[lldb] Make lldbassert fire only once per instance (#134343)

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

Author: JDevlieghere

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,

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) {