my_alloc.cc

/* Copyright (c) 2000, 2024, Oracle and/or its affiliates.
   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License, version 2.0,
   as published by the Free Software Foundation.

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   as designated in a particular file or component or in included license
   documentation.  The authors of MySQL hereby grant you an additional
   permission to link the program and your derivative works with the
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   Without limiting anything contained in the foregoing, this file,
   which is part of C Driver for MySQL (Connector/C), is also subject to the
   Universal FOSS Exception, version 1.0, a copy of which can be found at
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   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License, version 2.0, for more details.

   You should have received a copy of the GNU General Public License
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   Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301  USA */

/**
 * @file mysys/my_alloc.cc
 * Implementation of MEM_ROOT.
 *
 * This file follows Google coding style.
 */

#include <stdarg.h>
#include <string.h>
#include <sys/types.h>
#include <iterator>

#include "my_alloc.h"
#include "my_compiler.h"
#include "my_dbug.h"
#include "my_inttypes.h"
#include "my_pointer_arithmetic.h"
#include "my_sys.h"
#include "mysql/service_mysql_alloc.h"
#include "mysys_err.h"
#include "template_utils.h"

// For instrumented code: Always use malloc(); never reuse a chunk.
// This gives a lot more memory chunks, each with a red-zone around them.
#if defined(HAVE_VALGRIND) || defined(HAVE_ASAN)
static constexpr bool MEM_ROOT_SINGLE_CHUNKS = true;
#else
static constexpr bool MEM_ROOT_SINGLE_CHUNKS = false;
#endif

MEM_ROOT::Block *MEM_ROOT::AllocBlock(size_t wanted_length,
                                      size_t minimum_length) {
  DBUG_TRACE;

  size_t length = wanted_length;
  if (m_max_capacity != 0) {
    size_t bytes_left;
    if (m_allocated_size > m_max_capacity) {
      bytes_left = 0;
    } else {
      bytes_left = m_max_capacity - m_allocated_size;
    }
    if (wanted_length > bytes_left) {
      if (m_error_for_capacity_exceeded) {
        my_error(EE_CAPACITY_EXCEEDED, MYF(0),
                 static_cast<ulonglong>(m_max_capacity));
        // NOTE: No early return; we will abort the query at the next safe
        // point. We also don't go down to minimum_length, as this will give a
        // new block on every subsequent Alloc() (of which there might be
        // many, since we don't know when the next safe point will be).
      } else if (minimum_length <= bytes_left) {
        // Make one final chunk with all that we have left.
        length = bytes_left;
      } else {
        // We don't have enough memory left to satisfy minimum_length.
        return nullptr;
      }
    }
  }

  const size_t bytes_to_alloc = length + ALIGN_SIZE(sizeof(Block));
  Block *new_block = static_cast<Block *>(
      my_malloc(m_psi_key, bytes_to_alloc, MYF(MY_WME | ME_FATALERROR)));
  if (new_block == nullptr) {
    if (m_error_handler) (m_error_handler)();
    return nullptr;
  }
  TRASH(new_block, bytes_to_alloc);
  new_block->end = pointer_cast<char *>(new_block) + bytes_to_alloc;

  m_allocated_size += length;

  // Make the default block size 50% larger next time.
  // This ensures O(1) total mallocs (assuming Clear() is not called).
  if (!MEM_ROOT_SINGLE_CHUNKS) {
    m_block_size += m_block_size / 2;
  }
  return new_block;
}

void *MEM_ROOT::AllocSlow(size_t length) {
  DBUG_TRACE;
  DBUG_PRINT("enter", ("root: %p", this));

  // We need to allocate a new block to satisfy this allocation;
  // otherwise, the fast path in Alloc() would not have sent us here.
  // We plan to allocate a block of <block_size> bytes; see if that
  // would be enough or not.
  if (length >= m_block_size || MEM_ROOT_SINGLE_CHUNKS) {
    // The next block we'd allocate would _not_ be big enough
    // (or we're in Valgrind/ASAN mode, and want everything in single chunks).
    // Allocate an entirely new block, not disturbing anything;
    // since the new block isn't going to be used for the next allocation
    // anyway, we can just as well keep the previous one.
    Block *new_block =
        AllocBlock(/*wanted_length=*/length, /*minimum_length=*/length);
    if (new_block == nullptr) return nullptr;

    if (m_current_block == nullptr) {
      // This is the only block, so it has to be the current block, too.
      // However, it will be full, so we won't be allocating from it
      // unless ClearForReuse() is called.
      new_block->prev = nullptr;
      m_current_block = new_block;
      m_current_free_end = new_block->end;
      m_current_free_start = m_current_free_end;
    } else {
      // Insert the new block in the second-to-last position.
      new_block->prev = m_current_block->prev;
      m_current_block->prev = new_block;
    }

    return pointer_cast<char *>(new_block) + ALIGN_SIZE(sizeof(*new_block));
  } else {
    // The normal case: Throw away the current block, allocate a new block,
    // and use that to satisfy the new allocation.
    if (ForceNewBlock(/*minimum_length=*/length)) {
      return nullptr;
    }
    char *new_mem = m_current_free_start;
    m_current_free_start += length;
    return new_mem;
  }
}

bool MEM_ROOT::ForceNewBlock(size_t minimum_length) {
  if (MEM_ROOT_SINGLE_CHUNKS) {
    assert(m_block_size == m_orig_block_size);
  }
  Block *new_block = AllocBlock(/*wanted_length=*/ALIGN_SIZE(m_block_size),
                                minimum_length);  // Will modify block_size.
  if (new_block == nullptr) return true;

  new_block->prev = m_current_block;
  m_current_block = new_block;

  char *new_mem =
      pointer_cast<char *>(new_block) + ALIGN_SIZE(sizeof(*new_block));
  m_current_free_start = new_mem;
  m_current_free_end = new_block->end;
  return false;
}

void MEM_ROOT::Clear() {
  DBUG_TRACE;
  DBUG_PRINT("enter", ("root: %p", this));

  // Already cleared, or memset() to zero, so just ignore.
  if (m_current_block == nullptr) return;

  Block *start = m_current_block;

  m_current_block = nullptr;
  m_block_size = m_orig_block_size;
  m_current_free_start = &s_dummy_target;
  m_current_free_end = &s_dummy_target;
  m_allocated_size = 0;

  FreeBlocks(start);
}

void MEM_ROOT::ClearForReuse() {
  DBUG_TRACE;

  if (MEM_ROOT_SINGLE_CHUNKS) {
    Clear();
    return;
  }

  // Already cleared, or memset() to zero, so just ignore.
  if (m_current_block == nullptr) return;

  // Keep the last block, which is usually the biggest one.
  m_current_free_start = pointer_cast<char *>(m_current_block) +
                         ALIGN_SIZE(sizeof(*m_current_block));
  Block *start = m_current_block->prev;
  m_current_block->prev = nullptr;
  m_allocated_size = m_current_free_end - m_current_free_start;
  TRASH(m_current_free_start, m_allocated_size);

  FreeBlocks(start);
}

void MEM_ROOT::FreeBlocks(Block *start) {
  // The MEM_ROOT might be allocated on itself, so make sure we don't
  // touch it after we've started freeing.
  for (Block *block = start; block != nullptr;) {
    Block *prev = block->prev;
    TRASH(block, std::distance(pointer_cast<char *>(block), block->end));
    my_free(block);
    block = prev;
  }
}

void MEM_ROOT::Claim(bool claim) {
  DBUG_TRACE;
  DBUG_PRINT("enter", ("root: %p", this));

  for (Block *block = m_current_block; block != nullptr; block = block->prev) {
    my_claim(block, claim);
  }
}

/*
 * Allocate many pointers at the same time.
 *
 * DESCRIPTION
 *   ptr1, ptr2, etc all point into big allocated memory area.
 *
 * SYNOPSIS
 *   multi_alloc_root()
 *     root               Memory root
 *     ptr1, length1      Multiple arguments terminated by a NULL pointer
 *     ptr2, length2      ...
 *     ...
 *     NULL
 *
 * RETURN VALUE
 *   A pointer to the beginning of the allocated memory block
 *   in case of success or NULL if out of memory.
 */

void *multi_alloc_root(MEM_ROOT *root, ...) {
  va_list args;
  char **ptr, *start, *res;
  size_t tot_length, length;
  DBUG_TRACE;

  va_start(args, root);
  tot_length = 0;
  while ((ptr = va_arg(args, char **))) {
    length = va_arg(args, uint);
    tot_length += ALIGN_SIZE(length);
  }
  va_end(args);

  if (!(start = static_cast<char *>(root->Alloc(tot_length))))
    return nullptr; /* purecov: inspected */

  va_start(args, root);
  res = start;
  while ((ptr = va_arg(args, char **))) {
    *ptr = res;
    length = va_arg(args, uint);
    res += ALIGN_SIZE(length);
  }
  va_end(args);
  return (void *)start;
}

char *strdup_root(MEM_ROOT *root, const char *str) {
  return strmake_root(root, str, strlen(str));
}

char *safe_strdup_root(MEM_ROOT *root, const char *str) {
  return str ? strdup_root(root, str) : nullptr;
}

char *strmake_root(MEM_ROOT *root, const char *str, size_t len) {
  char *pos;
  if ((pos = static_cast<char *>(root->Alloc(len + 1)))) {
    if (len > 0) memcpy(pos, str, len);
    pos[len] = 0;
  }
  return pos;
}

void *memdup_root(MEM_ROOT *root, const void *str, size_t len) {
  char *pos;
  if ((pos = static_cast<char *>(root->Alloc(len)))) {
    memcpy(pos, str, len);
  }
  return pos;
}

char MEM_ROOT::s_dummy_target;