[SDK] DefaultLogHandler to print errors to std::cerr, add LogLevel::None

CHANGELOG.md

@@ -19,6 +19,8 @@ Increment the:
   [#2530](https://github.com/open-telemetry/opentelemetry-cpp/pull/2530)
 * [EXPORTER] Support URL-encoded values for `OTEL_EXPORTER_OTLP_HEADERS`
   [#2579](https://github.com/open-telemetry/opentelemetry-cpp/pull/2579)
+* [SDK] DefaultLogHandler to print to std::cerr, add LogLevel::None
+  [#2622](https://github.com/open-telemetry/opentelemetry-cpp/pull/2622)
 
 Important changes:
 
@@ -32,6 +34,22 @@ Important changes:
     dependency on zlib.
 * [SDK] Change OTLP HTTP content_type default to binary
   [#2558](https://github.com/open-telemetry/opentelemetry-cpp/pull/2558)
+* [SDK] DefaultLogHandler to print to std::cerr, add LogLevel::None
+  [#2622](https://github.com/open-telemetry/opentelemetry-cpp/pull/2622)
+  * Change DefaultLogHandler output
+    * Before, the default internal logger, DefaultLogHandler,
+      used to print to std::cout, interfering with the user application.
+    * Now, DefaultLogHandler prints to std::cerr, as expected.
+    * Applications that expected to find the opentelemetry-cpp internal
+      error log in std::cout may need adjustments, either by looking
+      at std::cerr instead, or by using a custom log handler.
+  * Additional LogLevel::None
+    * LogLevel::None is a new supported log level, which does not print
+      any message.
+    * Custom log handlers may need to implement a new case, to avoid
+      compiler warnings.
+    * Numbering of log levels like OTEL_INTERNAL_LOG_LEVEL_ERROR
+      has changed, which requires to rebuild, as the SDK ABI differs.
 
 ## [1.14.2] 2024-02-27
 

exporters/ostream/test/ostream_log_test.cc

@@ -49,12 +49,12 @@ TEST(OStreamLogRecordExporter, Shutdown)
   auto exporter =
       std::unique_ptr<sdklogs::LogRecordExporter>(new exporterlogs::OStreamLogRecordExporter);
 
-  // Save cout's original buffer here
-  std::streambuf *original = std::cout.rdbuf();
+  // Save cerr original buffer here
+  std::streambuf *original = std::cerr.rdbuf();
 
-  // Redirect cout to our stringstream buffer
+  // Redirect cerr to our stringstream buffer
   std::stringstream output;
-  std::cout.rdbuf(output.rdbuf());
+  std::cerr.rdbuf(output.rdbuf());
 
   EXPECT_TRUE(exporter->Shutdown());
 
@@ -64,7 +64,7 @@ TEST(OStreamLogRecordExporter, Shutdown)
   exporter->Export(nostd::span<std::unique_ptr<sdklogs::Recordable>>(&record, 1));
 
   // Restore original stringstream buffer
-  std::cout.rdbuf(original);
+  std::cerr.rdbuf(original);
   std::string err_message =
       "[Ostream Log Exporter] Exporting 1 log(s) failed, exporter is shutdown";
   EXPECT_TRUE(output.str().find(err_message) != std::string::npos);

exporters/ostream/test/ostream_span_test.cc

@@ -46,8 +46,8 @@ TEST(OStreamSpanExporter, Shutdown)
   auto recordable = processor->MakeRecordable();
   recordable->SetName("Test Span");
 
-  // Capture the output of cout
-  const auto captured = WithOStreamCapture(std::cout, [&]() {
+  // Capture the output of cerr
+  const auto captured = WithOStreamCapture(std::cerr, [&]() {
     EXPECT_TRUE(processor->Shutdown());
     processor->OnEnd(std::move(recordable));
   });

functional/otlp/func_http_main.cc

@@ -128,6 +128,8 @@ class TestLogHandler : public opentelemetry::sdk::common::internal_log::LogHandl
 
     switch (level)
     {
+      case opentelemetry::sdk::common::internal_log::LogLevel::None:
+        break;
       case opentelemetry::sdk::common::internal_log::LogLevel::Error:
         std::cout << " - [E] " << msg << std::endl;
         parse_error_msg(&g_test_result, msg);

sdk/include/opentelemetry/sdk/common/global_log_handler.h

@@ -10,10 +10,11 @@
 #include "opentelemetry/sdk/common/attribute_utils.h"
 #include "opentelemetry/version.h"
 
-#define OTEL_INTERNAL_LOG_LEVEL_ERROR 0
-#define OTEL_INTERNAL_LOG_LEVEL_WARN 1
-#define OTEL_INTERNAL_LOG_LEVEL_INFO 2
-#define OTEL_INTERNAL_LOG_LEVEL_DEBUG 3
+#define OTEL_INTERNAL_LOG_LEVEL_NONE 0
+#define OTEL_INTERNAL_LOG_LEVEL_ERROR 1
+#define OTEL_INTERNAL_LOG_LEVEL_WARN 2
+#define OTEL_INTERNAL_LOG_LEVEL_INFO 3
+#define OTEL_INTERNAL_LOG_LEVEL_DEBUG 4
 #ifndef OTEL_INTERNAL_LOG_LEVEL
 // DEBUG by default, we can change log level on runtime
 #  define OTEL_INTERNAL_LOG_LEVEL OTEL_INTERNAL_LOG_LEVEL_DEBUG
@@ -29,16 +30,19 @@ namespace internal_log
 
 enum class LogLevel
 {
-  Error = 0,
-  Warning,
-  Info,
-  Debug
+  None    = OTEL_INTERNAL_LOG_LEVEL_NONE,
+  Error   = OTEL_INTERNAL_LOG_LEVEL_ERROR,
+  Warning = OTEL_INTERNAL_LOG_LEVEL_WARN,
+  Info    = OTEL_INTERNAL_LOG_LEVEL_INFO,
+  Debug   = OTEL_INTERNAL_LOG_LEVEL_DEBUG
 };
 
 inline std::string LevelToString(LogLevel level)
 {
   switch (level)
   {
+    case LogLevel::None:
+      return "None";
     case LogLevel::Error:
       return "Error";
     case LogLevel::Warning:

sdk/src/common/global_log_handler.cc

@@ -33,7 +33,7 @@ void DefaultLogHandler::Handle(LogLevel level,
   }
   output_s << std::endl;
   // TBD - print attributes
-  std::cout << output_s.str();  // thread safe.
+  std::cerr << output_s.str();  // thread safe.
 }
 
 void NoopLogHandler::Handle(LogLevel,

sdk/test/common/global_log_handle_test.cc

@@ -62,6 +62,14 @@ TEST(GlobalLogHandleTest, CustomLogHandler)
   OTEL_INTERNAL_LOG_WARN("Warning message");
   EXPECT_EQ(before_count + 5, static_cast<CustomLogHandler *>(custom_log_handler.get())->count);
 
+  opentelemetry::sdk::common::internal_log::GlobalLogHandler::SetLogLevel(
+      opentelemetry::sdk::common::internal_log::LogLevel::None);
+  OTEL_INTERNAL_LOG_ERROR("Error message");
+  OTEL_INTERNAL_LOG_DEBUG("Debug message. Headers:", attributes);
+  OTEL_INTERNAL_LOG_INFO("Info message");
+  OTEL_INTERNAL_LOG_WARN("Warning message");
+  EXPECT_EQ(before_count + 5, static_cast<CustomLogHandler *>(custom_log_handler.get())->count);
+
   opentelemetry::sdk::common::internal_log::GlobalLogHandler::SetLogHandler(backup_log_handle);
   opentelemetry::sdk::common::internal_log::GlobalLogHandler::SetLogLevel(backup_log_level);
 }