[3.x] Guard against NPE during early invocation of Span.current()

tracing/tracing/src/main/java/io/helidon/tracing/TracerProviderHelper.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2019, 2022 Oracle and/or its affiliates.
+ * Copyright (c) 2019, 2024 Oracle and/or its affiliates.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -58,18 +58,32 @@ private TracerProviderHelper() {
     }
 
     public static Optional<Span> currentSpan() {
-        return TRACER_PROVIDER.currentSpan();
+        /*
+        If a custom TracerProvider implementation indirectly tries to access the current span (for example, by triggering custom
+        logging that adds the current span to each message), then this method can be invoked before the static initializer has
+        completed and, therefore, before TRACER_PROVIDER is assigned.
+         */
+        return (TRACER_PROVIDER == null) ? Optional.empty() : TRACER_PROVIDER.currentSpan();
     }
 
     static Tracer global() {
+        if (TRACER_PROVIDER == null) {
+            throw new IllegalStateException("Use before initialization has completed");
+        }
         return TRACER_PROVIDER.global();
     }
 
     static void global(Tracer tracer) {
+        if (TRACER_PROVIDER == null) {
+            throw new IllegalStateException("Use before initialization has completed");
+        }
         TRACER_PROVIDER.global(tracer);
     }
 
     static TracerBuilder<?> findTracerBuilder() {
+        if (TRACER_PROVIDER == null) {
+            throw new IllegalStateException("Use before initialization has completed");
+        }
         return TRACER_PROVIDER.createBuilder();
     }
 }

tracing/tracing/src/test/java/io/helidon/tracing/TestEarlyUseOfSpanCurrent.java

@@ -0,0 +1,30 @@
+/*
+ * Copyright (c) 2024 Oracle and/or its affiliates.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.helidon.tracing;
+
+import org.junit.jupiter.api.Test;
+
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.notNullValue;
+class TestEarlyUseOfSpanCurrent {
+
+    @Test
+    void ensureCurrentSpanNonNull() {
+
+        Span.current(); // Trigger the service loading of the test provider which will itself invoke Span.current().
+        assertThat("Early current span", TestTracerProvider.earlyCurrentSpan(), notNullValue());
+    }
+}

tracing/tracing/src/test/java/io/helidon/tracing/TestTracerProvider.java

@@ -0,0 +1,41 @@
+/*
+ * Copyright (c) 2024 Oracle and/or its affiliates.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.helidon.tracing;
+
+import java.util.Optional;
+
+/**
+ * Approximates the behavior of a custom tracer provider with a constructor that invokes a logger which invokes Span.current().
+ */
+public class TestTracerProvider extends NoOpTracerProvider {
+
+    private static Optional<Span> earlyCurrentSpan;
+
+    public TestTracerProvider() {
+        Optional<Span> currentSpan;
+        try {
+            currentSpan = Span.current();
+        } catch (NullPointerException e) {
+            // silent
+            currentSpan = null;
+        }
+        earlyCurrentSpan = currentSpan;
+    }
+
+    static Optional<Span> earlyCurrentSpan() {
+        return earlyCurrentSpan;
+    }
+}

tracing/tracing/src/test/resources/META-INF/services/io.helidon.tracing.spi.TracerProvider

@@ -0,0 +1,16 @@
+#
+# Copyright (c) 2024 Oracle and/or its affiliates.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+io.helidon.tracing.TestTracerProvider