[Scudo] Improve ScopedString constructor

[vitalybuka]

Summary

Avoid referencing elements beyond internal vector size.

[vitalybuka]

undo accidental change

[vitalybuka]

Please stamp this one to avoid conflicts with D103725

[cryptoad]

Was it the DCHECK issues I fixed with https://reviews.llvm.org/D103716 or something else?

[vitalybuka]

In D103718#2800231, @cryptoad wrote:

Was it the DCHECK issues I fixed with https://reviews.llvm.org/D103716 or something else?

I stated that as a fix for DCHECK, but then I noticed that you already landed the fix.
However then I noticed .data() on just created string is still kind of not nice even if it has no DCHECK there.

compiler-rt/lib/scudo/standalone/string_utils.h

 #include "vector.h"
 
 #include <stdarg.h>
 
 namespace scudo {
 
 class ScopedString {
 public:
-  explicit ScopedString() : String() {}
+  explicit ScopedString() { String.push_back('\0'); }
   uptr length() { return Length; }
   const char *data() { return String.data(); }
   void clear() {
-    if (!String.empty())
-      String[0] = '\0';
+    String.clear();
+    String.push_back('\0');
     Length = 0;
   }
   void append(const char *Format, va_list Args);
   void append(const char *Format, ...);
   void output() const { outputRaw(String.data()); }
 
 private:
   Vector<char> String;

compiler-rt/lib/scudo/standalone/tests/strings_test.cpp

 //===-- strings_test.cpp ----------------------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "tests/scudo_unit_test.h"
 
 #include "string_utils.h"
 
 #include <limits.h>
 
+TEST(ScudoStringsTest, Constructor) {
+  scudo::ScopedString Str;
+  EXPECT_EQ(0, Str.length());
+  EXPECT_EQ('\0', *Str.data());
+}
+
 TEST(ScudoStringsTest, Basic) {
   scudo::ScopedString Str;
   Str.append("a%db%zdc%ue%zuf%xh%zxq%pe%sr", static_cast<int>(-1),
              static_cast<scudo::uptr>(-2), static_cast<unsigned>(-4),
              static_cast<scudo::uptr>(5), static_cast<unsigned>(10),
              static_cast<scudo::uptr>(11), reinterpret_cast<void *>(0x123),
              "_string_");
   EXPECT_EQ(Str.length(), strlen(Str.data()));
 
   std::string expectedString = "a-1b-2c4294967292e5fahbq0x";
   expectedString += std::string(SCUDO_POINTER_FORMAT_LENGTH - 3, '0');
   expectedString += "123e_string_r";
   EXPECT_EQ(Str.length(), strlen(Str.data()));
   EXPECT_STREQ(expectedString.c_str(), Str.data());
 }
 
+TEST(ScudoStringsTest, Clear) {
+  scudo::ScopedString Str;
+  Str.append("123");
+  Str.clear();
+  EXPECT_EQ(0, Str.length());
+  EXPECT_EQ('\0', *Str.data());
+}
+
+TEST(ScudoStringsTest, ClearLarge) {
+  scudo::ScopedString Str;
+  for (int i = 0; i < 10000; ++i)
+    Str.append("123");
+  Str.clear();
+  EXPECT_EQ(0, Str.length());
+  EXPECT_EQ('\0', *Str.data());
+}
+
 TEST(ScudoStringsTest, Precision) {
   scudo::ScopedString Str;
   Str.append("%.*s", 3, "12345");
   EXPECT_EQ(Str.length(), strlen(Str.data()));
   EXPECT_STREQ("123", Str.data());
   Str.clear();
   Str.append("%.*s", 6, "12345");
   EXPECT_EQ(Str.length(), strlen(Str.data()));