=====================================================================

BELOW IS A RUNNING STREAM OF NOTES WHICH DON'T HAVE TO MAKE ANY SENSE

=====================================================================

I/O is serialized with timing data

IIIIIIiiIIIIIIIiOOOOOoooOOOOooIIIIIiiIIII
|              ||     ||     ||         |

needs splitting the timing:
    both pieces start at the same time, but different messages, keeping order
    needs tracking absolute time of each piece to put in the next message
    can't format timing data right away, only when writing

    OR

    store last whitespace time, input/output position

    >25
    IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII
                 ^
    ^

We'd better not cut anything for the start



write window to log
write output to log
write input to log

read window from terminal
write window to slave

write output to terminal
write input to slave

read input from terminal
read output from slave

why do we even need to be terminated by a signal?
1. because we need to handle out-of-band SIGWINCH when waiting for output and
   waiting for input to be consumed
2. because we need to handle SIGINT, SIGTERM and SIGHUP timely to restore
   terminal, instead of simply dying

Can we suspend I/O transfer for writing the log entry?
We should - this provides flow control

However, if we delay data transfer, SIGWINCH might go out of sync with the
data. So some data in the kernel buffer and pending write on the slave will
be counting on one window size, while the actual window size will be different.

Do we really care about window size? Really?
Yes we do care about transferring it, so we might as well record it. *** Delay
with the transfer should coincide with delay in the log. *** But can it? We
have no idea if the slave has read all of its input buffers when it gets
SIGWINCH. So it will be out of sync. Still screen handles this anyway.

Can we do all this without terminating on a signal? With poll even on write?
Would it work on terminals?


WE NEED TO HANDLE INTERRUPTED I/O BECAUSE WE NEED TO HANDLE LATENCY LIMIT
TIMER

Log is always in UTF-8
Convert any other encoding to UTF-8
Write character counts in timing
Keep incomplete characters to the next write - don't flush them?
Or assume characters are always complete? THAT IS NOT TRUE!
OR WAIT! Perhaps it's our code breaking on incomplete characters?
VERIFY THIS!

Anyway, we can't be sure, we can have a buffer size which won't fit the
characters.

We have to assume I/O is encoded somehow and for UTF-8 it means multibyte
characters.

Valid UTF-8 is a smaller subset of arbitrary binary stream. Thus we can't
represent everything that's being pushed to the terminal.

However, if we need to have our terminal logs searcheable, we need that data
in a format that ElasticSearch can understand.

What if we add two more timing record types: binary input and binary output?
Have them encoded in base64?


Write both streams while delay is under 0, then flush their timing
What to do with interleaved binary data? We should keep the pieces before
flushing, or rather we should flush them as we switch forks.

Delay is put in by code above streams. Data lengths are put in by streams.

So when a new piece of input comes in, check if delay is still within zero.
If it is more than zero:
    Ask both streams to flush timing, giving them control of timing buffer,
    then append delay.
Hand off the newly received data to the corresponding stream giving it control
of remaining length and timing buffer

What should the stream reserve space for? Only "cutting" or "flushing" as well?
Only cutting, flushing will need more space.

if invalid byte encountered
    if sequence is empty
        write one byte from input buffer to binary fork
    else
        write binary sequence from utf8 buffer
else
    if sequence is incomplete
        add another byte
        if not added
            break
    else
        write text sequence from utf8 buffer

sequence incomplete and no invalid byte encountered: collecting sequence
or
invalid byte encountered: writing non-text sequence: either from utf8 buffer or one byte from the input buffer
or
sequence complete: writing text sequence from utf8 buffer

[ ] [ ] [ ] [ ] [ ]


ended   valid

0       0           sequence in progress

0       1           impossible

1       0           sequence terminated by invalid byte

1       1           sequence completed


ended   complete
0       0           sequence in progress
0       0           impossible
1       0           seqence terminated by invalid byte
1       1           sequence completed


sequence of bytes
    /      \
   /        \
 text     binary


while true
    if the sequence has ended
        If it is valid UTF-8
            finish writing it to text fork
        else
            finish writing it to binary fork
    get a sequence

the sequence:
binary or text
length
how much is written

What if we rename the current utf-8 object to a sequence object?
Then it doesn't have to contain only UTF-8 bytes. But would that be useful?

We only need to keep incomplete sequences between calls because UTF-8
characters are multi-byte and need to be written atomically.

What if we extend our sequence object to keeping words atomically?
We'll need to add word length limit as well. Otherwise the rest of the
machinery will be there already.

So, an incomplete text sequence can be kept between write calls, and can be
force-flushed.


------
Can we define the sequence?

A sequence can be started.
A sequence can be ended.

A sequence can be:

    binary,
    partial character (binary when incomplete),
    text (partial character when incomplete?),
    word (text when incomplete

A byte can belong to word, text, binary or unknown (partial character) sequence.


Sequence:

        __________________ character
      /              _____ word
     /              /
d0 b0 d0 b1 d0 b2 0a 32

a word is a character sequence
a character sequence is a binary sequence

------

        __________________ character
      /             ______ word
     /             /
d0 b0  d0 b1  d0 b2  0a


do we flush incomplete words?
do we flush incomplete characters?

store last position of a complete sequence type?



ff      binary started
        character not started
        word not started

d0      binary started
        character started
        word not started

Oa      binary started
        character complete
        word not started

d0 b0   binary started
        character complete
        word started

d0 b0  d0   binary started
            character started
            word started

d0 b0  d0 b1  d0 b2     binary started
                        character complete
                        word started

d0 b0  d0 b1  d0 b2     binary                  a0
                        character complete
                        word ended


A binary sequence is a sequence of any bytes

Non-character sequence is a binary sequence that doesn't contain a valid UTF-8
character.

Character sequence is a binary sequence that is a UTF-8 character.

Text sequence is a sequence of character sequences.

Word sequence is a text sequence that doesn't contain word-break characters.


--------------------------

How do we want to use it?


forever
    until we have a complete sequence
        add a byte to sequence
    write the sequence


        byte          character
atom    sequence      sequence

byte --- Non-character
     --- Character --- Non-word
                   --- Word

So, there are two dimensions: sequencing and alternating.

We can start with the first (byte) sequencing level and leave the second
(character) sequencing level for later.

A two level solution can be like this:

until we have a complete character sequence
    forever
        until the byte sequence ends
            add a byte to the sequence
        if it is a character
            add the character to the character sequence
            break
        write the bytes to binary fork
        empty the byte sequence
write the character sequence to the text fork
empty the character sequence


So a sequence object provides binary output: valid/non-valid, but only if the
sequence has ended.

top level stream writing functions return amount of data actually written
they write to their forks and also to global metadata
inside they need to do some atomic writes, sometimes to both forks at once

These second level functions can also accept the location for the remaining
length and write to the buffers opportunistically. However, when atomic
writing to a few buffers is necessary, we can have intermediate storage for
meta pointer and remaining length to ensure that we can roll-back the write
accounting

So on the second level at the moment we're writing a character, which can be
valid or invalid. A valid character is simply an atomic write to the text
fork. An invalid character is an atomic write both to text and to binary fork.
Either of them can write meta records when switching between valid and invalid
runs. These writes should also account for the necessary meta records, but not
write them. If the meta record cannot be fit the writing should say that no
data was written.

We need these operations:
    streams:
        tlog_stream_cut             write withheld incomplete characters
        tlog_stream_flush           write meta and reset runs
    io:
        tlog_io_cut                 write withheld incomplete stream characters
        tlog_io_flush               flush streams
    sink:
        tlog_sink_io_cut            write withheld incomplete I/O characters
        tlog_sink_io_flush          flush I/O and write it as a message

Abstract transaction:

    change some fields
    if some action changed
        rollback changes
        abort

Should we implement single transaction interface?
I.e. if a transaction started on upper levels, we don't start another one?
So there is no sub-transaction support? Would that be right? I.e. would there
ever be a transaction trying some sub-transaction variants, aborting them and
then settling on one variant, proceeding, and then aborting on a later action?
Nevertheless, can't we do this with a single transaction storage anyway?

An object should save only what makes sense to rollback according to all the
possible operations.

So, a tlog_io_write can look like this:
    tlog_io_trx_start(io)
        tlog_stream_trx_start(input)
        tlog_stream_trx_start(output)
    try writing delay to timing
    if failed:
        tlog_io_trx_abort(io)
            tlog_stream_trx_abort(input)
            tlog_stream_trx_abort(output)
    try writing stream
    if failed:
        tlog_io_trx_abort(io)
            tlog_stream_trx_abort(input)
            tlog_stream_trx_abort(output)
    tlog_io_trx_commit(io)
        tlog_stream_trx_commit(input)
        tlog_stream_trx_commit(output)


Copying the state should happen only at the top level transaction start.
Restoring the state should happen only at the top level transaction abort.
But this way we can't do sub-transactions. Yeah, and anyway, if we're doing
sub-transactions we're supposed to have another set of fields saved, so we can
roll-back properly. So, let's not try to support sub-transactions for now.
Let's just assume that only top level transactions save/rollback. So, we'll
need a transaction nesting counter. Any level can call start increasing it, or
abort/commit decreasing it. But only the top level will actually act on the
decision (abort/commit). Perhaps we can come up with a facility to make adding
transaction support to an object easier.

Ah, it seems we need sub-transactions for writing characters / binary pieces
atomically in streams. Or do we? Yeah, pretty much. We abort character /
binary piece writing, but we commit pieces written before these. I.e. we're
not aborting the whole stack.

Another challenge: pmeta. It's not a part of stream object, it's passed into
functions, all of them. But perhaps it should be? Then we can save it in a
transaction as well. The same about prem. Shan't we try implementing that
first? Hmm. It's not like they ever change, after all.

Ah, nope. While we can have these pointers kept in the stream objects for ease
of calling the object functions, we can't snapshot them as part of streams.
They're a part of I/O object and need to be preserved as part of its own
transactions, in tlog_io_write.

Alright, then the needed depth of the transaction stack is determined by the
upper layers. If they need a copy for their transactions, they're supposed to
allocate and own it. We shouldn't need to allocate anything dynamically.
Shall they be stored on the stack, then?

So let's store them on the stack, but copy there only if we need a
a new, or sub-transaction.

Let's define transaction storage type for each object type, which can simply
be the object type. Also let's define backup/restore operations for each
object, which can simply be a memcpy. Still, that storage type should be an
aggregate for the nested objects.

If a function can't fail, it shouldn't accept a transaction pointer.
If a function doesn't need to make backups and rollbacks, it shouldn't accept
a transaction pointer. However, every type expected to support transactions
should define backup/restore functions.

On a closer examination and attempting to do it, we'd better not pass pmeta
and prem to streams at creation time. This turns out to be more obscure and
harder to reason about.

If we remove separate window message, then we don't need separate packet type
- just supply window size each time, make the sink code or source user compare
  window sizes and detect changes. This can make the code much simpler.
OTOH, I/O data and window changes *do* come separately and such scheme would
require the user to cut the I/O themselves and take care of comparing window
sizes all the time. So, for a stream of events, having separate packet types,
as in "separate even types", makes sense. We can still have one message type
for the JSON sink.

We still need to preserve window size changes, likely. Can we put them inside
timing, as events? What would happen with the player, if there is no window
size at the start? How would the *partial* stream matches work?! Shall we
simply produce messages with empty I/O and new window sizes? These would
have timestamps. We shouldn't send such messages immediately, but only when
told to flush, or when we run out of I/O size limit. If window change comes
before I/O, the written message should have the window change time position,
and timing would have a delay record coming first - this needs a tlog_io
object change, so it starts when it's told it starts, not when first I/O comes
in. I.e. we can say that window change happens always at message start.

No, this is wrong. This means that we won't be able to merge messages across
window changes. To satisfy both the need to merge messages and the need to
process partial streams, window changes should be able to appear any place in
the message (i.e. in "timing"), and each message should have a window size
record at the start. Argh.

BTW, how are we even going to merge particular stream chunks with audit
messages for a specific session? Could Kibana plugin request audit data by
itself? Not likely, and it wouldn't be right, anyway. We can tell users:
search your messages and then add 'OR something that matches a bunch of
relevant audit messages' to the query, and the playback visualisation would
filter them out by sessionid and time ranges. That's ugly. Do we have
alternatives? I think it is time to actually try making a visualisation.

All the objects manipulated by a function are saved on the entrance to the
stack frame with depth zero, and restored on return.

We need backup/restore so we don't present intermediate state, but rather
completely changed or completely unchanged

- What do you need sub-transactions for?
- For moving in discrete states within another transaction

- Why do you need tracking which objects are saved and which are not?
- So cyclical ownerships can be saved. So that we can start a transaction at
  either of these levels:

     => A -.           A -.
        |  |           |  |
        B  |    AND    B  |
        |  |           |  |
        C -'        => C -'

Each function down the stack needs to store the objects explicitly, in case it
is called at the root of the transaction. However, if the function doesn't
leave any state inconsistent it doesn't need to accept the transaction
argument.

So we have object saving for root transaction, then stack increment, then
processing, then stack decrement, then object restoration for root
transaction.

---

We write pieces of input or output with time attached. We write window sizes
with time attached

We go over each byte sequence of submitted I/O and decide if it's a valid
character or not.

If it's a valid character, we encode it into text. Otherwise we encode it into
binary.

We can attach timestamp to each piece of data. Each piece of data has its
timestamp.

We can't encode data until we know it's a valid character or not.

The decision to write the data happens only then, so the decision to terminate
a meta record happens only then and the decision to insert a delay record can
only happen after that.

So how about we have forks again?

Can we remove the need for transactions?

We would need to be able to back out of writing a delay and subsequent records
on flush. To be precise, we would need to be able to play these forward and
decide if we want them to do or not. That would mean doing everything twice
for all operations, whereas we only need to back out once in a while.

What is more complicated? Transactions or backing out? Transactions are easier
to think about. Otherwise we would need to pass our temporary copies to all
the arbitrary function depths and care about copying/applying them properly in
each stack frame. That's what transactions could do for us anyway.

---

So if we want to support multiple object roots per transaction we need to
separate transaction beginning/ending from transaction object
saving/restoring. Also, to be reliable we'll need to remove the need to
specify objects to restore for transaction abortion.

We can specify objects to backup/restore via initialization of an array
variable in a macro. We can then backup these objects in transaction begin and
restore them in transaction abort.

Can we use the stack for indirect transaction store? First we need to describe
object's transaction interface.

    store_size  The size of the object's transaction store
    pmask       Location of the object's transaction mask
    xfr         Transaction data transfer function

So each function frame would contain an array describing which objects should
be backed up/restored, which would be built from both static, direct
object information and dynamic, indirect object information. Or perhaps we
should simply resort to dynamic information always. Even with dynamic
information, first level can be allocated on the stack and lower levels can do
heap allocation for indirect objects, if necessary.

We can then remove transaction objects and functions from headers and put it
into object files, making them tidier.

---

What if we allocate transaction storage within the objects themselves
according to the maximum transaction depth? This way we won't have to mess
with allocation/deallocation at transaction backup/restore/cleanup time!

Then the transaction interface becomes:
    xfr         Transaction data transfer function

If we manage to separate them, then it becomes:
    backup      Transaction backup function
    restore     Transaction restore function

----

iface
    act
        mask
        store

root header
    trx - transaction management function
        TRX_BEGIN
        TRX_ABORT
        TRX_COMMIT
    trx_misc - miscellaneous transaction functions and macros
    trx_state - transaction state passed between functions
        trx_state_get_depth
    trx_iface - object's interface definition
    trx_basic - macros for basic interface implementation
        trx_basic_iface - define interface
            trx_basic_act - define acting function
        trx_basic_store - basic store definition macros
            trx_basic_store_sig
        trx_basic_members - define fields for object structure
            trx_iface - interface
            trx_mask - mask
            trx_store - store list

----

        1                                   3
input >-------------+-------------------------> input
                    |
         3          |                        1
output <----------- | ------+-----------------< output
                    |       |
                    |       |
                  2 |     2 |
                   \|/     \|/

when to log I/O

* after receiving, before sending
    single timestamp
    more likely to get out

* after sending
    timestamps may need to be set to the time after sending
    less likely to get out

Should the timestamp be the time we received data, or the time we delivered
the data?

Neither is guaranteed to be exact - network latency can happen on either side,
so no advantage here.

So, after receiving and before sending then.

Shall we allow interruption of log writing by signal handlers?
What shall we do when that happens?

    SIGTERM/INT/HUP
        we don't need to log anymore, since data will not be delivered anyway
        don't write data
    SIGALRM
        flush log - that can be done as well after writing, since we're
        already trying to deliver the log as hard as we can.
    SIGWINCH
        that can as well be handled after we finish log writing, if a few
        resizes are missed, it's not a big deal - delivering the last resize
        before passing any data is what's important.

So if we do partial log writing it is only to handle aborting log writing and
not writing extra. This can be useful with large buffers, slow logging
backend and overly chatty user application.

Shall we mix logging and writing? I.e. deliver what we logged if logging is
interrupted and then try to deliver logs again?

Shall we actually deliver what we read so far before acting on
SIGTERM/INT/HUP? Or shall we simply exit ASAP? It's likely the sysadmins would
appreciate the latter more. As well as not finishing logging of undelivered
data.

With that in mind, there's no use to proceed to writing partially logged data.
Because otherwise, SIGWINCH... Wait.

What happens when we delay the data in transit and the window gets resized?
The data that we've read already was expecting the old window size, we can't
push it back so it needs to be delivered anyway. We'll read next portion only
after we've delivered the resize. So it's fine.

So, going back, we don't need to do anything with SIGWINCH until we're ready
to read some more and that's after we've logged the data. Wait. What shall we
write first: the SIGWINCH, or the input we've read already? SIGWINCH would be
better, as otherwise delivering input first can make the application fill the
buffers with response formatted for the old window size. Once again, it
doesn't matter what we'll deliver first: the output we read already or the
SIGWINCH we received.

So it seems it is better to have I/O log writing interrupted to log and
deliver window resizes before resuming logging and delivery of I/O.

Thus our TTY source needs to drop support for reading window sizes as they
need to have higher priority. Since we need to record timing of our window
resize deliveries outside the source, we better remove timestamping from the
source. Thus we'll have timing neither in source nor in sink. However, sink
can have window writing.

OTOH, input sent before resize was done with the expectation of the old size.
E.g. a command to go to the bottom of the window in vim (L) would need to move
the cursor to the old window bottom, not to the new one. So, for the sake of
simplicity and, it seems, for the sake of consistency it is better to send the
input received before SIGWINCH before sending the new window size. Thus we can
preserve the ordering and keep window size retrieval in the source.

As to writing as soon as the (interrupted) data logging is complete (i.e.
logging/writing interleaving), it might improve interactivity and make the
program easier/faster to kill.

---

Configuration parameters

    Log latency (seconds)
    Maximum message payload size (bytes)
    Shell to start (path)
    Log target (file / syslog / journald?)
    Log target options:
        file:
            path
        syslog:
            facility
            level
    Recording warning to output on startup

Command line-specific parameters:
    argv[0] starting with '-' - login shell - passed the same way to the shell
    '-c' - command to execute - passed the same way to the shell
    -?tlog-rec-(.*) - shell, perhaps

Mandatory environment parameters:
    shell

It would be good to have all configuration options available via all sources:
    configuration file
    command line
    environment

We can parse a particular environment variable value as an option string.

We can map command-line options to JSON configuration structure, or the other
way around.

We can make a mechanism to parse all of these into a common format (JSON?) and
merging them according to the priority.

E.g. (last overrides first): configuration, environment, command-line.

We'll also need some sort of schema verification for configuration.

Then we can make a module taking that merged configuration and running the
program.

What should be the common format?

Choices:

    Custom structure, generic perhaps
        Hmm, not really, no.
    JSON
        merging:
            Generic repeated walking and merging according to priority.
        validation:
            Random access with accumulated state.
        conversion:
            from ARGV list:
                Split into levels on dashes within option names, perhaps?
                Hmm, options often have very short names, which will have to
                be normalized. Some sort of mapping, then? How to tell the
                user about that mapping? Build-time documentation generation?
                M4, ha-ha?
            environment:
                if command-line options, convert to ARGV list - need to handle
                shell quoting
                otherwise JSON
    ARGV list
        merging:
            Just concatenate lists from various sources according to priority.
        validation:
            Regular getopt loop with parsing and accumulated state
        conversion:
            from JSON:
                Walk and convert automatically, say by concatenating with
                dashes between levels. Still needs schema to provide
                JSON-oriented error messages (i.e. line number, field
                reference).
            from environment:
                Split into ARGV list - need to handle shell quoting

Values of specific parameters and permitted parameter set have to be validated
upon parsing the source format. The overall configuration, required parameter
set should be validated only after merging. The (missing or invalid)
parameters should be referred by a name common to the configuration sources.

So, we can have an m4 file describing containers and parameters, then generate
documentation from that, then generate JSON validation, option reading,
environment variable reading from that.

-----

program name - argv[0]
    This is not configuration, i.e. not something that the user would like to
    specify, that's why we don't want it in our options or config file. OTOH,
    a system administrator or a distro package maintainer might like to change
    it. Still he's not a user and this is done outside the normal
    configuration path.

    Shall we perhaps transfer it out-of-band? But it's sooo convenient to
    transfer it via JSON.

shell - cfg, env, argv[0], -s/--shell option

login flag - argv[0], -l/--login option

positional arguments argv[x..]


If we take away the program name, then we can split those into three origins:

    name    - login flag, shell
    opts    - login flag, shell
    args    - arguments only

Then the login flag and shell would be documented properly and overridable by
the command-line options.

-----

We need to signal the sink to terminate cached characters. That needs to be
done with a timestamp.

Since in tlog-rec we don't operate on timestamps anymore, getting them with
packets from the source, it would be good to have these terminating packets
from the source too. The most logical way would be to send these packets when
the corresponding streams in the source are ending.

There's the problem of the TTY source stream end - it usually happens when
either the child or the terminal go away, which are reported as errors, and
the source reports all errors to the caller. To return the void packet the
source would have to interpret the errors, which might be different between
TTYs, PTYs, pipes and files, and interpreting them incorrectly might hide a
problem. Let's see what we can get:

    EBADF   - can also be returned if the FD was closed, thus interpreting it
              as EOF can hide a programming mistake
    EIO     - can also occur on low level I/O error, thus hiding a serious
              hardware issue

We have the void packet signifying the end of the whole source stream now. And
it can't have a timestamp as of now.

Let's consider the following options:
    Add a timestamp to the void packet
        We have to have a completely meaningless packet type to denote an
        empty, meaningless packet
        - No
    Add another type of packet signifying the end of the whole source stream
        When the source would send such a packet? It can send it only when
        both input and output are closed. Can we put the function of keeping
        track of the stream status on the source? Seems alright since it
        manages and encapsulates them.
    Add another type of packet allowing specifying the end of input/output
    stream separately
        Now what about the potential window size stream? Shall we add that
        too? Wouldn't it make the whole thing too complicated?
    Add a new meaning to the I/O packet type which signifies the end of stream
    (e.g. zero length data)

Now, what kind of timestamp would other types of sources specify? E.g.
ElasticSearch? It doesn't have anywhere to pull the EOF timestamp from!

We shall rethink this. There shouldn't be a timestamp for EOF and it can
still be signified by a void packet.

We have to accept the fact that sometimes we need to write data that was
received before the latest data written to the chunk, and that there can't
always be a timestamp associated with that event. E.g. when converting from
one storage format to another (e.g. sudo session recording to JSON log and
having incomplete characters at the end). To properly handle that we would
need to only allow encoding the packet stream as a whole and that's not what
we can do.

Which timestamp should we have for such data, then? Well, there's nothing left
but to use the latest timestamp written to the timing (metadata).

Now, how do we signal the cut-off? Shall we keep the function, since we don't
need the timestamp anymore, or shall we move to using the void packet?

Void packet
    Symmetrical to EOF from source.
    Somewhat obscure code.
    All the non-UTF-8 sinks would need to explicitly ignore the void packet.
    More code to write, a little code to remove.
Function
    Separate function which complicates the interface and is only needed by
    one sink so far.
    Clearer code.
    All the non-UTF-8 sinks can just not define the cutting function.
    No code to write.

Let's leave it as function for now.
Let's have the JSON chunk use the latest timestamp written to the timing for
cutting.

Wait. We can't use the latest timestamp in the chunk. Because it is possible
that a stream will be written only the incomplete character and there will be
no timestamp reported to the chunk at all until we actually "cut" the stream.

Therefore we must allow the chunk to receive timestamps in the past and assume
the latest timestamp in these cases. The streams, though, will still be able
to enforce the time succession.

So, streams will have to report the last received data timestamp and if chunk
sees them as too old, it will use the latest time instead.

-----

Do we create a PTY or a pair of pipes for the child?
    First, effects:
        If a child uses PTY and we are using non-TTY FDs, then:
            Child input won't contain expected terminal control symbols
            Child output will contain unexpected terminal control symbols
        If a child uses pipes and we are using TTY FDs, then:
            Child input will contain unexpected terminal control symbols
            Child output won't contain expected terminal control symbols
    Next, decision logic:
        SSH allocates PTY if input is TTY, or if forced
        What would user expect?
        Tlog-rec is supposed to start a shell, either interactively or not.
        When started as the login shell (the primary use case), tlog-rec will
        either get all descriptors tied to a terminal or all to a non-terminal.
        What's the use case for starting as a non-login shell?
            Testing?
                We won't be outputting to an actual terminal, but will be
                capturing everything, so we can automate.
                We will test whatever the user needs.
                - Not applicable
            Just the shell being started again, where we need to simply exec
            the actual user shell.
                -Not applicable.
        So what kind of logic can we have?
            If at least one of in/out/err is a terminal, spawn PTY?
            If all of in/out/err are terminals, spawn PTY?
            Which one would be easier for testing?
                If we're running under expect as we plan, then all FDs will be
                TTYs.
                If we want to test non-TTY mode, then we will spawn tlog-rec
                under pipes, perhaps from shell, Python, or Lua?
                - Not applicable
            Which would be more useful?
                Considering the above the difference is marginal.
            Which would be easier to implement?
                If at least one (stdin?) is not a TTY, don't spawn a PTY.
                However, considering we need to handle open/closed descriptor
                state and propagate it to the child anyway we can as well
                check all of the descriptors anyway.
        The POSIX shell standard says that the shell is considered
        interactive if both its stdin and stderr are connected to a
        terminal. We can employ the same logic for deciding whether to spawn
        the PTY or not.
        I.e. spawn the PTY only if both stdin and stderr are terminals, ignore
        stdout.
        No, tlog-rec is not a shell and in fact any program can be ran under
        tlog-rec. So we need to employ our own logic.
        Leaving it at "if any FD is a TTY, spawn PTY" can work as well for
        now.

Do we create two pipes, one, or none?
    We create two pipes to match the PTY case.
    We send both child stdout and stderr to our stdout.

What do we do when any of stdin/stdout/stderr are closed at the start?
    We ought to pass the closed/opened state to the child.
    If our stdout is closed, the child's stdout and stderr are closed.
    If our stdin is closed, the child's stdin is closed.
    The child is not affected by our stderr state.
    We need to ensure that the closed FD is not taken over by another FD.
    So, for example, error messages directed to the initially closed stderr
    don't get written somewhere else.
    How? Perhaps by opening /dev/null?
    What do we do when only input or output is closed?
    Perhaps switch to pipes and propagate the status?

What do we do when we get an EOF on any of the FDs during operation?
    We continue the transfer until all FDs get EOF.

----
How can we implement recording and playback to/from journal?

We can add a journal sink, which, in addition to the JSON message, can log
additional fields, duplicating the data already in JSON, letting us filter
journal messages by them. It remains to be seen how that will perform.

We can let users configure if tlog-rec should put extra fields into journal,
or just the message. The journal source can then accept an arbitrary filter
string, similar to what journalctl accepts to filter out the required session.

One problem can be specifying the time from which we should play. The journal
API provides sd_journal_seek_realtime_usec and sd_journal_seek_monotonic_usec,
which we can perhaps use by taking a separate journal source option.
---
So, rate limiting is not letting us log more than a specified amount of I/O
bytes per time unit. Let's say per hour.

The way we calculate that could be having an averaging time window, or just
calculate from the start of the recording.

current_rate = bytes / hours

How would we choose what to drop, though?
Should we just do

    if current_rate > target_rate then
        drop
    else
        pass

What should be the quantum of dropping, then? A packet?

----------------
Leaky bucket for raw data, both input and output
Either drop or hold the packets
Consider providing maximum packet size configuration
Consider doing it for encoded data later
Add configuration parameter/option for bytes per second

----------------
The pass decision is for a packet.
We can simply say every piece of it is written when dropping.
We can simply wait on the first piece when delaying.
----------------

We need to either assume that packet timestamps are in local time,
or take our own time samples. Because we cannot advance abstract packet
recording time, if it is not local time. For that matter, to make the rate
limit sink more universal we need to ignore packet timestamps and take our own
timestamps.

---------------

So, we don't need to know local time if we are just dropping packets, we only
need packet timestamps.

We can drop packets based on their timestamps, or based on times when they're
written. We can only delay packets based on writing timestamps.

Having limit based on packet timestamps would be useful for testing.

We would need rate limit set on the log sink, right? Right.

What if we give the rate limit sink some time source function instead of
letting it get its own time?

What timestamps should delayed packets have? Original ones, or delayed? Should
the recording reflect timestamps the packets were output by the recorded
program and input by the user, or the timestamps they were delivered at?
If we follow the idea of "recording what users saw on the screen", then it
should be delivery time.

Ha! Should we actually rate-limit the source instead? Then we can adjust
packet timestamps and take time samples freely! For the start we can just give
the rate-limit source a clock ID, but in the future we can pass it a function
which would generate timestamps, and let us do testing at any actual speed
required. We would also have no problem with packet fragments, as sources
don't have any.

The rate limit sink will have to assume that the time source for packets is
the same time source it was given. We can later introduce a notion of
"abstract time", which would have timestamp retrieval and time-advancing
(sleeping) functions. We can have one specific time for
clock_gettime/nanosleep, and one for testing time.

Right. We can't rate-limit source, because we will be dropping data sent to
the user terminal and entered by the user. We have to rate-limit the log sink.

Would it be right to change the timestamp of packets sent to the log?
No it likely wouldn't.

Should we have delay at the TTY source, but dropping at the log sink?
Even if we had delay at the TTY source, would it be right to change packet
timestamp? For that matter, what the packet timestamp should be? Time of
arrival, or time of departure? And should input and output rather be both
measured at the tlog<->program interface, and not input at tty<->tlog and
output at tlog<->program interfaces? Or both at tty<->tlog interface, as the
user saw it? From the point of correlating with other logs, the right way
would be to record time at the tlog<->program interface, as that would be
closer to other events in the system. The tty<->tlog interface is just a
display, after all. What about window resizes? That should be the same as
input.

For that matter, if we did throttling (delaying) at the source, then only
input and window timestamps should be updated, and we wouldn't really be able
to use packet timestamps for rate control. However, since we don't let any
packets pass until we log them, and we can't really control logging latency,
we can only log source timestamps.

But should we log only before passing the I/O? If not, then when should we log
it? Should we log it after passing a single packet? Why a single packet then?
Why not two? What if it's a single *big* packet? No, we should log *before*
passing the packet, or rather we log any data, before passing it on.

To avoid the mess of timestamps, we can just assume that timestamp is opaque
packet data, and deal only with rate related to the provided clock. Then we
can implement rate-limit sink, simply, covering all the features.

What if we're asked to write only a part of a packet, and it doesn't pass,
but a later part passes? Do we write a fragment only? Apparently yes.
So dropping/delaying quanta becomes a packet fragment.

Should we allow a part of packet fragment through, if only a part fits?

We should let whatever fits through immediately, and drop/delay the rest.

We need to do the pkt_pos math properly. Support adding and subtracting. Both
should round to the closest possible value, just like floating point does.
Should they cap at maximum?

ppos + free, cap at end
if more than zero, write
if less than end, delay until what's left can fit

First purpose of pkt_pos: track progress of writing packets.
I.e. we need to mark where we were and where we've gone until all of the
packet is written
Second purpose of pkt_pos: calculating payload size and converting payload
size to position to write from and until

No, we're not going to be rate-limiting at the packet level. First of all,
encoded size can be wildly different, then it will be hard to tell if anything
was dropped or not. We need to rate-limit at the JSON writer sink.

------------

So, tests, even unit tests are essentially an outside entity.
Or, perhaps, not entirely. If we ship a library, and we hide some of the
internal interfaces, we might need to access them for testing purposes.
Thus unit tests will be a privileged entity, with access to internal
interfaces. Still, we can build a separate library, with those interfaces
exposed, so that the unit tests can reach them.

Regardless, unit tests and integration tests are not a part of the
release distribution, they're outside of it, working on the product of it.
Ideally, they need to be in a separate directory, beside tlog.

Yet, having two directories side-by-side in the root of a git repo is
generally unheard of, and we shouldn't really have a separate repo for the
tests, as it will make it difficult to keep them in sync with the code.

Shall we make subdirs under top directories instead? E.g.

    include
        tlog
        tltest
    lib
        tlog
        tltest
    src
        tlog
        tltest
    m4
        autotools
        tlog
    man
        tlog
    doc
        tlog

Let's try.