[Linux] Force the inclusion of the backtracing code when static linking.

When we're statically linking the standard library, we need to force
the inclusion of the backtracing code in the runtime, otherwise we don't
get on-crash backtraces.

Also, add a test to make sure that this works.

rdar://115774613

Author: al45tair

stdlib/public/runtime/SwiftRT-ELF-WASM.cpp

@@ -12,6 +12,7 @@
 
 #include "ImageInspectionCommon.h"
 #include "swift/shims/MetadataSections.h"
+#include "swift/Runtime/Backtrace.h"
 
 #include <cstddef>
 #include <new>
@@ -24,6 +25,11 @@ extern "C" const char __dso_handle[];
 static constexpr const void *__dso_handle = nullptr;
 #endif
 
+// Drag in a symbol from the backtracer, to force the static linker to include
+// the code.
+static const void *__backtraceRef __attribute__((used))
+  = (const void *)swift::runtime::backtrace::_swift_backtrace_isThunkFunction;
+
 // Create empty sections to ensure that the start/stop symbols are synthesized
 // by the linker.  Otherwise, we may end up with undefined symbol references as
 // the linker table section was never constructed.

test/Backtracing/CrashStatic.swift

@@ -0,0 +1,60 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-build-swift %s -parse-as-library %import-libdispatch -Onone -static-stdlib -g -o %t/CrashStatic
+// RUN: %target-codesign %t/CrashStatic
+// RUN: (env SWIFT_BACKTRACE=enable=yes,cache=no,swift-backtrace=%backtracer %target-run %t/CrashStatic 2>&1 || true) | %FileCheck %s
+
+// UNSUPPORTED: use_os_stdlib
+// UNSUPPORTED: back_deployment_runtime
+// UNSUPPORTED: asan
+// REQUIRES: executable_test
+// REQUIRES: backtracing
+// REQUIRES: static_stdlib
+// REQUIRES: OS=linux-gnu
+
+func level1() {
+  level2()
+}
+
+func level2() {
+  level3()
+}
+
+func level3() {
+  level4()
+}
+
+func level4() {
+  level5()
+}
+
+func level5() {
+  print("About to crash")
+  let ptr = UnsafeMutablePointer<Int>(bitPattern: 4)!
+  ptr.pointee = 42
+}
+
+@main
+struct CrashStatic {
+  static func main() {
+    level1()
+  }
+}
+
+// CHECK: *** Program crashed: Bad pointer dereference at 0x{{0+}}4 ***
+
+// CHECK: Thread 0 {{(".*" )?}}crashed:
+
+// CHECK: 0               0x{{[0-9a-f]+}} level5() + {{[0-9]+}} in CrashStatic at {{.*}}/CrashStatic.swift:33:15
+// CHECK-NEXT: 1 [ra]          0x{{[0-9a-f]+}} level4() + {{[0-9]+}} in CrashStatic at {{.*}}/CrashStatic.swift:27:3
+// CHECK-NEXT: 2 [ra]          0x{{[0-9a-f]+}} level3() + {{[0-9]+}} in CrashStatic at {{.*}}/CrashStatic.swift:23:3
+// CHECK-NEXT: 3 [ra]          0x{{[0-9a-f]+}} level2() + {{[0-9]+}} in CrashStatic at {{.*}}/CrashStatic.swift:19:3
+// CHECK-NEXT: 4 [ra]          0x{{[0-9a-f]+}} level1() + {{[0-9]+}} in CrashStatic at {{.*}}/CrashStatic.swift:15:3
+// CHECK-NEXT: 5 [ra]          0x{{[0-9a-f]+}} static CrashStatic.main() + {{[0-9]+}} in CrashStatic at {{.*}}/CrashStatic.swift:39:5
+// CHECK-NEXT: 6 [ra] [system] 0x{{[0-9a-f]+}} static CrashStatic.$main() + {{[0-9]+}} in CrashStatic at {{.*}}/<compiler-generated>
+// CHECK-NEXT: 7 [ra] 0x{{[0-9a-f]+}} main + {{[0-9]+}} in CrashStatic at {{.*}}/CrashStatic.swift
+
+// CHECK: Registers:
+
+// CHECK: Images ({{[0-9]+}} omitted):
+
+// CHECK: {{0x[0-9a-f]+}}–{{0x[0-9a-f]+}}{{ +}}{{([0-9a-f]+|<no build ID>)}}{{ +}}CrashStatic{{ +}}{{.*}}/CrashStatic

test/lit.cfg

@@ -354,6 +354,7 @@ config.scale_test = make_path(config.swift_utils, 'scale-test')
 config.PathSanitizingFileCheck = make_path(config.swift_utils, 'PathSanitizingFileCheck')
 config.swift_bin_dir = make_path(config.swift, '..', '..', 'bin')
 config.swift_lib_dir = make_path(config.swift, '..', '..', 'lib')
+config.swift_libexec_dir = make_path(config.swift, '..', '..', 'libexec')
 config.swift_share_dir = make_path(config.swift, '..', '..', 'share')
 config.round_trip_syntax_test = make_path(config.swift_utils, 'round-trip-syntax-test')
 config.refactor_check_compiles = make_path(config.swift_utils, 'refactor-check-compiles.py')
@@ -588,6 +589,7 @@ else:
 config.substitutions.append( ('%llvm_obj_root', config.llvm_obj_root) )
 config.substitutions.append( ('%swift-bin-dir', config.swift_bin_dir) )
 config.substitutions.append( ('%swift-lib-dir', config.swift_lib_dir) )
+config.substitutions.append( ('%swift-libexec-dir', config.swift_lib_dir) )
 config.substitutions.append( ('%swift-share-dir', config.swift_share_dir) )
 config.substitutions.append( ('%swift-plugin-dir', config.swift_plugin_dir) )
 config.substitutions.append( ('%llvm_src_root', config.llvm_src_root) )
@@ -1996,6 +1998,10 @@ if run_vendor == 'apple':
         config.available_features.add('back_deploy_concurrency')
         concurrency_back_deploy_path = os.path.join(os.path.dirname(swift_obj_root), os.path.basename(swift_obj_root).replace("swift-", "backdeployconcurrency-"), 'lib', 'swift-5.5', xcrun_sdk_name)
 
+backtracer_path = make_path(config.swift_libexec_dir, 'swift',
+                            config.target_sdk_name, 'swift-backtrace')
+config.substitutions.append(('%backtracer', backtracer_path))
+
 def os_stdlib_paths():
     if run_vendor == 'apple':
         if run_os == 'maccatalyst':