "")
+    set(file_position 0)
+    foreach(file ${files})
+        math(EXPR file_shard "${file_position} % ${test_shard_count}")
+        if(file_shard EQUAL test_shard_index)
+            list(APPEND shard_files ${file})
+        endif()
+        math(EXPR file_position "${file_position} + 1")
+    endforeach()
+    set(files ${shard_files})
+    list(LENGTH files shard_file_count)
+    message(STATUS "Test shard ${JSON_TestShard}: ${shard_file_count} of ${test_file_count} unit test files")
+endif()
+
 foreach(file ${files})
     json_test_add_test_for(${file} MAIN test_main CXX_STANDARDS ${test_cxx_standards} ${test_force})
 endforeach()
@@ -305,6 +333,9 @@ if(json_32bit_test_only)
     return()
 endif()
 
+# the following variants of single test files are only built in the first shard
+if(test_shard_index EQUAL 0)
+
 # test legacy comparison of discarded values
 json_test_set_test_options(test-comparison_legacy
     COMPILE_DEFINITIONS JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON=1
@@ -332,6 +363,8 @@ json_test_add_test_for(src/unit-diagnostic-positions.cpp
     MAIN test_main CXX_STANDARDS ${test_cxx_standards} ${test_force}
 )
 
+endif()
+
 # *DO NOT* use json_test_set_test_options() below this line
 
 #############################################################################