feat(otel_thread_ctx): thread-level ctx publication

libdd-library-config/src/otel_process_ctx.rs

@@ -1,4 +1,4 @@
-// Copyright 2021-Present Datadog, Inc. https://www.datadoghq.com/
+// Copyright 2026-Present Datadog, Inc. https://www.datadoghq.com/
 // SPDX-License-Identifier: Apache-2.0
 
 //! Implementation of the publisher part of the [OTEL process
@@ -8,7 +8,8 @@
 //!
 //! Process context sharing implies concurrently writing to a memory area that another process
 //! might be actively reading. However, reading isn't done as direct memory accesses but goes
-//! through the OS, so the Rust definition of race conditions doesn't really apply.
+//! through the OS, so the Rust definition of race conditions doesn't really apply. We also use
+//! atomics and fences, see MappingHeader's documentation.
 
 #[cfg(target_os = "linux")]
 #[cfg(target_has_atomic = "64")]

libdd-profiling/Cargo.toml

@@ -81,4 +81,5 @@ strum = { version = "0.26", features = ["derive"] }
 tempfile = "3"
 
 [build-dependencies]
+cc = "1.1.31"
 cxx-build = { version = "1.0", optional = true }

libdd-profiling/build.rs

@@ -11,4 +11,20 @@ fn main() {
 
         println!("cargo:rerun-if-changed=src/cxx.rs");
     }
+
+    // Only compile the TLS shim on Linux; the thread-level context feature is Linux-only.
+    #[cfg(target_os = "linux")]
+    {
+        let mut build = cc::Build::new();
+
+        // - On aarch64, TLSDESC is already the only dynamic TLS model so no flag is needed.
+        // - On x86-64, we use `-mtls-dialect=gnu2` (supported since GCC 4.4 and Clang 19+) to force
+        //   the use of TLSDESC as mandated by the spec. If it's not supported, this build will
+        //   fail.
+        #[cfg(target_arch = "x86_64")]
+        build.flag("-mtls-dialect=gnu2");
+
+        build.file("src/tls_shim.c").compile("tls_shim");
+        println!("cargo:rerun-if-changed=src/tls_shim.c");
+    }
 }

libdd-profiling/src/lib.rs

@@ -14,5 +14,6 @@ pub mod cxx;
 pub mod exporter;
 pub mod internal;
 pub mod iter;
+pub mod otel_thread_ctx;
 pub mod pprof;
 pub mod profiles;