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mf_iocache.cc
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/* Copyright (c) 2000, 2025, 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.
This program is designed to work with certain software (including
but not limited to OpenSSL) that is licensed under separate terms,
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
separately licensed software that they have either included with
the program or referenced in the documentation.
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
http://oss.oracle.com/licenses/universal-foss-exception.
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
along with this program; if not, write to the Free Software
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA */
/**
@file mysys/mf_iocache.cc
Cashing of files with only does (sequential) read or writes of fixed-
length records. A read isn't allowed to go over file-length. A read is ok
if it ends at file-length and next read can try to read after file-length
(and get a EOF-error).
Possibly use of asyncronic io.
macros for read and writes for faster io.
Used instead of FILE when reading or writing whole files.
This code makes mf_rec_cache obsolete (currently only used by ISAM).
One can change info->pos_in_file to a higher value to skip bytes in the file
if also info->read_pos is set to info->read_end. If called through
open_cached_file(), then the temporary file will only be created if a write
exceeds the file buffer or if one calls my_b_flush_io_cache().
If one uses SEQ_READ_APPEND, then two buffers are allocated, one for
reading and another for writing. Reads are first done from disk and
then done from the write buffer. This is an efficient way to read
from a log file when one is writing to it at the same time.
For this to work, the file has to be opened in append mode!
Note that when one uses SEQ_READ_APPEND, one MUST write using
my_b_append ! This is needed because we need to lock the mutex
every time we access the write buffer.
TODO:
When one SEQ_READ_APPEND and we are reading and writing at the same time,
each time the write buffer gets full and it's written to disk, we will
always do a disk read to read a part of the buffer from disk to the
read buffer.
This should be fixed so that when we do a my_b_flush_io_cache() and
we have been reading the write buffer, we should transfer the rest of the
write buffer to the read buffer before we start to reuse it.
*/
#include <errno.h>
#include <fcntl.h>
#include <math.h>
#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <algorithm>
#include "my_byteorder.h"
#include "my_compiler.h"
#include "my_dbug.h"
#include "my_inttypes.h"
#include "my_io.h"
#include "my_macros.h"
#include "my_sys.h"
#include "my_systime.h"
#include "my_thread_local.h"
#include "mysql/components/services/bits/psi_bits.h"
#include "mysql/psi/mysql_cond.h"
#include "mysql/psi/mysql_file.h"
#include "mysql/psi/mysql_mutex.h"
#include "mysql/service_mysql_alloc.h"
#include "mysql/strings/int2str.h"
#include "mysys/mysys_priv.h"
#include "mysys/stream_cipher.h"
#include "thr_mutex.h"
PSI_file_key key_file_io_cache;
#ifndef NDEBUG
bool binlog_cache_temporary_file_is_encrypted = false;
#endif
#define lock_append_buffer(info) mysql_mutex_lock(&(info)->append_buffer_lock)
#define unlock_append_buffer(info) \
mysql_mutex_unlock(&(info)->append_buffer_lock)
#define IO_ROUND_UP(X) (((X) + IO_SIZE - 1) & ~(IO_SIZE - 1))
#define IO_ROUND_DN(X) ((X) & ~(IO_SIZE - 1))
/*
Setup internal pointers inside IO_CACHE
SYNOPSIS
setup_io_cache()
info IO_CACHE handler
NOTES
This is called on automatically on init or reinit of IO_CACHE
It must be called externally if one moves or copies an IO_CACHE
object.
*/
void setup_io_cache(IO_CACHE *info) {
/* Ensure that my_b_tell() and my_b_bytes_in_cache works */
if (info->type == WRITE_CACHE) {
info->current_pos = &info->write_pos;
info->current_end = &info->write_end;
} else {
info->current_pos = &info->read_pos;
info->current_end = &info->read_end;
}
}
static void init_functions(IO_CACHE *info) {
const enum cache_type type = info->type;
switch (type) {
case READ_NET:
/*
Must be initialized by the caller. The problem is that
_my_b_net_read has to be defined in sql directory because of
the dependency on THD, and therefore cannot be visible to
programs that link against mysys but know nothing about THD, such
as myisamchk
*/
break;
case SEQ_READ_APPEND:
info->read_function = _my_b_seq_read;
info->write_function = nullptr; /* Force a core if used */
break;
default:
info->read_function = info->share ? _my_b_read_r : _my_b_read;
info->write_function = _my_b_write;
}
setup_io_cache(info);
}
/*
Initialize an IO_CACHE object
SYNOPSIS
init_io_cache_ext()
info cache handler to initialize
file File that should be associated to to the handler
If == -1 then real_open_cached_file()
will be called when it's time to open file.
cachesize Size of buffer to allocate for read/write
If == 0 then use my_default_record_cache_size
type Type of cache
seek_offset Where cache should start reading/writing
use_async_io Set to 1 of we should use async_io (if available)
cache_myflags Bitmap of different flags
MY_WME | MY_FAE | MY_NABP | MY_FNABP |
MY_DONT_CHECK_FILESIZE
file_key Instrumented file key for temporary cache file
RETURN
0 ok
# error
*/
int init_io_cache_ext(IO_CACHE *info, File file, size_t cachesize,
enum cache_type type, my_off_t seek_offset,
bool use_async_io, myf cache_myflags,
PSI_file_key file_key) {
size_t min_cache;
my_off_t pos;
my_off_t end_of_file = ~(my_off_t)0;
DBUG_TRACE;
DBUG_PRINT("enter", ("cache: %p type: %d pos: %ld", info, (int)type,
(ulong)seek_offset));
DBUG_EXECUTE_IF("simulate_init_io_cache_failure", return 1;);
info->file = file;
info->file_key = file_key;
info->type = TYPE_NOT_SET; /* Don't set it until mutex are created */
info->pos_in_file = seek_offset;
info->pre_close = info->pre_read = info->post_read = nullptr;
info->arg = nullptr;
info->alloced_buffer = false;
info->buffer = nullptr;
info->seek_not_done = false;
if (file >= 0) {
pos = mysql_file_tell(file, MYF(0));
if ((pos == (my_off_t)-1) && (my_errno() == ESPIPE)) {
/*
This kind of object doesn't support seek() or tell(). Don't set a
flag that will make us again try to seek() later and fail.
*/
info->seek_not_done = false;
/*
Additionally, if we're supposed to start somewhere other than the
the beginning of whatever this file is, then somebody made a bad
assumption.
*/
assert(seek_offset == 0);
} else
info->seek_not_done = (seek_offset != pos);
}
info->disk_writes = 0;
info->share = nullptr;
if (!cachesize && !(cachesize = my_default_record_cache_size))
return 1; /* No cache requested */
min_cache = use_async_io ? IO_SIZE * 4 : IO_SIZE * 2;
if (type == READ_CACHE ||
type == SEQ_READ_APPEND) { /* Assume file isn't growing */
if (!(cache_myflags & MY_DONT_CHECK_FILESIZE)) {
/* Calculate end of file to avoid allocating oversized buffers */
end_of_file = mysql_encryption_file_seek(info, 0L, MY_SEEK_END, MYF(0));
/* Need to reset seek_not_done now that we just did a seek. */
info->seek_not_done = !(end_of_file == seek_offset);
if (end_of_file < seek_offset) end_of_file = seek_offset;
/* Trim cache size if the file is very small */
if ((my_off_t)cachesize > end_of_file - seek_offset + IO_SIZE * 2 - 1) {
cachesize = (size_t)(end_of_file - seek_offset) + IO_SIZE * 2 - 1;
use_async_io = false; /* No need to use async */
}
}
}
cache_myflags &= ~MY_DONT_CHECK_FILESIZE;
if (type != READ_NET && type != WRITE_NET) {
/* Retry allocating memory in smaller blocks until we get one */
cachesize = ((cachesize + min_cache - 1) & ~(min_cache - 1));
for (;;) {
size_t buffer_block;
/*
Unset MY_WAIT_IF_FULL bit if it is set, to prevent conflict with
MY_ZEROFILL.
*/
myf flags = (myf)(cache_myflags & ~(MY_WME | MY_WAIT_IF_FULL));
if (cachesize < min_cache) cachesize = min_cache;
buffer_block = cachesize;
if (type == SEQ_READ_APPEND) buffer_block *= 2;
if (cachesize == min_cache) flags |= (myf)MY_WME;
if ((info->buffer = (uchar *)my_malloc(key_memory_IO_CACHE, buffer_block,
flags)) != nullptr) {
info->write_buffer = info->buffer;
if (type == SEQ_READ_APPEND)
info->write_buffer = info->buffer + cachesize;
info->alloced_buffer = true;
break; /* Enough memory found */
}
if (cachesize == min_cache) return 2; /* Can't alloc cache */
/* Try with less memory */
cachesize = (cachesize * 3 / 4 & ~(min_cache - 1));
}
}
DBUG_PRINT("info", ("init_io_cache: cachesize = %lu", (ulong)cachesize));
info->read_length = info->buffer_length = cachesize;
info->myflags = cache_myflags & ~(MY_NABP | MY_FNABP);
info->request_pos = info->read_pos = info->write_pos = info->buffer;
if (type == SEQ_READ_APPEND) {
info->append_read_pos = info->write_pos = info->write_buffer;
info->write_end = info->write_buffer + info->buffer_length;
mysql_mutex_init(key_IO_CACHE_append_buffer_lock, &info->append_buffer_lock,
MY_MUTEX_INIT_FAST);
}
#if defined(SAFE_MUTEX)
else {
/* Clear mutex so that safe_mutex will notice that it's not initialized */
new (&info->append_buffer_lock) mysql_mutex_t();
}
#endif
if (type == WRITE_CACHE)
info->write_end =
info->buffer + info->buffer_length - (seek_offset & (IO_SIZE - 1));
else
info->read_end = info->buffer; /* Nothing in cache */
/* End_of_file may be changed by user later */
info->end_of_file = end_of_file;
info->error = 0;
info->type = type;
init_functions(info);
return 0;
} /* init_io_cache_ext */
/*
Initialize an IO_CACHE object
SYNOPSIS
init_io_cache() - Wrapper for init_io_cache_ext()
NOTE
This function should be used if the IO_CACHE tempfile is not instrumented.
However, if a PSI key other than PSI_NOT_INSTRUMENTED is set explicitly,
that key will be used for instrumentation.
*/
int init_io_cache(IO_CACHE *info, File file, size_t cachesize,
enum cache_type type, my_off_t seek_offset, bool use_async_io,
myf cache_myflags) {
return init_io_cache_ext(
info, file, cachesize, type, seek_offset, use_async_io, cache_myflags,
info->file_key != PSI_NOT_INSTRUMENTED ? info->file_key
: key_file_io_cache);
}
/*
Use this to reset cache to re-start reading or to change the type
between READ_CACHE <-> WRITE_CACHE
If we are doing a reinit of a cache where we have the start of the file
in the cache, we are reusing this memory without flushing it to disk.
*/
bool reinit_io_cache(IO_CACHE *info, enum cache_type type, my_off_t seek_offset,
bool use_async_io [[maybe_unused]], bool clear_cache) {
DBUG_TRACE;
DBUG_PRINT("enter", ("cache: %p type: %d seek_offset: %lu clear_cache: %d",
info, type, (ulong)seek_offset, (int)clear_cache));
/* One can't do reinit with the following types */
assert(type != READ_NET && info->type != READ_NET && type != WRITE_NET &&
info->type != WRITE_NET && type != SEQ_READ_APPEND &&
info->type != SEQ_READ_APPEND);
/* If the whole file is in memory, avoid flushing to disk */
if (!clear_cache && seek_offset >= info->pos_in_file &&
seek_offset <= my_b_tell(info)) {
/* Reuse current buffer without flushing it to disk */
uchar *pos;
if (info->type == WRITE_CACHE && type == READ_CACHE) {
info->read_end = info->write_pos;
info->end_of_file = my_b_tell(info);
/*
Trigger a new seek only if we have a valid
file handle.
*/
info->seek_not_done = (info->file != -1);
} else if (type == WRITE_CACHE) {
if (info->type == READ_CACHE) {
info->write_end = info->write_buffer + info->buffer_length;
info->seek_not_done = true;
}
info->end_of_file = ~(my_off_t)0;
}
pos = info->request_pos + (seek_offset - info->pos_in_file);
if (type == WRITE_CACHE)
info->write_pos = pos;
else
info->read_pos = pos;
} else {
/*
If we change from WRITE_CACHE to READ_CACHE, assume that everything
after the current positions should be ignored
*/
if (info->type == WRITE_CACHE && type == READ_CACHE)
info->end_of_file = my_b_tell(info);
/* flush cache if we want to reuse it */
if (!clear_cache && my_b_flush_io_cache(info, 1)) return true;
info->pos_in_file = seek_offset;
/* Better to do always do a seek */
info->seek_not_done = true;
info->request_pos = info->read_pos = info->write_pos = info->buffer;
if (type == READ_CACHE) {
info->read_end = info->buffer; /* Nothing in cache */
} else {
info->write_end =
(info->buffer + info->buffer_length - (seek_offset & (IO_SIZE - 1)));
info->end_of_file = ~(my_off_t)0;
}
}
info->type = type;
info->error = 0;
init_functions(info);
if (DBUG_EVALUATE_IF("fault_injection_reinit_io_cache", true, false))
return true;
return false;
} /* reinit_io_cache */
/*
Read buffered.
SYNOPSIS
_my_b_read()
info IO_CACHE pointer
Buffer Buffer to retrieve count bytes from file
Count Number of bytes to read into Buffer
NOTE
This function is only called from the my_b_read() macro when there
isn't enough characters in the buffer to satisfy the request.
WARNING
When changing this function, be careful with handling file offsets
(end-of_file, pos_in_file). Do not cast them to possibly smaller
types than my_off_t unless you can be sure that their value fits.
Same applies to differences of file offsets.
When changing this function, check _my_b_read_r(). It might need the
same change.
RETURN
0 we succeeded in reading all data
1 Error: couldn't read requested characters. In this case:
If info->error == -1, we got a read error.
Otherwise info->error contains the number of bytes in Buffer.
*/
int _my_b_read(IO_CACHE *info, uchar *Buffer, size_t Count) {
size_t length, diff_length, left_length, max_length;
my_off_t pos_in_file;
DBUG_TRACE;
/* If the buffer is not empty yet, copy what is available. */
if ((left_length = (size_t)(info->read_end - info->read_pos))) {
assert(Count >= left_length); /* User is not using my_b_read() */
memcpy(Buffer, info->read_pos, left_length);
Buffer += left_length;
Count -= left_length;
}
/* pos_in_file always point on where info->buffer was read */
pos_in_file = info->pos_in_file + (size_t)(info->read_end - info->buffer);
/*
Whenever a function which operates on IO_CACHE flushes/writes
some part of the IO_CACHE to disk it will set the property
"seek_not_done" to indicate this to other functions operating
on the IO_CACHE.
*/
if (info->seek_not_done) {
if ((mysql_encryption_file_seek(info, pos_in_file, MY_SEEK_SET, MYF(0)) !=
MY_FILEPOS_ERROR)) {
/* No error, reset seek_not_done flag. */
info->seek_not_done = false;
} else {
/*
If the seek failed and the error number is ESPIPE, it is because
info->file is a pipe or socket or FIFO. We never should have tried
to seek on that. See Bugs#25807 and #22828 for more info.
*/
assert(my_errno() != ESPIPE);
info->error = -1;
return 1;
}
}
/*
Calculate, how much we are within a IO_SIZE block. Ideally this
should be zero.
*/
diff_length = (size_t)(pos_in_file & (IO_SIZE - 1));
/*
If more than a block plus the rest of the current block is wanted,
we do read directly, without filling the buffer.
*/
if (Count >=
(size_t)(IO_SIZE +
(IO_SIZE - diff_length))) { /* Fill first intern buffer */
size_t read_length;
if (info->end_of_file <= pos_in_file) {
/* End of file. Return, what we did copy from the buffer. */
info->error = (int)left_length;
return 1;
}
/*
Crop the wanted count to a multiple of IO_SIZE and subtract,
what we did already read from a block. That way, the read will
end aligned with a block.
*/
length = (Count & (size_t) ~(IO_SIZE - 1)) - diff_length;
if ((read_length = mysql_encryption_file_read(info, Buffer, length,
info->myflags)) != length) {
/*
If we didn't get, what we wanted, we either return -1 for a read
error, or (it's end of file), how much we got in total.
*/
info->error =
(read_length == (size_t)-1 ? -1 : (int)(read_length + left_length));
return 1;
}
Count -= length;
Buffer += length;
pos_in_file += length;
left_length += length;
diff_length = 0;
}
/*
At this point, we want less than one and a partial block.
We will read a full cache, minus the number of bytes, we are
within a block already. So we will reach new alignment.
*/
max_length = info->read_length - diff_length;
/* We will not read past end of file. */
if (info->type != READ_FIFO && max_length > (info->end_of_file - pos_in_file))
max_length = (size_t)(info->end_of_file - pos_in_file);
/*
If there is nothing left to read,
we either are done, or we failed to fulfill the request.
Otherwise, we read max_length into the cache.
*/
if (!max_length) {
if (Count) {
/* We couldn't fulfil the request. Return, how much we got. */
info->error = (int)left_length;
return 1;
}
length = 0; /* Didn't read any chars */
} else if ((length = mysql_encryption_file_read(
info, info->buffer, max_length, info->myflags)) < Count ||
length == (size_t)-1) {
/*
We got an read error, or less than requested (end of file).
If not a read error, copy, what we got.
*/
if (length != (size_t)-1) memcpy(Buffer, info->buffer, length);
info->pos_in_file = pos_in_file;
/* For a read error, return -1, otherwise, what we got in total. */
info->error = length == (size_t)-1 ? -1 : (int)(length + left_length);
info->read_pos = info->read_end = info->buffer;
return 1;
}
/*
Count is the remaining number of bytes requested.
length is the amount of data in the cache.
Read Count bytes from the cache.
*/
info->read_pos = info->buffer + Count;
info->read_end = info->buffer + length;
info->pos_in_file = pos_in_file;
memcpy(Buffer, info->buffer, Count);
return 0;
}
/*
Prepare IO_CACHE for shared use.
SYNOPSIS
init_io_cache_share()
read_cache A read cache. This will be copied for
every thread after setup.
cshare The share.
write_cache If non-NULL a write cache that is to be
synchronized with the read caches.
num_threads Number of threads sharing the cache
including the write thread if any.
DESCRIPTION
The shared cache is used so: One IO_CACHE is initialized with
init_io_cache(). This includes the allocation of a buffer. Then a
share is allocated and init_io_cache_share() is called with the io
cache and the share. Then the io cache is copied for each thread. So
every thread has its own copy of IO_CACHE. But the allocated buffer
is shared because cache->buffer is the same for all caches.
One thread reads data from the file into the buffer. All threads
read from the buffer, but every thread maintains its own set of
pointers into the buffer. When all threads have used up the buffer
contents, one of the threads reads the next block of data into the
buffer. To accomplish this, each thread enters the cache lock before
accessing the buffer. They wait in lock_io_cache() until all threads
joined the lock. The last thread entering the lock is in charge of
reading from file to buffer. It wakes all threads when done.
Synchronizing a write cache to the read caches works so: Whenever
the write buffer needs a flush, the write thread enters the lock and
waits for all other threads to enter the lock too. They do this when
they have used up the read buffer. When all threads are in the lock,
the write thread copies the write buffer to the read buffer and
wakes all threads.
share->running_threads is the number of threads not being in the
cache lock. When entering lock_io_cache() the number is decreased.
When the thread that fills the buffer enters unlock_io_cache() the
number is reset to the number of threads. The condition
running_threads == 0 means that all threads are in the lock. Bumping
up the number to the full count is non-intuitive. But increasing the
number by one for each thread that leaves the lock could lead to a
solo run of one thread. The last thread to join a lock reads from
file to buffer, wakes the other threads, processes the data in the
cache and enters the lock again. If no other thread left the lock
meanwhile, it would think it's the last one again and read the next
block...
The share has copies of 'error', 'buffer', 'read_end', and
'pos_in_file' from the thread that filled the buffer. We may not be
able to access this information directly from its cache because the
thread may be removed from the share before the variables could be
copied by all other threads. Or, if a write buffer is synchronized,
it would change its 'pos_in_file' after waking the other threads,
possibly before they could copy its value.
However, the 'buffer' variable in the share is for a synchronized
write cache. It needs to know where to put the data. Otherwise it
would need access to the read cache of one of the threads that is
not yet removed from the share.
RETURN
void
*/
void init_io_cache_share(IO_CACHE *read_cache, IO_CACHE_SHARE *cshare,
IO_CACHE *write_cache, uint num_threads) {
DBUG_TRACE;
DBUG_PRINT("io_cache_share", ("read_cache: %p share: %p "
"write_cache: %p threads: %u",
read_cache, cshare, write_cache, num_threads));
assert(num_threads > 1);
assert(read_cache->type == READ_CACHE);
assert(!write_cache || (write_cache->type == WRITE_CACHE));
mysql_mutex_init(key_IO_CACHE_SHARE_mutex, &cshare->mutex,
MY_MUTEX_INIT_FAST);
mysql_cond_init(key_IO_CACHE_SHARE_cond, &cshare->cond);
mysql_cond_init(key_IO_CACHE_SHARE_cond_writer, &cshare->cond_writer);
cshare->running_threads = num_threads;
cshare->total_threads = num_threads;
cshare->error = 0; /* Initialize. */
cshare->buffer = read_cache->buffer;
cshare->read_end = nullptr; /* See function comment of lock_io_cache(). */
cshare->pos_in_file = 0; /* See function comment of lock_io_cache(). */
cshare->source_cache = write_cache; /* Can be NULL. */
read_cache->share = cshare;
read_cache->read_function = _my_b_read_r;
read_cache->current_pos = nullptr;
read_cache->current_end = nullptr;
if (write_cache) write_cache->share = cshare;
}
/*
Remove a thread from shared access to IO_CACHE.
SYNOPSIS
remove_io_thread()
cache The IO_CACHE to be removed from the share.
NOTE
Every thread must do that on exit for not to deadlock other threads.
The last thread destroys the pthread resources.
A writer flushes its cache first.
RETURN
void
*/
void remove_io_thread(IO_CACHE *cache) {
IO_CACHE_SHARE *cshare = cache->share;
uint total;
DBUG_TRACE;
/* If the writer goes, it needs to flush the write cache. */
if (cache == cshare->source_cache) flush_io_cache(cache);
mysql_mutex_lock(&cshare->mutex);
DBUG_PRINT(
"io_cache_share",
("%s: %p", (cache == cshare->source_cache) ? "writer" : "reader", cache));
/* Remove from share. */
total = --cshare->total_threads;
DBUG_PRINT("io_cache_share", ("remaining threads: %u", total));
/* Detach from share. */
cache->share = nullptr;
/* If the writer goes, let the readers know. */
if (cache == cshare->source_cache) {
DBUG_PRINT("io_cache_share", ("writer leaves"));
cshare->source_cache = nullptr;
}
/* If all threads are waiting for me to join the lock, wake them. */
if (!--cshare->running_threads) {
DBUG_PRINT("io_cache_share", ("the last running thread leaves, wake all"));
mysql_cond_signal(&cshare->cond_writer);
mysql_cond_broadcast(&cshare->cond);
}
mysql_mutex_unlock(&cshare->mutex);
if (!total) {
DBUG_PRINT("io_cache_share", ("last thread removed, destroy share"));
mysql_cond_destroy(&cshare->cond_writer);
mysql_cond_destroy(&cshare->cond);
mysql_mutex_destroy(&cshare->mutex);
}
}
/*
Lock IO cache and wait for all other threads to join.
SYNOPSIS
lock_io_cache()
cache The cache of the thread entering the lock.
pos File position of the block to read.
Unused for the write thread.
DESCRIPTION
Wait for all threads to finish with the current buffer. We want
all threads to proceed in concert. The last thread to join
lock_io_cache() will read the block from file and all threads start
to use it. Then they will join again for reading the next block.
The waiting threads detect a fresh buffer by comparing
cshare->pos_in_file with the position they want to process next.
Since the first block may start at position 0, we take
cshare->read_end as an additional condition. This variable is
initialized to NULL and will be set after a block of data is written
to the buffer.
RETURN
1 OK, lock in place, go ahead and read.
0 OK, unlocked, another thread did the read.
*/
static int lock_io_cache(IO_CACHE *cache, my_off_t pos) {
IO_CACHE_SHARE *cshare = cache->share;
DBUG_TRACE;
/* Enter the lock. */
mysql_mutex_lock(&cshare->mutex);
cshare->running_threads--;
DBUG_PRINT("io_cache_share",
("%s: %p pos: %lu running: %u",
(cache == cshare->source_cache) ? "writer" : "reader", cache,
(ulong)pos, cshare->running_threads));
if (cshare->source_cache) {
/* A write cache is synchronized to the read caches. */
if (cache == cshare->source_cache) {
/* The writer waits until all readers are here. */
while (cshare->running_threads) {
DBUG_PRINT("io_cache_share", ("writer waits in lock"));
mysql_cond_wait(&cshare->cond_writer, &cshare->mutex);
}
DBUG_PRINT("io_cache_share", ("writer awoke, going to copy"));
/* Stay locked. Leave the lock later by unlock_io_cache(). */
return 1;
}
/* The last thread wakes the writer. */
if (!cshare->running_threads) {
DBUG_PRINT("io_cache_share", ("waking writer"));
mysql_cond_signal(&cshare->cond_writer);
}
/*
Readers wait until the data is copied from the writer. Another
reason to stop waiting is the removal of the write thread. If this
happens, we leave the lock with old data in the buffer.
*/
while ((!cshare->read_end || (cshare->pos_in_file < pos)) &&
cshare->source_cache) {
DBUG_PRINT("io_cache_share", ("reader waits in lock"));
mysql_cond_wait(&cshare->cond, &cshare->mutex);
}
/*
If the writer was removed from the share while this thread was
asleep, we need to simulate an EOF condition. The writer cannot
reset the share variables as they might still be in use by readers
of the last block. When we awake here then because the last
joining thread signalled us. If the writer is not the last, it
will not signal. So it is safe to clear the buffer here.
*/
if (!cshare->read_end || (cshare->pos_in_file < pos)) {
DBUG_PRINT("io_cache_share", ("reader found writer removed. EOF"));
cshare->read_end = cshare->buffer; /* Empty buffer. */
cshare->error = 0; /* EOF is not an error. */
}
} else {
/*
There are read caches only. The last thread arriving in
lock_io_cache() continues with a locked cache and reads the block.
*/
if (!cshare->running_threads) {
DBUG_PRINT("io_cache_share", ("last thread joined, going to read"));
/* Stay locked. Leave the lock later by unlock_io_cache(). */
return 1;
}
/*
All other threads wait until the requested block is read by the
last thread arriving. Another reason to stop waiting is the
removal of a thread. If this leads to all threads being in the
lock, we have to continue also. The first of the awaken threads
will then do the read.
*/
while ((!cshare->read_end || (cshare->pos_in_file < pos)) &&
cshare->running_threads) {
DBUG_PRINT("io_cache_share", ("reader waits in lock"));
mysql_cond_wait(&cshare->cond, &cshare->mutex);
}
/* If the block is not yet read, continue with a locked cache and read. */
if (!cshare->read_end || (cshare->pos_in_file < pos)) {
DBUG_PRINT("io_cache_share", ("reader awoke, going to read"));
/* Stay locked. Leave the lock later by unlock_io_cache(). */
return 1;
}
/* Another thread did read the block already. */
}
DBUG_PRINT(
"io_cache_share",
("reader awoke, going to process %u bytes",
(uint)(cshare->read_end ? (size_t)(cshare->read_end - cshare->buffer)
: 0)));
/*
Leave the lock. Do not call unlock_io_cache() later. The thread that
filled the buffer did this and marked all threads as running.
*/
mysql_mutex_unlock(&cshare->mutex);
return 0;
}
/*
Unlock IO cache.
SYNOPSIS
unlock_io_cache()
cache The cache of the thread leaving the lock.
NOTE
This is called by the thread that filled the buffer. It marks all
threads as running and awakes them. This must not be done by any
other thread.
Do not signal cond_writer. Either there is no writer or the writer
is the only one who can call this function.
The reason for resetting running_threads to total_threads before
waking all other threads is that it could be possible that this
thread is so fast with processing the buffer that it enters the lock
before even one other thread has left it. If every awoken thread
would increase running_threads by one, this thread could think that
he is again the last to join and would not wait for the other
threads to process the data.
RETURN
void
*/
static void unlock_io_cache(IO_CACHE *cache) {
IO_CACHE_SHARE *cshare = cache->share;
DBUG_TRACE;
DBUG_PRINT("io_cache_share",
("%s: %p pos: %lu running: %u",
(cache == cshare->source_cache) ? "writer" : "reader", cache,
(ulong)cshare->pos_in_file, cshare->total_threads));
cshare->running_threads = cshare->total_threads;
mysql_cond_broadcast(&cshare->cond);
mysql_mutex_unlock(&cshare->mutex);
}
/*
Read from IO_CACHE when it is shared between several threads.
SYNOPSIS
_my_b_read_r()
cache IO_CACHE pointer
Buffer Buffer to retrieve count bytes from file
Count Number of bytes to read into Buffer
NOTE
This function is only called from the my_b_read() macro when there
isn't enough characters in the buffer to satisfy the request.
IMPLEMENTATION
It works as follows: when a thread tries to read from a file (that
is, after using all the data from the (shared) buffer), it just
hangs on lock_io_cache(), waiting for other threads. When the very
last thread attempts a read, lock_io_cache() returns 1, the thread
does actual IO and unlock_io_cache(), which signals all the waiting
threads that data is in the buffer.
WARNING
When changing this function, be careful with handling file offsets
(end-of_file, pos_in_file). Do not cast them to possibly smaller
types than my_off_t unless you can be sure that their value fits.
Same applies to differences of file offsets. (Bug #11527)
When changing this function, check _my_b_read(). It might need the
same change.
RETURN
0 we succeeded in reading all data
1 Error: can't read requested characters
*/
int _my_b_read_r(IO_CACHE *cache, uchar *Buffer, size_t Count) {
my_off_t pos_in_file;
size_t length, diff_length, left_length;
IO_CACHE_SHARE *cshare = cache->share;
DBUG_TRACE;
if ((left_length = (size_t)(cache->read_end - cache->read_pos))) {
assert(Count >= left_length); /* User is not using my_b_read() */
memcpy(Buffer, cache->read_pos, left_length);
Buffer += left_length;
Count -= left_length;
}
while (Count) {
size_t cnt, len;
pos_in_file = cache->pos_in_file + (cache->read_end - cache->buffer);
diff_length = (size_t)(pos_in_file & (IO_SIZE - 1));
length = IO_ROUND_UP(Count + diff_length) - diff_length;
length = ((length <= cache->read_length)
? length + IO_ROUND_DN(cache->read_length - length)
: length - IO_ROUND_UP(length - cache->read_length));
if (cache->type != READ_FIFO &&
(length > (cache->end_of_file - pos_in_file)))
length = (size_t)(cache->end_of_file - pos_in_file);
if (length == 0) {
cache->error = (int)left_length;
return 1;
}
if (lock_io_cache(cache, pos_in_file)) {
/* With a synchronized write/read cache we won't come here... */
assert(!cshare->source_cache);
/*
... unless the writer has gone before this thread entered the
lock. Simulate EOF in this case. It can be distinguished by
cache->file.
*/
if (cache->file < 0)
len = 0;
else {
/*
Whenever a function which operates on IO_CACHE flushes/writes
some part of the IO_CACHE to disk it will set the property
"seek_not_done" to indicate this to other functions operating
on the IO_CACHE.
*/
if (cache->seek_not_done) {
if (mysql_encryption_file_seek(cache, pos_in_file, MY_SEEK_SET,
MYF(0)) == MY_FILEPOS_ERROR) {
cache->error = -1;
unlock_io_cache(cache);
return 1;
}
}
len = mysql_encryption_file_read(cache, cache->buffer, length,
cache->myflags);
}
DBUG_PRINT("io_cache_share", ("read %lu bytes", (ulong)len));
cache->read_end = cache->buffer + (len == (size_t)-1 ? 0 : len);
cache->error = (len == length ? 0 : (int)len);
cache->pos_in_file = pos_in_file;
/* Copy important values to the share. */
cshare->error = cache->error;
cshare->read_end = cache->read_end;
cshare->pos_in_file = pos_in_file;
/* Mark all threads as running and wake them. */
unlock_io_cache(cache);
} else {
/*
With a synchronized write/read cache readers always come here.
Copy important values from the share.
*/
cache->error = cshare->error;
cache->read_end = cshare->read_end;
cache->pos_in_file = cshare->pos_in_file;