# Current sbuff failings

## The major issues with the current sbuffs are

- Markers need to be released before the function they're created in returns.  This means we often create temporary `fr_sbuff_t` on the stack which is inefficient.
- Using pointers often results in abuse, i.e. people storing pointers pointing into the underlying char buffer.
- Sbuffs don't support streaming properly.  Large parts of the buffer ends up getting pinned in place and we're unable to shift it out.
- Shifting data out of sbuffs is expensive because we need to update every nested sbuff, and every marker associated with them.
- Passing terminals and unescape callbacks into functions leads to massive complexity, and needing to allocate memory to allocate merged tables of token sequences.
- Last matched token isn't stored anywhere, so the caller of a parsing function needs to check the current position of an sbuff against its tokens.
- Returning negative offsets to indicate error positions dosn't work when there are transform functions in the sbuff chain.

## Fixes
### Marker release

Current sbuffs require markers to be tracked because the marker's pointer needs to be updated is the sbuff is shifted,

Using offsets relative to the start of the current sbuff removes the tracking requirement and means that the markers no longer need to be updated.

[source,c]
----
typedef struct {
        sbuff_t         *parent;
        size_t          offset;
} sbuff_marker_t;
----

### Pointer abuse

In almost every instance a "legacy" C function that's operating on a standard C buffer takes a `buffer` and a `length`.

Direct access into the sbuff should be restricted, and a macro should be provided that provides both the length of the
data and a pointer into the sbuff as consective function arguments.  This prevents assignments to C pointers as this will
result in a syntax error.

[source,c]
----
#define sbuff_ptr_and_len(_sbuff_or_marker, _max_len) (sbuff->buff + sbuff_offset(_sbuff_or_marker)), sbuff_max_len(_max_len)
----

Standard copy functions could copy data from one "parsing" sbuff to a "printing" sbuff which wrapped a buffer on the stack.
Printing sbuffs could also be setup to do standard escaping for \0, making the string "c safe".

### Overzealous pinning

The key to fixing the pinning problem is to allow backtracking.

Not all source sbuffs support backtracking, so we'd need a type of sbuff which can grow a temporary buffer and will allow
backtracking within a given range.  This sbuff can be inserted into the stack of sbuffs to buffer a unidirectional stream
in parsing functions that require it.

We probably want an initialisation macro that can determine whether the parent sbuff supports backtracking, and if not allocate
a "backtrack buffer" of a given size.  Only the function that wants to backtrack knows when data can be released from the
backtrack buffer, so it needs to be responsible for doing that.

It is up to the developer to add these "backtrack" buffers in functions where processing the token stream doesn't move forward
monotonically.

A good example of one of these non-monotonic functions is `tmpl_afrom_substr`, where multiple parsing functions attempt matches
against known data types, like mac addresses and integers.

In this case `tmpl_afrom_substr` must insert a backtrack sbuff into the sbuff chain which will guarantee sufficient bytes are
retainted so that it can always backtrack after calling each of the data parsing functions.

### Backtrackable child buffers

As with parsing functions today, whenever a parse function is entered, it should allocate a new sbuff.  This sbuff records the
current offset of its parent and sets its current offset to the same value.

[source,c]
----
typedef struct {
        char                    *p;                             //!< Cached pointer into stream->buff, used to avoid performing arithmetic
                                                                ///< on each call.
        char                    *end;                           //!< Cached pointer to the end of valid data.
} sbuff_cache_t;

typedef struct {
        sbuff_stream_t          *stream;                        //!< Where we acquire data.

        size_t                  offset_at_creation;             //!< sbuff's offset when we were created.  Lets us seek back
                                                                ///< to the same position as our start, so we can seek forward to a given
                                                                ///< offset.

        sbuff_t                 *parent;                        //!< Our direct parent.

        size_t                  current;                        //!< Our current offset.

        sbuff_cache_t           cache;                          //!< Cached pointers.
} sbuff_t;

typedef struct {
        sbuff_t                 sbuff;                          //!< Our sbuff.

        sbuff_seek_func_t       forward;                        //!< Function used to shift data leftwards.
        sbuff_seek_func_t       back;                           //!< Function used to shift data rightwards.

        size_t                  window_start;                   //!< Offset representing the start of valid data in the buffer.
        size_t                  window_end;                     //!< Offset representing the end of valid data in the buffer.

        uint8_t                 is_const:1;                     //!< Buffer may not be written to (if true).
} sbuff_stream_t

/** Stream variants which read/write to different sources
 *
 */
typedef struct {
        sbuff_stream_t          stream;                         //! common stream fields.

        char                    *buff;                          //!< A normal buffer on the stack.
} sbuff_buff_t;

typedef struct {
        sbuff_stream_t          stream;                         //!< stream manipulation functions.

        FILE                    *file;                          //!< File to read from.

        size_t                  size;                           //!< How big the buff is.
        char                    buff[];                         //!< Temporary buffer for storing data.
} sbuff_file_t;

typedef struct {
        sbuff_stream_t          stream;                         //!< stream manipulation functions.

        TALLOC_CTX              *ctx;                           //!< ctx to alloc/realloc the buffer in.
        char                    *buff;                          //!< the talloced print buffer.
} sbuff_talloc_t;

typedef struct {
        sbuff_t                 *sbuff;

        size_t                  offset;
} sbuff_marker_t;

/* Call fr_sbuff_done or fr_sbuff_error to restore the pointer cache in the parent */
/* FR_SBUFF() should invalidate the cache so one of these functions is required */
----

All markers and positions in the child sbuff start from the parent's offset.  If the underlying buffer has data shifted
through it, `window_start` and `window_end` reflect the range of offsets the data in the buffer represents.

It is impossible to seek a child sbuff past its starting point of zero because we can't map this to an offset in its parent
sbuff.

Consider a chain of sbuffs: `<FILE * sbuff> -> <transform sbuff> -> <parse sbuff>`.  The offsets of `parse sbuff``, will likely
not map 1:1 with `FILE * sbuff`.  This is because `<transform sbuff>` is transforming the text.  As soon as one escape sequence
is removed during the work done by the `<transform sbuff>`, offsets between `FILE * sbuff` and `parse sbuff` are no longer relative.

In order to be able to map any offset of `<parse sbuff>` to any offset of `FILE * sbuff` we'd need a lookup table. This isn't feasible.

As backtracking will be relatively rare, the mechanism we use can be expensive.

If a backtrack is requested to an offset < `start`, we request a backtrack in the parent to `parent_start`, the parent sbuff
can request a backtrack in its parent to its `parent_start`.  In this way the entire chain of sbuffs can backtrack to
an earlier position in the stream being read.

This will likely result in work being redone, to retransform text which had previously been read in from the file.

One concern is a `parent_start` offset falling in the middle of an atom in the parent, but if this happens, then it means the
transform function is broken.  i.e. if a transform function provides the `\` from `\t` in its output buffer, then the transform
function must be rewritten. The output buffer of a transform sbuff must contain only complete, transformed, atoms.

### Shifting data out of sbuffs is expensive

The reason why it's expensive is because the entire sbuff chain needs to have its pointers updated.  If every sbuff maintains
positions using offsets, only the start and end offsets need to be changed.  This gives a real advantage to using markers,
as they will never need to be updated during shifts.  marker's offsets remain relative to `sbuff->start`, even if
`sbuff->start` changes .i.e. `char *current_pos = sbuff->buff + (marker->offset - sbuff->start)`.

If `sbuff->offset < sbuff->start`, this event would trigger a backtrack.

### Pasing terminal sequences and escape rules into functions is awful

Text transformation must be transparent to the parsing function, anything else will not function correctly, or will add
horendous complexity.

A trasnformation sbuff takes data from its parent, transforms it, and places it in and output buffer.  The transform sbuff's
offsets are valid for its output buffer only.

A terminal sbuff takes requests for more data and scans the stream coming from its parent sbuff for terminal sequences.
If a terminal sequence is found, it notes the last terminal found in its internal structure, and will refuse to allow data
to be read past the terminal sequence.

Both these functions could be combined into a single sbuff type, or implemented separately.

Consider parsing a double quoted string

[source,c]
----
int my_parse_function(fr_sbuff_t *in)
{
        fr_sbuff_switch(in) {
        case '"':
        {
                sbuff_term_table[] = {
                        { "\"" , DOUBLE_QUOTE }
                };

                sbuff_t dquote = FR_SBUFF_TRANSFORM_TERM(in, dquote_transform_func, quote_terminal_table);

                tmpl_afrom_substr(ctx, &dquote, TMPL_TYPE_DOUBLE_QUOTE);

                if (sbuff_last_token(&dquote) != DOUBLE_QUOTE) /* ERROR */
        }

        default:
                ...
        }
}
----

Here `tmpl_afrom_substr` would be completely unaware it was processing a double quoted string unless we told it explicitly
(we need to do that so it can find xlats).  All unescaping and terminal sequence location is done transparently, and
`tmpl_afrom_substr` only sees the unescaped byte stream.

### Negative offsets don't work when transform sbuffs are used

One key realisation is that returning negative offsets up the call stack will not work when the offsets don't map 1:1
between parent and child.

As we only use negative offsets for error printing, it makes most sense to duplicate some of the code from the `fr_strerror`
API, and create a sbuff error stack in thread local storage.

[source,c]
----
/** Pushes an error onto the error stack
 *
 * @param[in] subject           that experienced the error
 * @param[in] ctx_len           Number of bytes to store before the current position.  Might want to have this accept a marker or len.
 * @param[in] subject_len       Maximum number of bytes to store after the current position.  Might want to have this accept a marker or len.
 * @param[in] err_fmt           The error message.
 * @param[in] ...               Arguments for err_fmt.
 */
void sbuff_error_printf(sbuff_t *subject, size_t ctx_len, size_t subject_len, char const *err_fmt, ...);
void sbuff_error_printf_push(sbuff_t *sbuff, size_t ctx_len, size_t subject_len, char const *err_fmt, ...);
void sbuff_error(void (error_print_t *)(char const *ptr, size_t len, void *uctx), void *uctx)
#define sbuff_error_foreach(...)
----

Behind the scenes, when `sbuff_error_printf` is called, it copies `ctx_len` bytes from before the current position,
and `subject_len` bytes after the current position,  records the offset of the error in the string it just copied,
and creates/stored the error string from `err_fmt`, and `...`.

In this way we still get rich, contextful errors, but without passing an offset back up the stack.

All parse functions can revert to returning `int (0, -1)` instead of `fr_slen_t` (as they do now).
