build release tarball when nlohmann-json not installed

CMakeLists.txt

@@ -278,6 +278,7 @@ if((NOT WITH_API_ONLY)
    OR WITH_ETW)
   # nlohmann_json package is required for most SDK build configurations
   find_package(nlohmann_json QUIET)
+  set(nlohmann_json_clone FALSE)
   if(NOT nlohmann_json_FOUND)
     if(EXISTS ${PROJECT_SOURCE_DIR}/.git)
       message("Trying to use local nlohmann::json from submodule")
@@ -294,15 +295,14 @@ if((NOT WITH_API_ONLY)
         include_directories(
           ${PROJECT_SOURCE_DIR}/third_party/nlohmann-json/single_include)
       else()
-        message(
-          FATAL_ERROR
-            "\nnlohmann_json package was not found. Please either provide it manually or clone with submodules. "
-            "To initialize, fetch and checkout any nested submodules, you can use the following command:\n"
-            "git submodule update --init --recursive")
+        set(nlohmann_json_clone TRUE)
+        include(cmake/nlohmann-json.cmake)
+        message("\nnlohmann_json package was not found. Cloning from github")
       endif()
     else()
-      # Let's fail with default find_package error
-      find_package(nlohmann_json REQUIRED)
+      set(nlohmann_json_clone TRUE)
+      include(cmake/nlohmann-json.cmake)
+      message("\nnlohmann_json package was not found. Cloning from github")
     endif()
   else()
     message("Using external nlohmann::json")

cmake/nlohmann-json.cmake

@@ -0,0 +1,34 @@
+if("${nlohmann-json}" STREQUAL "")
+    set(nlohmann-json "develop")
+endif()
+include(ExternalProject)
+ExternalProject_Add(nlohmann_json_download
+    PREFIX third_party
+    GIT_REPOSITORY https://github.com/nlohmann/json.git
+    GIT_TAG
+        "${nlohmann-json}"
+    UPDATE_COMMAND ""
+    CMAKE_ARGS
+      -DJSON_BuildTests=OFF
+      -DJSON_Install=ON
+      -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}
+    TEST_AFTER_INSTALL
+        0
+    DOWNLOAD_NO_PROGRESS
+        1
+    LOG_CONFIGURE
+        1
+    LOG_BUILD
+        1
+    LOG_INSTALL
+        1
+)
+
+ExternalProject_Get_Property(nlohmann_json_download INSTALL_DIR)
+SET(NLOHMANN_JSON_INCLUDE_DIR ${INSTALL_DIR}/third_party/src/nlohmann_json_download/single_include)
+add_library(nlohmann_json_ INTERFACE)
+target_include_directories(nlohmann_json_ INTERFACE
+    "$<BUILD_INTERFACE:${NLOHMANN_JSON_INCLUDE_DIR}>"
+    "$<INSTALL_INTERFACE:include>")
+add_dependencies(nlohmann_json_ nlohmann_json_download)
+add_library(nlohmann_json::nlohmann_json ALIAS nlohmann_json_)

exporters/etw/CMakeLists.txt

@@ -10,6 +10,9 @@ set_target_properties(opentelemetry_exporter_etw PROPERTIES EXPORT_NAME
 
 target_link_libraries(opentelemetry_exporter_etw
                       INTERFACE nlohmann_json::nlohmann_json)
+if(nlohmann_json_clone)
+  add_dependencies(opentelemetry_exporter_etw nlohmann_json::nlohmann_json)
+endif()
 
 install(
   TARGETS opentelemetry_exporter_etw

exporters/otlp/CMakeLists.txt

@@ -48,6 +48,11 @@ if(WITH_OTLP_HTTP)
     opentelemetry_exporter_otlp_http_client
     PUBLIC opentelemetry_sdk opentelemetry_proto http_client_curl
            nlohmann_json::nlohmann_json)
+  if(nlohmann_json_clone)
+    add_dependencies(opentelemetry_exporter_otlp_http_client
+                     nlohmann_json::nlohmann_json)
+    list(APPEND OPENTELEMETRY_OTLP_TARGETS nlohmann_json_)
+  endif()
   target_include_directories(
     opentelemetry_exporter_otlp_http_client
     PUBLIC "$<BUILD_INTERFACE:${CMAKE_CURRENT_LIST_DIR}/include>"

exporters/zipkin/CMakeLists.txt

@@ -18,8 +18,9 @@ add_definitions(-DWITH_CURL)
 add_library(opentelemetry_exporter_zipkin_trace src/zipkin_exporter.cc
                                                 src/recordable.cc)
 
-target_link_libraries(opentelemetry_exporter_zipkin_trace
-                      PUBLIC opentelemetry_trace http_client_curl)
+target_link_libraries(
+  opentelemetry_exporter_zipkin_trace
+  PUBLIC opentelemetry_trace http_client_curl nlohmann_json::nlohmann_json)
 
 install(
   TARGETS opentelemetry_exporter_zipkin_trace

ext/test/w3c_tracecontext_test/CMakeLists.txt

@@ -10,4 +10,8 @@ else()
     PRIVATE ${CMAKE_THREAD_LIBS_INIT} opentelemetry_trace http_client_curl
             opentelemetry_exporter_ostream_span ${CURL_LIBRARIES}
             nlohmann_json::nlohmann_json)
+  if(lohmann_json_clone)
+    add_dependencies(w3c_tracecontext_test nlohmann_json::nlohmann_json)
+  endif()
+
 endif()