Memory Allocations
==================

C language requires explicitly allocating and freeing memory. The main two
problems with this are:

 1. A lot of allocations and frees cause memory fragmentation. The longer a
    process runs, the more it could have leaked memory because there are tiny
    unused free spots all around in heap.
 2. Freeing memory is easy to forget, causing memory leaks. Sometimes it can be
    accidentally done multiple times, causing a potential security hole. A lot
    of free() calls all around in the code also makes the code more difficult
    to read and write.

The second problem could be solved with Boehm garbage collector, which Dovecot
can use optionally (prior to 2.3), but it's not very efficient. It also doesn't
help with the first problem.

To reduce the problems caused by these issues, Dovecot has several ways to do
memory management.

Common Design Decisions
-----------------------

All memory allocations (with some exceptions in data stack) return memory
filled with NULs. This is also true for new memory when growing an allocated
memory with realloc. The zeroing reduces accidental use of uninitialized memory
and makes the code simpler since there is no need to explicitly set all fields
in allocated structs to zero/NULL. (I guess assuming that this works correctly
for NULLs isn't strictly ANSI-C compliant, but I don't see this assumption
breaking in any system anyone would really use Dovecot.) The zeroing is cheap
anyway.

In out-of-memory situations memory allocation functions die internally by
calling 'i_fatal_status(FATAL_OUTOFMEM, ..)'. There are several reasons for
this:

 * Trying to handle malloc() failures explicitly would add a lot of error
   handling code paths and make the code much more complex than necessary.
 * In most systems malloc() rarely actually fails because the system has run
   out of memory. Instead the kernel will just start killing processes.
 * Typically when malloc() does fail, it's because the process's address space
   limit is reached. Dovecot enforces these limits by default. Reaching it
   could mean that the process was leaking memory and it should be killed. It
   could also mean that the process is doing more work than anticipated and
   that the limit should probably be increased.
 * Even with perfect out-of-memory handling, the result isn't much better
   anyway than the process dying. User isn't any happier by seeing "out of
   memory" error than "server disconnected".

When freeing memory, most functions usually also change the pointer to NULL.
This is also the reason why most APIs' deinit functions take pointer-to-pointer
parameter, so that when they're done they can change the original pointer to
NULL.

malloc() Replacements
---------------------

'lib/imem.h' has replacements for all the common memory allocation functions:

 * 'malloc', 'calloc' -> 'i_malloc()'
 * 'realloc()' -> 'i_realloc()'
 * 'strdup()' -> 'i_strdup()'
 * 'free()' -> 'i_free'
 * etc.

All memory allocation functions that begin with 'i_' prefix require that the
memory is later freed with 'i_free()'. If you actually want the freed pointer
to be set to NULL, use 'i_free_and_null()'. Currently 'i_free()' also changes
the pointer to NULL, but in future it might change to something else.

Memory Pools
------------

'lib/mempool.h' defines API for allocating memory through memory pools. All
memory allocations actually go through memory pools. Even the 'i_*()' functions
get called through 'default_pool', which by default is 'system_pool' but can be
changed to another pool if wanted. All memory allocation functions that begin
with 'p_' prefix have a memory pool parameter, which it uses to allocate the
memory.

Dovecot has many APIs that require you to specify a memory pool. Usually (but
not always) they don't bother freeing any memory from the pool, instead they
assume that more memory can be just allocated from the pool and the whole pool
is freed later. These pools are usually alloconly-pools, but can also be data
stack pools. See below.

Alloc-only Pools
----------------

'pool_alloconly_create()' creates an allocate-only memory pool with a given
initial size.

As the name says, alloconly-pools only support allocating more memory. As a
special case its last allocation can be freed.'p_realloc()' also tries to grow
the existing allocation only if it's the last allocation, otherwise it'll just
allocates new memory area and copies the data there.

Initial memory pool sizes are often optimized in Dovecot to be set large enough
that in most situations the pool doesn't need to be grown. To make this easier,
when Dovecot is configured with --enable-devel-checks, it logs a warning each
time a memory pool is grown. The initial pool size shouldn't of course be made
too large, so usually I just pick some small initial guessed value and later if
I get too many "growing memory pool" warnings I start growing the pool sizes.
Sometimes there's just no good way to set the initial pool size and avoid the
warnings, in that situation you can prefix the pool's name with MEMPOOL_GROWING
and it doesn't log warnings.

Alloconly-pools are commonly used for an object that builds its state from many
memory allocations, but doesn't change (much of) its state. It's a lot easier
when you can do a lot of small memory allocations and when destroying the
object you'll just free the memory pool.

Data Stack
----------

'lib/data-stack.h' describes the low-level data stack functions. Data stack
works a bit like C's control stack.'alloca()' is quite near to what it does,
but there's one major difference: In data stack the stack frames are explicitly
defined, so functions can return values allocated from data
stack.'t_strdup_printf()' call is an excellent example of why this is useful.
Rather than creating some arbitrary sized buffer and using snprintf(), which
might truncate the value, you can just use t_strdup_printf() without worrying
about buffer sizes being large enough.

Try to keep the allocations from data stack small, since the data stack's
highest memory usage size is kept for the rest of the process's lifetime. The
initial data stack size is 32kB, which should be enough in normal use. If
Dovecot is configured with --enable-devel-checks, it logs a warning each time
the data stack needs to be grown.

Stack frames are preferably created using T_BEGIN/T_END block, for example:

---%<-------------------------------------------------------------------------
T_BEGIN {
  string_t *str1 = t_str_new(256);
  string_t *str2 = t_str_new(256);
  /* .. */
} T_END;
---%<-------------------------------------------------------------------------

In the above example two strings are allocated from data stack. They get freed
once the code goes past T_END, that's why the variables are preferably declared
inside the T_BEGIN/T_END block so they won't accidentally be used after they're
freed.

T_BEGIN and T_END expand to 't_push()' and 't_pop()' calls and they must be
synchronized. Returning from the block without going past T_END is going to
cause Dovecot to panic in next T_END call with "Leaked t_pop() call" error.

Memory allocations have similar disadvantages to alloc-only memory pools.
Allocations can't be grown, so with the above example if str1 grows past 256
characters, it needs to be reallocated, which will cause it to forget about the
original 256 bytes and allocate 512 bytes more.

Memory allocations from data stack often begin with 't_' prefix, meaning
"temporary". There are however many other functions that allocate memory from
data stack without mentioning it. Memory allocated from data stack is usually
returned as a const pointer, so that the caller doesn't try to free it (which
would cause a compiler warning).

When should T_BEGIN/T_END used and when not? This is kind of black magic. In
general they shouldn't be used unless it's really necessary, because they make
the code more complex. But if the code is going through loops with many
iterations, where each iteration is allocating memory from data stack, running
each iteration inside its own stack frame would be a good idea to avoid
excessive memory usage. It's also difficult to guess how public APIs are being
used, so I've tried to make such API functions use their own private stack
frames. Dovecot's ioloop code also wraps all I/O callbacks and timeout
callbacks into their own stack frames, so you don't need to worry about them.

You can create temporary memory pools from data stack too. Usually you should
be calling 'pool_datastack_create()' to generate a new pool, which also tries
to track that it's not being used unsafely across different stack frames. Some
low-level functions can also use 'unsafe_data_stack_pool' as the pool, which
doesn't do such tracking.

Data stack's advantages over malloc():

 * FAST, most of the time allocating memory means only updating a couple of
   pointers and integers. Freeing memory all at once also is a fast operation.
 * No need to 'free()' each allocation resulting in prettier code
 * No memory leaks
 * No memory fragmentation

It also has some disadvantages:

 * Allocating memory inside loops can accidentally allocate a lot of memory
 * Memory allocated from data stack can be accidentally stored into a permanent
   location and accessed after it's already been freed.
 * Debugging invalid memory usage may be difficult using existing tools

(This file was created from the wiki on 2019-06-19 12:42)
