Add OTEP #4947 OTEL-compatible context storage with custom attributes

Add alternative context storage mode (ctxstorage=otel) that exposes
trace context via an OTEP #4947-compliant TLS pointer discoverable
by external profilers through the ELF dynsym table.

Each thread gets a 640-byte OtelThreadContextRecord with W3C trace_id,
span_id, and custom attributes encoded in attrs_data[612]. A ContextApi
abstraction routes reads/writes to the appropriate storage backend.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

Author: jbachorik

ddprof-lib/src/main/cpp/arguments.cpp

@@ -374,6 +374,15 @@ Error Arguments::parse(const char *args) {
           }
       }
 
+      CASE("ctxstorage")
+      if (value != NULL) {
+          if (strcmp(value, "otel") == 0) {
+              _context_storage = CTX_STORAGE_OTEL;
+          } else {
+              _context_storage = CTX_STORAGE_PROFILER;
+          }
+      }
+
       DEFAULT()
       if (_unknown_arg == NULL)
         _unknown_arg = arg;

ddprof-lib/src/main/cpp/arguments.h

@@ -92,6 +92,17 @@ enum Clock {
     CLK_MONOTONIC
 };
 
+/**
+ * Context storage mode for trace/span context.
+ *
+ * PROFILER: Use existing TLS-based storage (default, proven async-signal safe)
+ * OTEL: Use OTEP #4947 TLS pointer storage (discoverable by external profilers)
+ */
+enum ContextStorageMode {
+    CTX_STORAGE_PROFILER,  // Default: TLS-based storage
+    CTX_STORAGE_OTEL       // OTEP #4947 TLS pointer storage
+};
+
 // Keep this in sync with JfrSync.java
 enum EventMask {
     EM_CPU          = 1,
@@ -189,6 +200,7 @@ class Arguments {
   bool _lightweight;
   bool _enable_method_cleanup;
   bool _remote_symbolication;  // Enable remote symbolication for native frames
+  ContextStorageMode _context_storage;  // Context storage mode (profiler TLS or OTEL TLS pointer)
 
   Arguments(bool persistent = false)
       : _buf(NULL),
@@ -223,7 +235,8 @@ class Arguments {
         _wallclock_sampler(ASGCT),
         _lightweight(false),
         _enable_method_cleanup(true),
-        _remote_symbolication(false) {}
+        _remote_symbolication(false),
+        _context_storage(CTX_STORAGE_PROFILER) {}
 
   ~Arguments();
 

ddprof-lib/src/main/cpp/context_api.cpp

@@ -0,0 +1,266 @@
+/*
+ * Copyright 2026, Datadog, Inc
+ *
+ * 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.
+ */
+
+#include "context_api.h"
+#include "context.h"
+#include "guards.h"
+#include "otel_context.h"
+#include "otel_process_ctx.h"
+#include "profiler.h"
+#include "thread.h"
+#include <cstring>
+
+// Static member initialization
+ContextStorageMode ContextApi::_mode = CTX_STORAGE_PROFILER;
+bool ContextApi::_initialized = false;
+char* ContextApi::_attribute_keys[MAX_ATTRIBUTE_KEYS] = {};
+int ContextApi::_attribute_key_count = 0;
+
+bool ContextApi::initialize(const Arguments& args) {
+    if (__atomic_load_n(&_initialized, __ATOMIC_ACQUIRE)) {
+        return true;
+    }
+
+    ContextStorageMode mode = args._context_storage;
+    if (mode == CTX_STORAGE_OTEL) {
+        if (!OtelContexts::initialize()) {
+            mode = CTX_STORAGE_PROFILER;
+            __atomic_store_n(&_mode, mode, __ATOMIC_RELEASE);
+            return false;
+        }
+    }
+
+    __atomic_store_n(&_mode, mode, __ATOMIC_RELEASE);
+    __atomic_store_n(&_initialized, true, __ATOMIC_RELEASE);
+    return true;
+}
+
+void ContextApi::shutdown() {
+    if (!__atomic_load_n(&_initialized, __ATOMIC_ACQUIRE)) {
+        return;
+    }
+
+    if (__atomic_load_n(&_mode, __ATOMIC_ACQUIRE) == CTX_STORAGE_OTEL) {
+        OtelContexts::shutdown();
+    }
+
+    // Free registered attribute keys
+    for (int i = 0; i < _attribute_key_count; i++) {
+        free(_attribute_keys[i]);
+        _attribute_keys[i] = nullptr;
+    }
+    _attribute_key_count = 0;
+
+    __atomic_store_n(&_initialized, false, __ATOMIC_RELEASE);
+}
+
+bool ContextApi::isInitialized() {
+    return __atomic_load_n(&_initialized, __ATOMIC_ACQUIRE);
+}
+
+ContextStorageMode ContextApi::getMode() {
+    return __atomic_load_n(&_mode, __ATOMIC_ACQUIRE);
+}
+
+/**
+ * Initialize OTel TLS for the current thread on first use.
+ * Allocates a per-thread OtelThreadContextRecord and caches it in ProfiledThread.
+ * Must be called with signals blocked to prevent musl TLS deadlock.
+ */
+static OtelThreadContextRecord* initializeOtelTls(ProfiledThread* thrd) {
+    // Block profiling signals during first TLS access (same pattern as context_tls_v1)
+    SignalBlocker blocker;
+
+    // Allocate one record per thread — freed in ProfiledThread destructor/releaseFromBuffer
+    OtelThreadContextRecord* record = new OtelThreadContextRecord();
+    record->valid = 0;
+    record->_reserved = 0;
+    record->attrs_data_size = 0;
+
+    // First access to custom_labels_current_set_v2 triggers TLS init
+    custom_labels_current_set_v2 = nullptr;
+
+    // Cache in ProfiledThread for signal-safe cross-thread access
+    thrd->markOtelContextInitialized(record);
+
+    return record;
+}
+
+void ContextApi::set(u64 span_id, u64 root_span_id) {
+    // For Datadog 64-bit trace IDs, trace_id_high is zero. This is valid per OTEP
+    // since trace_id_low (rootSpanId) is non-zero — the full 128-bit trace_id is
+    // not all-zeros, so the record is considered active.
+    setOtel(0, root_span_id, span_id);
+}
+
+void ContextApi::setOtel(u64 trace_id_high, u64 trace_id_low, u64 span_id) {
+    ContextStorageMode mode = __atomic_load_n(&_mode, __ATOMIC_ACQUIRE);
+
+    if (mode == CTX_STORAGE_OTEL) {
+        // All-zero IDs = context detachment per OTEP — use NULL pointer
+        if (trace_id_high == 0 && trace_id_low == 0 && span_id == 0) {
+            OtelContexts::clear();
+            return;
+        }
+
+        // Ensure TLS + record are initialized on first use
+        ProfiledThread* thrd = ProfiledThread::current();
+        if (thrd == nullptr) return;
+
+        if (!thrd->isOtelContextInitialized()) {
+            initializeOtelTls(thrd);
+        }
+
+        OtelContexts::set(trace_id_high, trace_id_low, span_id);
+    } else {
+        // Profiler mode: use existing TLS
+        Context& ctx = Contexts::get();
+
+        __atomic_store_n(&ctx.checksum, 0ULL, __ATOMIC_RELEASE);
+        __atomic_store_n(&ctx.spanId, span_id, __ATOMIC_RELAXED);
+        __atomic_store_n(&ctx.rootSpanId, trace_id_low, __ATOMIC_RELAXED);
+
+        u64 newChecksum = Contexts::checksum(span_id, trace_id_low);
+        __atomic_store_n(&ctx.checksum, newChecksum, __ATOMIC_RELEASE);
+    }
+}
+
+bool ContextApi::get(u64& span_id, u64& root_span_id) {
+    ContextStorageMode mode = __atomic_load_n(&_mode, __ATOMIC_ACQUIRE);
+
+    if (mode == CTX_STORAGE_OTEL) {
+        u64 trace_high, trace_low;
+        if (OtelContexts::get(trace_high, trace_low, span_id)) {
+            root_span_id = trace_low;
+            return true;
+        }
+        return false;
+    } else {
+        Context& ctx = Contexts::get();
+        u64 checksum1 = __atomic_load_n(&ctx.checksum, __ATOMIC_ACQUIRE);
+        span_id = __atomic_load_n(&ctx.spanId, __ATOMIC_RELAXED);
+        root_span_id = __atomic_load_n(&ctx.rootSpanId, __ATOMIC_RELAXED);
+        return checksum1 != 0 && checksum1 == Contexts::checksum(span_id, root_span_id);
+    }
+}
+
+
+void ContextApi::clear() {
+    ContextStorageMode mode = __atomic_load_n(&_mode, __ATOMIC_ACQUIRE);
+
+    if (mode == CTX_STORAGE_OTEL) {
+        OtelContexts::clear();
+    } else {
+        set(0, 0);
+    }
+}
+
+bool ContextApi::setAttribute(uint8_t key_index, const char* value, uint8_t value_len) {
+    ContextStorageMode mode = __atomic_load_n(&_mode, __ATOMIC_ACQUIRE);
+
+    if (mode == CTX_STORAGE_OTEL) {
+        // Ensure TLS + record are initialized on first use
+        ProfiledThread* thrd = ProfiledThread::current();
+        if (thrd == nullptr) return false;
+
+        if (!thrd->isOtelContextInitialized()) {
+            initializeOtelTls(thrd);
+        }
+
+        // Pre-register the value string in the Dictionary from this JNI thread.
+        // The signal handler (writeCurrentContext) will later call bounded_lookup
+        // to find the encoding — by pre-registering here, the signal handler
+        // only does a read (no malloc), which is async-signal-safe.
+        Profiler::instance()->contextValueMap()->bounded_lookup(
+            value, value_len, 1 << 16);
+
+        return OtelContexts::setAttribute(key_index, value, value_len);
+    } else {
+        // Profiler mode: register the string and write encoding to Context.tags[]
+        if (key_index >= DD_TAGS_CAPACITY) return false;
+
+        u32 encoding = Profiler::instance()->contextValueMap()->bounded_lookup(
+            value, value_len, 1 << 16);
+        if (encoding == INT_MAX) return false;
+
+        Context& ctx = Contexts::get();
+        ctx.tags[key_index].value = encoding;
+        return true;
+    }
+}
+
+void ContextApi::registerAttributeKeys(const char** keys, int count) {
+    // Free any previously registered keys
+    for (int i = 0; i < _attribute_key_count; i++) {
+        free(_attribute_keys[i]);
+        _attribute_keys[i] = nullptr;
+    }
+
+    _attribute_key_count = count < MAX_ATTRIBUTE_KEYS ? count : MAX_ATTRIBUTE_KEYS;
+    for (int i = 0; i < _attribute_key_count; i++) {
+        _attribute_keys[i] = strdup(keys[i]);
+    }
+
+    // If in OTEL mode, re-publish process context with thread_ctx_config
+    ContextStorageMode mode = __atomic_load_n(&_mode, __ATOMIC_ACQUIRE);
+    if (mode == CTX_STORAGE_OTEL) {
+        // Build NULL-terminated key array for the process context config
+        const char* key_ptrs[MAX_ATTRIBUTE_KEYS + 1];
+        for (int i = 0; i < _attribute_key_count; i++) {
+            key_ptrs[i] = _attribute_keys[i];
+        }
+        key_ptrs[_attribute_key_count] = nullptr;
+
+        otel_thread_ctx_config_data config = {
+            .schema_version = "tlsdesc_v1_dev",
+            .attribute_key_map = key_ptrs,
+        };
+
+#ifndef OTEL_PROCESS_CTX_NO_READ
+        // Re-publish the process context with thread_ctx_config
+        // We need to read the current context and re-publish with the config
+        otel_process_ctx_read_result read_result = otel_process_ctx_read();
+        if (read_result.success) {
+            otel_process_ctx_data data = {
+                .deployment_environment_name = read_result.data.deployment_environment_name,
+                .service_instance_id = read_result.data.service_instance_id,
+                .service_name = read_result.data.service_name,
+                .service_version = read_result.data.service_version,
+                .telemetry_sdk_language = read_result.data.telemetry_sdk_language,
+                .telemetry_sdk_version = read_result.data.telemetry_sdk_version,
+                .telemetry_sdk_name = read_result.data.telemetry_sdk_name,
+                .resource_attributes = read_result.data.resource_attributes,
+                .extra_attributes = read_result.data.extra_attributes,
+                .thread_ctx_config = &config,
+            };
+
+            otel_process_ctx_publish(&data);
+            otel_process_ctx_read_drop(&read_result);
+        }
+#endif
+    }
+}
+
+const char* ContextApi::getAttributeKey(int index) {
+    if (index < 0 || index >= _attribute_key_count) {
+        return nullptr;
+    }
+    return _attribute_keys[index];
+}
+
+int ContextApi::getAttributeKeyCount() {
+    return _attribute_key_count;
+}