This commit adds a function that retrieves the number of records stored
in a light hash table and makes light users use it instead of accessing
the light count directly. This gives us more freedom of refactoring the
light internals without modifying the code using it.

Needed for #7192

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring

prbuf_check, which is called by prbuf_open, proceeds to scanning the
buffer even if it's empty. On debug build, this results in prbuf_open
reporting that the buffer is corrupted, because we trash the buffer in
prbuf_create. On a release build, this may lead to a hang, in case the
buffer is zeroed out. Let's fix this by returning success from
prbuf_check if the buffer is empty. Note, prbuf_iterator_next doesn't
call prbuf_first_record if the buffer is empty, either.

Needed for https://github.com/tarantool/tarantool-ee/issues/187

NO_DOC=bug fix
NO_CHANGELOG=will be added to EE

The main decision made in this patch is how large the public
`box_decimal_t` type should be. Let's look on some calculations.

We're interested in the following values.

* How much decimal digits is stored?
* Size of an internal decimal type (`sizeof(decimal_t)`).
* Size of a buffer to store a string representation of any valid
  `decimat_t` value.
* Largest signed integer type fully represented in decimal_t (number of
  bits).
* Largest unsigned integer type fully represented in decimal_t (number
  of bits).

Now `decimal_t` is defined to store 38 decimal digits. It means the
following values:

| digits | sizeof | string | int???_t | uint???_t |
| ------ | ------ | ------ | -------- | --------- |
| 38     | 36     | 52     | 126      | 127       |

In fact, decNumber (the library we currently use under the hood) allows
to vary the 'decimal digits per unit' parameter, which is 3 by default,
so we can choose density of the representation. For example, for given
38 digits the sizeof is 36 by default, but it may vary from 28 to 47
bytes:

| digits | sizeof     | string | int???_t | uint???_t |
| ------ | ---------- | ------ | -------- | --------- |
| 38     | 36 (28-47) | 52     | 126      | 127       |

If we'll want to store `int128_t` and `uint128_t` ranges, we'll need 39
digits:

| digits | sizeof     | string | int???_t | uint???_t |
| ------ | ---------- | ------ | -------- | --------- |
| 39     | 36 (29-48) | 53     | 130      | 129       |

If we'll want to store `int256_t` and `uint256_t` ranges:

| digits | sizeof     | string | int???_t | uint???_t |
| ------ | ---------- | ------ | -------- | --------- |
| 78     | 62 (48-87) | 92     | 260      | 259       |

If we'll want to store `int512_t` and `uint512_t` ranges:

| digits | sizeof       | string | int???_t | uint???_t |
| ------ | ------------ | ------ | -------- | --------- |
| 155    | 114 (84-164) | 169    | 515      | 514       |

The decision here is what we consdider as possible and what as unlikely.
The patch freeze the maximum amount of bytes in `decimal_t` as 64. So
we'll able to store 256 bit integers and will NOT able to store 512 bit
integers in a future (without the ABI breakage at least).

The script, which helps to calculate those tables, is at end of the
commit message.

Next, how else `box_decimal_*()` library is different from the internal
`decimal_*()`?

* Added a structure that may hold any decimal value from any current or
  future tarantool version.
* Added `box_decimal_copy()`.
* Left `strtodec()` out of scope -- we can add it later.
* Left `decimal_str()` out of scope -- it looks dangerous without at
  least a good explanation when data in the static buffer are
  invalidated. There is `box_decimal_to_string()` that writes to an
  explicitly provided buffer.
* Added `box_decimal_mp_*()` for encoding to/decoding from msgpack.
  Unlike `mp_decimal.h` functions, here we always have `box_decimal_t`
  as the first parameter.
* Left `decimal_pack()` out of scope, because a user unlikely wants to
  serialize a decimal value piece-by-piece.
* Exposed `decimal_unpack()` as `box_decimal_mp_decode_data()` to keep a
  consistent terminogoly around msgpack encoding/decoding.
* More detailed API description, grouping by functionality.

The script, which helps to calculate sizes around `decimal_t`:

```lua
-- See notes in decNumber.h.

-- DECOPUN: DECimal Digits Per UNit
local function unit_size(DECOPUN)
    assert(DECOPUN > 0 and DECOPUN < 10)
    if DECOPUN <= 2 then
        return 1
    elseif DECOPUN <= 4 then
        return 2
    end
    return 4
end

function sizeof_decimal_t(digits, DECOPUN)
    -- int32_t digits;
    -- int32_t exponent;
    -- uint8_t bits;
    -- <..padding..>
    -- <..units..>
    local us = unit_size(DECOPUN)
    local padding = us - 1
    local unit_count = math.ceil(digits / DECOPUN)
    return 4 + 4 + 1 + padding + us * unit_count
end

function string_buffer(digits)
    -- -9.{9...}E+999999999# (# is '\0')
    -- ^ ^      ^^^^^^^^^^^^
    return digits + 14
end

function binary_signed(digits)
    local x = 1
    while math.log10(2 ^ (x - 1)) < digits do
        x = x + 1
    end
    return x - 1
end

function binary_unsigned(digits)
    local x = 1
    while math.log10(2 ^ x) < digits do
        x = x + 1
    end
    return x - 1
end

function digits_for_binary_signed(x)
    return math.ceil(math.log10(2 ^ (x - 1)))
end

function digits_for_binary_unsigned(x)
    return math.ceil(math.log10(2 ^ x))
end

function summary(digits)
    print('digits', digits)
    local sizeof_min = math.huge
    local sizeof_max = 0
    local DECOPUN_sizeof_min
    local DECOPUN_sizeof_max
    for DECOPUN = 1, 9 do
        local sizeof = sizeof_decimal_t(digits, DECOPUN)
        print('sizeof', sizeof, 'DECOPUN', DECOPUN)
        if sizeof < sizeof_min then
            sizeof_min = sizeof
            DECOPUN_sizeof_min = DECOPUN
        end
        if sizeof > sizeof_max then
            sizeof_max = sizeof
            DECOPUN_sizeof_max = DECOPUN
        end
    end
    print('sizeof min', sizeof_min, 'DECOPUN', DECOPUN_sizeof_min)
    print('sizeof max', sizeof_max, 'DECOPUN', DECOPUN_sizeof_max)
    print('string', string_buffer(digits))
    print('int???_t', binary_signed(digits))
    print('uint???_t', binary_unsigned(digits))
end
```

Part of #7228

@TarantoolBot document
Title: Module API for decimals

See the declarations in `src/box/decimal.h` in tarantool sources.

In most cases, NaN was treated as NULL. But in case NaN was returned as
a result of a Lua or C user defined function, it was considered a
double. After this patch, NaN will also be considered NULL in the
specified cases.

Closes #6374
Closes #6572

NO_DOC=bugfix

This patch makes the is_res_neg flag false in the sql_rem_int() function
if the left value is negative and the result is 0. Prior to this patch,
the value of the flag was true, which resulted in an assertion during
encoding 0 as MP_INT.

Closes #6575

NO_DOC=bugfix

The smallest positive double value is 2.225E-307, and the value before
the exponent has a maximum of 15 digits after the decimal point. This
means that double values cannot have more than 307 + 15 digits after
the decimal point.

After this patch, ROUND() will return its first argument unchanged if
the first argument is DOUBLE and the second argument is INTEGER greater
than 322.

Closes #6650

NO_DOC=bugfix

The patch introduces opportunity for user to reset
slice of current fiber execution. It allows to limit
iteration in space with SIGURG.

NO_CHANGELOG=see later commits
NO_DOC=see later commits

Currently, there is no way to interrupt a long execution of a
request (such as s:select(nil)). This patch introduces this
opportunity.

Box will use fiber deadline timeout as a timeout for DML usage.
Thus, when deadline of current fiber is up, all DML requests will
end with a particular error.

Closes #6085

NO_CHANGELOG=see later commits
NO_DOC=see later commits

Part of #6085

NO_TEST=no behavior changes
NO_CHANGELOG=no behavior changes
NO_DOC=no behavior changes

This patch introduces execution time slice for fiber. Later, we will use
this mechanism to limit iteration in space.

Part of #6085

NO_CHANGELOG=see later commits
NO_DOC=see later commits

The Rust module [1] leans on several internal symbols. They were open in
Tarantool 2.8 (see #2971 and #5932), but never were in the public API.
Tarantool 2.10.0 hides the symbols and we need a way to get them back to
use in the module.

We have the following options:

1. Design and expose a module API for fiber channels.
2. Export the symbols with a prefix like `tnt_internal_` (to don't spoil
   the global namespace).
3. Provide a `dlsym()` alike function to get an address of an internal
   symbol for users who knows what they're doing.

I think that the third way offers the best compromise between amount of
effort, quality of the result and opportunities to extend. In this
commit I hardcoded the list of functions to make the change as safe as
possible. Later I'll return here to autogenerate the list.

Exported the following function from the tarantool executable:

```c
void *
tnt_internal_symbol(const char *name);
```

I don't add it into the module API headers, because the function is to
perform a dark magic and we don't suggest it for users.

While I'm here, added `static` to a couple of fiber channel functions,
which are only used within the compilation unit.

[1]: https://github.com/picodata/tarantool-module

Part of #7228
Related to #6372

NO_DOC=don't advertize the dangerous API
NO_CHANGELOG=don't advertize the dangerous API

The valid range for thread_id is [0, iproto_threads_count - 1].

Closes #7196

NO_DOC=bugfix

Add a missed check to on_replace_dd_truncate, similar to
on_replace_dd_space and on_replace_dd_index.

Closes #7309

NO_DOC=bugfix

The problem is that even if we block all signals on all
threads except the main thread, the signals still can be
delivered to other threads (#7206). And another problem
is that user can spawn his own thread and not block
signals.

That is why the patch introduces tt_sigaction function that
guarantees that all signals will be handled only by the main
thread. We use this helper in clock_lowres module.
This is supposed to solve the problem, described in #7408.

NO_CHANGELOG=internal
NO_DOC=internal

Added a function (see the API in the documentation request below), which
reflects the `tuple[json_path]` Lua API (see #1285).

Part of #7228

@TarantoolBot document
Title: tuple: access a field using JSON path via module API

The following function is added into the module API:

```c
/**
 * Return a raw tuple field in the MsgPack format pointed by
 * a JSON path.
 *
 * The JSON path includes the outmost field. For example, "c" in
 * ["a", ["b", "c"], "d"] can be accessed using "[2][2]" path (if
 * index_base is 1, as in Lua). If index_base is set to 0, the
 * same field will be pointed by the "[1][1]" path.
 *
 * The first JSON path token may be a field name if the tuple
 * has associated format with named fields. A field of a nested
 * map can be accessed in the same way: "foo.bar" or ".foo.bar".
 *
 * The return value is valid until the tuple is destroyed, see
 * box_tuple_ref().
 *
 * Return NULL if the field does not exist or if the JSON path is
 * malformed or invalid. Multikey JSON path token [*] is treated
 * as invalid in this context.
 *
 * \param tuple a tuple
 * \param path a JSON path
 * \param path_len a length of @a path
 * \param index_base 0 if array element indexes in @a path are
 *        zero-based (like in C) or 1 if they're one-based (like
 *        in Lua)
 * \retval a pointer to a field data if the field exists or NULL
 */
API_EXPORT const char *
box_tuple_field_by_path(box_tuple_t *tuple, const char *path,
			uint32_t path_len, int index_base);
```

In Lua field's numbers are counted from base 1, however currently
space:format() and space.foreign_key return zero-based foreign key
fields, which leads to an error on space:format(space:format()).

Closes #7350

NO_DOC=bugfix

Currently a foreign_key field in the `format` argument, passed to
normalize_format, can be changed inside normalize_foreign_key_one.
Fix this by using a local copy of def.field.

NO_DOC=bugfix
NO_CHANGELOG=minor bug

It is inconvenient to create self-referencing FK constraints, as the
space ID will only be generated during space creation. This is
especially useful for SQL, since the format for the space is created
at compile time, and the space ID is only obtained at run time.

Closes #7200

@TarantoolBot document
Title: Describe foreign keys referring to the same space
Since: 2.11
Root document: https://www.tarantool.io/en/doc/latest/book/box/data_model/#foreign-keys

It is possible to create a foreign key that refers to the same space (a
child space equals to the parent space).
To do that, omit `space` in the `foreign_key` parameter, or set it to
the id or to the name of the current space.

According to man 3 exec: "The first argument, by convention, should point
to the filename associated with the file being executed.". Using empty
string as the first argument while calling `true` program, provided by
coreutils led to error message being printed to stderr, which failed the
test.

This patch passes 'true' as the first argument.

Closes #7452.

NO_DOC=test
NO_CHANGELOG=test

Currently net.box's methods doesn't check types of passed options.
This can lead to Lua's internal errors, which are not self-explaining.

Let's add this functionality and raise errors with meaningful messages
in case of incorrect options.

Closes #6063
Closes #6530

NO_DOC=Not a visible change

Test is needed to be updated every time net.box's internals are patched
(e.g. in case of adding new functions of deleting old ones), as the order
of suggested autocomplete options changes.

Let's replace `assert_equals` with `assert_items_equals`, which is not
order dependent.

NO_DOC=test
NO_CHANGELOG=test

Created simplistic unit test for checking boundary conditions
for `datetime_unpack` function.

NO_DOC=internal
NO_CHANGELOG=internal

Introduced fuzzing testing for datetime decoder.

NO_DOC=internal
NO_CHANGELOG=internal

NO_DOC=refactoring
NO_CHANGELOG=refactoring

This patch fixes the ctrl+c behavior in multiline mode.
The old behavior was ignoring sigint in this mode.
The new behavior is quitting this mode and printing the new
default prompt.
Added a new test for covering the mentioned case.

Follows up #2717
Closes #7109

NO_DOC=BUGFIX

This commit replaces enter/leave_delayed_free_mode() MemtxAllocator
methods with open/close_read_view(). The open_read_view() method
returns a pointer to the new read view, which is supposed to be pssed
to close_read_view() when the read view is no longer needed. The new
API will allow us to optimize garbage collection, see #7185.

Currently, open_read_view() always returns nullptr and both ReadView
and memtx_read_view_opts are empty structs. We will extend them in
the future.

Closes #7364

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring

Checking #7185 with functional tests is complicated. Let's add a unit
test for this.

Needed for #7185

NO_DOC=test
NO_CHANGELOG=test

Now we can make interactive transactions like in case of
local or txt remote consoles if peer supports streams.

Closes #7413

NO_DOC=minor change

The issue is if in binary remote console a error is thrown in expression
like "box.begin() error('something') box.commit()" then it is
overwritten by iproto check for active transactions at the end of eval.
The solution is to rollback active transaction in this case in console
code before iproto check.

Let's also assert in tests a behaviour that after successful transaction
begin it stays active in next evaluations (successful or not) until
explicit rollback.

Closes #7288

NO_DOC=minor change

Starting server is failed after chdir(3) for example if we call
box.cfg{} with work_dir set.

NO_DOC=fix testing harness
NO_TEST=fix testing harness
NO_CHANGELOG=fix testing harness

We keep per server directories there too, not only socket files.

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring

The ; separator was missed after select (check the first hunk) and as
a result the rollback was not executed. So the next test was failed merely
due to the unfinished transaction of the previous test.

We also need to fix the third test to use a different tuple.
Also fix a couple of comments.

NO_DOC=test fix
NO_TEST=test fix
NO_CHANGELOG=test fix

If MP_ERROR contains an error message with invalid UTF-8 sequences,
encode it in Base64 to obtain printable string.

Closes #6781
Closes #6934

NO_DOC=bugfix

The upgrade script first tries to determine if the node is booted from
old snaps not recoverable on current Tarantool versions. If this is the
case, it sets up special triggers so that snaps are automatically
converted to a suitable format.

This happens before box.cfg{}, so the workdir is not set at this point
in time, and the upgrade script should take configured work_dir into
account explicitly. Fix this.

Closes #7232

NO_DOC=bugfix

Prior to this patch, some opcodes could use a pointer to struct space
that was set during parsing. However, the pointer to struct space is not
really something that defines spaces. A space can be identified by its
ID or name. In most cases, specifying space by pointer works fine, but
truncate() changes the pointer to space, resulting in a sigfault for
prepared statements using the above opcodes. To avoid this problem, a
new opcode has been introduced. This opcode uses the space ID to
determine the pointer to the struct space at runtime and stores it in
the MEM, which is later used in the mentioned opcodes.

Closes #7358

NO_DOC=bugfix

Functional index keys are allocated and freed with MemtxAllocator's
alloc and free methods. In contrast to tuples, which are allocated and
freed with alloc_tuple and free_tuple, freeing a functional index key
happens immediately, irrespective of whether there's a snapshot in
progress or not. It's acceptable, because snapshot only uses primary
indexes, which can't be functional. However, to reuse the snapshot
infrastructure for creating general purpose user read views, we will
need to guarantee that functional index keys stay alive until all read
views using them are closed.

To achieve that, this commit turns functional index keys into tuples,
which automatically makes them linger if there's an open read view.
We use the same global tuple format for allocating functional keys,
because the key format is checked in key_list_iterator_next.

Closes #7376

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring

Currently the latch doesn't guarantee the order in which it is acquired
by fibers, which requested it. E.g. it is possible to wake up spuriously
a fiber which is yielding in the latch_lock, it will be removed from
l->queue by fiber_make_ready, then it will be inserted to l->queue
again, but for this time, to the head of the list instead of its
original place in the queue.

Fix this by using latch_waiter structure, which is linked into l->queue.

Part of #7166

@TarantoolBot document
Title: Update box_latch_lock description
Since: 2.11

Add "Locks are acquired in the strict order as they were requested." to
the box_latch_lock description in C API reference - Module latch.

This patch fixes the following error while tarantool build on
FreeBSD 13.1:

    [100%] Linking CXX executable watcher.test
    ld: error: undefined symbol: set_sigint_cb
    >>> referenced by console.c:261 (lua/console.c:261)
    >>>               console.c.o:(lbox_console_readline) in
                        archive ../../src/box/libbox.a
    >>> referenced by console.c:342 (lua/console.c:342)
    >>>               console.c.o:(lbox_console_readline) in
                        archive ../../src/box/libbox.a
    >>> referenced by console.c:352 (lua/console.c:352)
    >>>               console.c.o:(lbox_console_readline) in
                        archive ../../src/box/libbox.a
    >>> referenced 1 more times
    c++: error: linker command failed with exit code 1
        (use -v to see invocation)
    gmake[2]: *** [test/unit/CMakeFiles/watcher.test.dir/build.make:152:
        test/unit/watcher.test] Error 1
    gmake[1]: *** [CMakeFiles/Makefile2:10528:
        test/unit/CMakeFiles/watcher.test.dir/all] Error 2
    gmake: *** [Makefile:156: all] Error 2

NO_DOC=minor changes
NO_TEST=minor changes
NO_CHANGELOG=minor changes

The gh_6565 test doesn't stop the hot standby replica it started,
because the replica should fail to initialize and exit eventually
anyway. However, if the replica lingers until the next test due to
https://github.com/tarantool/test-run/issues/345, the next test may
successfully connect to it, which is likely to lead to a failure,
because UNIX socket paths used by luatest servers are not randomized.

For example, here gh_6568 test fails after gh_6565, because it uses the
same alias for the test instance ('replica'):

NO_WRAP
[008] vinyl-luatest/gh_6565_hot_standby_unsupported_>                 [ pass ]
[008] vinyl-luatest/gh_6568_replica_initial_join_rem>                 [ fail ]
[008] Test failed! Output from reject file /tmp/t/rejects/vinyl-luatest/gh_6568_replica_initial_join_removal_of_compacted_run_files.reject:
[008] TAP version 13
[008] 1..1
[008] # Started on Fri Jul  8 15:30:47 2022
[008] # Starting group: gh-6568-replica-initial-join-removal-of-compacted-run-files
[008] not ok 1  gh-6568-replica-initial-join-removal-of-compacted-run-files.test_replication_compaction_cleanup
[008] #   builtin/fio.lua:242: fio.pathjoin(): undefined path part 1
[008] #   stack traceback:
[008] #         builtin/fio.lua:242: in function 'pathjoin'
[008] #         ...ica_initial_join_removal_of_compacted_run_files_test.lua:43: in function 'gh-6568-replica-initial-join-removal-of-compacted-run-files.test_replication_compaction_cleanup'
[008] #         ...
[008] #         [C]: in function 'xpcall'
[008] replica | 2022-07-08 15:30:48.311 [832856] main/103/default.lua F> can't initialize storage: unlink, called on fd 30, aka unix/:(socket), peer of unix/:(socket): Address already in use
[008] # Ran 1 tests in 0.722 seconds, 0 succeeded, 1 errored
NO_WRAP

Let's fix this by explicitly killing the hot standby replica. Since it
could have exited voluntarily, we need to use pcall, because server.stop
fails if the instance is already dead.

This issue is similar to the one fixed by commit 85040161 ("test:
stop server started by vinyl-luatest/update_optimize test").

NO_DOC=test
NO_CHANGELOG=test

Handle status header response like 'HTTP/2 200' with version without
dot.

Closes #7319

NO_DOC=bugfix