[Sanitizer] Internal Printf string precision argument + padding.

[alekseyshl]

Summary

Example:

Printf("%.*s", 5, "123");

should yield:

'123  '

In case Printf's requested string precision is larger than the string
argument, the resulting string should be padded up to the requested
precision.

For the simplicity sake, implementing right padding only.

lib/sanitizer_common/sanitizer_printf.cc

static int AppendString(char **buff, const char *buff_end, int precision,
   if (!s)
     s = "<null>";
   int result = 0;
   for (; *s; s++) {
     if (precision >= 0 && result >= precision)
       break;
     result += AppendChar(buff, buff_end, *s);
   }
+  while (result < precision)
+    result += AppendChar(buff, buff_end, ' ');
   return result;
 }
 
 static int AppendPointer(char **buff, const char *buff_end, u64 ptr_value) {
   int result = 0;
   result += AppendString(buff, buff_end, -1, "0x");
   result += AppendUnsigned(buff, buff_end, ptr_value, 16,
                            SANITIZER_POINTER_FORMAT_LENGTH,

lib/sanitizer_common/tests/sanitizer_printf_test.cc

 }
 
 TEST(Printf, Precision) {
   char buf[1024];
   uptr len = internal_snprintf(buf, sizeof(buf), "%.*s", 3, "12345");
   EXPECT_EQ(3U, len);
   EXPECT_STREQ("123", buf);
   len = internal_snprintf(buf, sizeof(buf), "%.*s", 6, "12345");
-  EXPECT_EQ(5U, len);
-  EXPECT_STREQ("12345", buf);
+  EXPECT_EQ(6U, len);
+  EXPECT_STREQ("12345 ", buf);
+  // CHeck that precision does not overflow the smaller buffer, although
+  // 10 chars is requested, it stops at the buffer size, 8.
+  len = internal_snprintf(buf, 8, "%.*s", 10, "12345");
+  EXPECT_EQ(10U, len);  // The required size reported.
+  EXPECT_STREQ("12345  ", buf);
 }
 
 }  // namespace __sanitizer