Add throw-site stack trace capture with Linux implementation

Summary:
Hooks into __cxa_throw to capture stack traces at the throw site on Linux using
folly::exception_tracer

Reviewed By: vilatto, janezhang10

Differential Revision: D96944408

fbshipit-source-id: 5439280516bc430a329a94f65c4d55d4060a6b9c

Author: Chandhana Solainathan

eden/fs/telemetry/BUCK

@@ -115,6 +115,22 @@ cpp_library(
     ],
 )
 
+cpp_library(
+    name = "throw_trace_capture",
+    srcs = [
+        "ThrowTraceCapture.cpp",
+    ],
+    headers = [
+        "ThrowTraceCapture.h",
+    ],
+    deps = [
+        "//folly/debugging/exception_tracer:exception_tracer_base",
+    ],
+    exported_deps = [
+        "//folly/debugging/exception_tracer:exception_tracer",  # @manual
+    ],
+)
+
 cpp_library(
     name = "error_info",
     srcs = [

eden/fs/telemetry/ThrowTraceCapture.cpp

@@ -0,0 +1,55 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This software may be used and distributed according to the terms of the
+ * GNU General Public License version 2.
+ */
+
+#include "eden/fs/telemetry/ThrowTraceCapture.h"
+
+#ifdef __linux__
+// =============================================================================
+// Linux: folly::exception_tracer
+//
+// Uses __cxa_throw hook (via --wrap linker flag) to capture stack frames at
+// throw time. Frames are stored in thread-local StackTraceStack and symbolized
+// via folly's ELF/DWARF symbolizer.
+// =============================================================================
+
+#include <sstream>
+
+#include <folly/debugging/exception_tracer/ExceptionTracer.h>
+
+namespace facebook::eden {
+
+std::optional<std::string> getThrowSiteStackTrace() {
+  auto exceptions = folly::exception_tracer::getCurrentExceptions();
+  if (!exceptions.empty()) {
+    std::ostringstream ss;
+    for (const auto& info : exceptions) {
+      ss << info;
+    }
+    auto trace = ss.str();
+    if (!trace.empty()) {
+      return trace;
+    }
+  }
+  return std::nullopt;
+}
+
+} // namespace facebook::eden
+
+#else
+// =============================================================================
+// Unsupported platform — no throw-site stack traces available.
+// =============================================================================
+
+namespace facebook::eden {
+
+std::optional<std::string> getThrowSiteStackTrace() {
+  return std::nullopt;
+}
+
+} // namespace facebook::eden
+
+#endif // __linux__

eden/fs/telemetry/ThrowTraceCapture.h

@@ -0,0 +1,26 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This software may be used and distributed according to the terms of the
+ * GNU General Public License version 2.
+ */
+
+#pragma once
+
+#include <optional>
+#include <string>
+
+namespace facebook::eden {
+
+/**
+ * Returns the throw-site stack trace for the current exception.
+ * Must be called inside a catch block.
+ *
+ * Platform-specific implementations:
+ *   Linux:   folly::exception_tracer (hooks __cxa_throw via --wrap)
+ *   macOS:   Not yet implemented (returns std::nullopt)
+ *   Windows: Not yet implemented (returns std::nullopt)
+ */
+std::optional<std::string> getThrowSiteStackTrace();
+
+} // namespace facebook::eden

eden/fs/telemetry/test/BUCK

@@ -62,6 +62,19 @@ cpp_benchmark(
     ],
 )
 
+cpp_unittest(
+    name = "throw_trace_capture_test",
+    srcs = [
+        "ThrowTraceCaptureTest.cpp",
+    ],
+    supports_static_listing = False,
+    deps = [
+        "fbsource//third-party/googletest:gtest",
+        "//eden/fs/telemetry:throw_trace_capture",
+        "//folly:c_portability",
+    ],
+)
+
 cpp_unittest(
     name = "error_info_builder_test",
     srcs = [

eden/fs/telemetry/test/ThrowTraceCaptureTest.cpp

@@ -0,0 +1,51 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This software may be used and distributed according to the terms of the
+ * GNU General Public License version 2.
+ */
+
+#include "eden/fs/telemetry/ThrowTraceCapture.h"
+
+#include <stdexcept>
+
+#include <folly/CPortability.h>
+#include <gtest/gtest.h>
+
+using namespace facebook::eden;
+
+#ifdef __linux__
+namespace {
+
+[[noreturn]] FOLLY_NOINLINE void innerThrowingFunc() {
+  throw std::runtime_error("inner error");
+}
+
+[[noreturn]] FOLLY_NOINLINE void outerThrowingFunc() {
+  innerThrowingFunc();
+}
+
+} // namespace
+#endif
+
+TEST(ThrowTraceCaptureTest, CapturesThrowSiteStackTrace) {
+#ifndef __linux__
+  GTEST_SKIP() << "Stack trace capture not yet implemented on this platform";
+#else
+  try {
+    outerThrowingFunc();
+  } catch (const std::exception&) {
+    auto trace = getThrowSiteStackTrace();
+    ASSERT_TRUE(trace.has_value()) << "Stack trace capture is broken";
+    EXPECT_NE(trace->find("innerThrowingFunc"), std::string::npos)
+        << "Expected throw-site function in trace, got: " << *trace;
+    EXPECT_NE(trace->find("outerThrowingFunc"), std::string::npos)
+        << "Expected caller function in trace, got: " << *trace;
+  }
+#endif
+}
+
+TEST(ThrowTraceCaptureTest, ReturnsNulloptOutsideCatchBlock) {
+  auto trace = getThrowSiteStackTrace();
+  EXPECT_FALSE(trace.has_value());
+}