[Support] Trim trailing null character in PrettyStackTraceFormat.

[jonmeow]

Space for the \0 is added on line 247, but the value is not trimmed. SmallVector prints the entire contents, including the \0. To fix, pop the back value to remove the \0.

Sample program, and cat -v can be used to show the null (as ^@):

#include "llvm/Support/PrettyStackTrace.h"

auto main() -> int {
  auto msg = llvm::PrettyStackTraceFormat("x");
  llvm::EnablePrettyStackTrace();
  assert(false);
}

Before:

$ ./crash_test |& cat -v
crash_test: toolchain/driver/crash_test.cpp:6: int main(): Assertion `false' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.
Stack dump:
0.	x^@

After:

$ ./crash_test |& cat -v
crash_test: toolchain/driver/crash_test.cpp:6: int main(): Assertion `false' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.
Stack dump:
0.	x

Fixes #81273

[llvmbot]

@llvm/pr-subscribers-llvm-support

Author: Jon Ross-Perkins (jonmeow)

Changes

Space for the \0 is added on line 247, but the value is not trimmed. SmallVector prints the entire contents, including the \0. To fix, pop the back value to remove the \0.

Sample program, and cat -v can be used to show the null (as ^@):

#include "llvm/Support/PrettyStackTrace.h"

auto main() -> int {
  auto msg = llvm::PrettyStackTraceFormat("x");
  llvm::EnablePrettyStackTrace();
  assert(false);
}

Before:

$ ./crash_test |& cat -v
crash_test: toolchain/driver/crash_test.cpp:6: int main(): Assertion `false' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.
Stack dump:
0.	x^@

After:

$ ./crash_test |& cat -v
crash_test: toolchain/driver/crash_test.cpp:6: int main(): Assertion `false' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.
Stack dump:
0.	x

---

Full diff: https://github.com/llvm/llvm-project/pull/77351.diff

1 Files Affected:

• (modified) llvm/lib/Support/PrettyStackTrace.cpp (+2)

diff --git a/llvm/lib/Support/PrettyStackTrace.cpp b/llvm/lib/Support/PrettyStackTrace.cpp
index f9f1b8a419b820..fc3e08e3662217 100644
--- a/llvm/lib/Support/PrettyStackTrace.cpp
+++ b/llvm/lib/Support/PrettyStackTrace.cpp
@@ -249,6 +249,8 @@ PrettyStackTraceFormat::PrettyStackTraceFormat(const char *Format, ...) {
   va_start(AP, Format);
   vsnprintf(Str.data(), Size, Format, AP);
   va_end(AP);
+  // Remove the '\0' for printing.
+  Str.pop_back();
 }
 
 void PrettyStackTraceFormat::print(raw_ostream &OS) const { OS << Str << "\n"; }

[jonmeow]

AFAICT the buildkite issues aren't related, but I'm having trouble getting them to cleanly run. Is there something I should be doing here for review?