[lldb] Make lldbassert fire only once per instance

[JDevlieghere]

The lldbassert macro in LLDB behaves like a regular assert when assertions are enabled, and otherwise prints a pretty backtrace and prompts the user to file a bug. By default, this is emitted as a diagnostic event, but vendors can provide their own behavior, for example pre-populating a bug report.

Recently, we ran into an issue where an lldbassert (in the Swift language plugin) would fire excessively, to the point that it was interfering with the usability of the debugger.

Once an lldbasser has fired, there's no point in bothering the user over and over again for the same problem. This PR solves the problem by introducing a static std::once_flag in the macro. This way, every lldbasser will fire at most once per lldb instance.

rdar://148520448

[llvmbot]

@llvm/pr-subscribers-lldb

Author: Jonas Devlieghere (JDevlieghere)

Changes

The lldbassert macro in LLDB behaves like a regular assert when assertions are enabled, and otherwise prints a pretty backtrace and prompts the user to file a bug. By default, this is emitted as a diagnostic event, but vendors can provide their own behavior, for example pre-populating a bug report.

Recently, we ran into an issue where an lldbassert (in the Swift language plugin) would fire excessively, to the point that it was interfering with the usability of the debugger.

Once an lldbasser has fired, there's no point in bothering the user over and over again for the same problem. This PR solves the problem by introducing a static std::once_flag in the macro. This way, every lldbasser will fire at most once per lldb instance.

rdar://148520448

---

Full diff: https://github.com/llvm/llvm-project/pull/134343.diff

2 Files Affected:

• (modified) lldb/include/lldb/Utility/LLDBAssert.h (+8-3)
• (modified) lldb/source/Utility/LLDBAssert.cpp (+21-16)

diff --git a/lldb/include/lldb/Utility/LLDBAssert.h b/lldb/include/lldb/Utility/LLDBAssert.h
index 21dbdb3b3202d..cee30b81402ca 100644
--- a/lldb/include/lldb/Utility/LLDBAssert.h
+++ b/lldb/include/lldb/Utility/LLDBAssert.h
@@ -10,6 +10,7 @@
 #define LLDB_UTILITY_LLDBASSERT_H
 
 #include "llvm/ADT/StringRef.h"
+#include <mutex>
 
 #ifndef NDEBUG
 #define lldbassert(x) assert(x)
@@ -19,8 +20,11 @@
 // __FILE__ but only renders the last path component (the filename) instead of
 // an invocation dependent full path to that file.
 #define lldbassert(x)                                                          \
-  lldb_private::_lldb_assert(static_cast<bool>(x), #x, __FUNCTION__,           \
-                             __FILE_NAME__, __LINE__)
+  do {                                                                         \
+    static std::once_flag _once_flag;                                          \
+    lldb_private::_lldb_assert(static_cast<bool>(x), #x, __FUNCTION__,         \
+                               __FILE_NAME__, __LINE__, _once_flag)            \
+  } while (0)
 #else
 #define lldbassert(x)                                                          \
   lldb_private::_lldb_assert(static_cast<bool>(x), #x, __FUNCTION__, __FILE__, \
@@ -33,7 +37,8 @@ namespace lldb_private {
 /// Don't use _lldb_assert directly. Use the lldbassert macro instead so that
 /// LLDB asserts become regular asserts in NDEBUG builds.
 void _lldb_assert(bool expression, const char *expr_text, const char *func,
-                  const char *file, unsigned int line);
+                  const char *file, unsigned int line,
+                  std::once_flag &once_flag);
 
 /// The default LLDB assert callback, which prints to stderr.
 typedef void (*LLDBAssertCallback)(llvm::StringRef message,
diff --git a/lldb/source/Utility/LLDBAssert.cpp b/lldb/source/Utility/LLDBAssert.cpp
index b84c581ccf822..611ad43cd071b 100644
--- a/lldb/source/Utility/LLDBAssert.cpp
+++ b/lldb/source/Utility/LLDBAssert.cpp
@@ -11,6 +11,7 @@
 #include "llvm/Support/FormatVariadic.h"
 #include "llvm/Support/Signals.h"
 #include "llvm/Support/raw_ostream.h"
+#include <mutex>
 
 #if LLVM_SUPPORT_XCODE_SIGNPOSTS
 #include <os/log.h>
@@ -33,29 +34,33 @@ static std::atomic<LLDBAssertCallback> g_lldb_assert_callback =
     &DefaultAssertCallback;
 
 void _lldb_assert(bool expression, const char *expr_text, const char *func,
-                  const char *file, unsigned int line) {
+                  const char *file, unsigned int line,
+                  std::once_flag &once_flag) {
   if (LLVM_LIKELY(expression))
     return;
 
+  std::call_once(once_flag, [&]() {
 #if LLVM_SUPPORT_XCODE_SIGNPOSTS
-  if (__builtin_available(macos 10.12, iOS 10, tvOS 10, watchOS 3, *)) {
-    os_log_fault(OS_LOG_DEFAULT,
-                 "Assertion failed: (%s), function %s, file %s, line %u\n",
-                 expr_text, func, file, line);
-  }
+    if (__builtin_available(macos 10.12, iOS 10, tvOS 10, watchOS 3, *)) {
+      os_log_fault(OS_LOG_DEFAULT,
+                   "Assertion failed: (%s), function %s, file %s, line %u\n",
+                   expr_text, func, file, line);
+    }
 #endif
 
-  std::string buffer;
-  llvm::raw_string_ostream backtrace(buffer);
-  llvm::sys::PrintStackTrace(backtrace);
+    std::string buffer;
+    llvm::raw_string_ostream backtrace(buffer);
+    llvm::sys::PrintStackTrace(backtrace);
 
-  (*g_lldb_assert_callback.load())(
-      llvm::formatv("Assertion failed: ({0}), function {1}, file {2}, line {3}",
-                    expr_text, func, file, line)
-          .str(),
-      buffer,
-      "Please file a bug report against lldb and include the backtrace, the "
-      "version and as many details as possible.");
+    (*g_lldb_assert_callback.load())(
+        llvm::formatv(
+            "Assertion failed: ({0}), function {1}, file {2}, line {3}",
+            expr_text, func, file, line)
+            .str(),
+        buffer,
+        "Please file a bug report against lldb and include the backtrace, the "
+        "version and as many details as possible.");
+  });
 }
 
 void SetLLDBAssertCallback(LLDBAssertCallback callback) {