Hello,
To demonstrate the abilities of PVS-Studio analyzer, we decided to find several bugs.

Status verifySystemIndexes(OperationContext* opCtx) {
  ....
  IndexCatalog* indexCatalog = collection->getIndexCatalog();
  std::vector<IndexDescriptor*> indexes;
  indexCatalog->findIndexesByKeyPattern(opCtx,                    // <=
                                        v1SystemUsersKeyPattern, 
                                        false, 
                                        &indexes);
  if (indexCatalog && !indexes.empty()) {                         // <=
    ....
  }
  ....
}

A link to the source code on GitHub

PVS-Studio warning: V595 The 'indexCatalog' pointer was utilized before it was verified against nullptr. Check lines: 100, 102. auth_index_d.cpp 100

In the conditional expression of the if statement there is a check that the pointer indexCatalog is non-null. But earlier in the code we see a call of the method findIndexesByKeyPattern via the pointer indexCatalog. Thus, if indexCatalog is a null pointer, then in the line 100 we'll have a null pointer dereference before the necessary check. If the pointer indexCatalog can never be null, then the statement indexCatalog && !indexes.empty() is redundant.

string IndexBoundsBuilder::simpleRegex(....) {
  ....
  if (c == 'Q') {
    ....
  } else if ((c >= 'A' && c <= 'Z') || 
             (c >= 'a' && c <= 'z') || 
             (c >= '0' && c <= '0') ||          // <=
             (c == '\0')) {
    ....
  }
  ....
}

A link to the source code on GitHub

PVS-Studio warning: V590 Consider inspecting the 'c >= '0' && c <= '0'' expression. The expression is excessive or contains a misprint. index_bounds_builder.cpp 145

Most likely, the subexpression c >= '0' && c <= '0' has an error, there is no range check of the symbol (this subexpression will be true only in case c == '0'). Judging by other subexpressions, supposedly it should be as follows: c >= '0' && c <= '9'.

void* MemoryMappedFile::map(....) {
  ....
  size_t len = strlen(filename);
  for (size_t i = len - 1; i >= 0; i--) {            // <=
    if (filename[i] == '/' || filename[i] == '\\')
      break;

    if (filename[i] == ':')
      filename[i] = '_';
  }
  ....
}

A link to the source code on GitHub

PVS-Studio warning: V547 Expression 'i >= 0' is always true. Unsigned type value is always >= 0. mmap_windows.cpp 197

The conditional statement of the loop (i >= 0) should always be true, as the loop counter ( i ) is of unsigned type (size_t).

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Best regards,
Sergey Vasiliev