Source

ocaml-core / base / async / scheduler / lib / writer.mli

(** Writer provides an interface to writing strings that batches together writes into a
    single underlying write system call. *)
open Core.Std
open Import

module Id : Unique_id

type t with sexp_of

(** [io_stats] Overall IO statistics for all writers *)
val io_stats : Io_stats.t

val to_string : t -> string

(** [stdout] and [stderr] are writers for file descriptors 1 and 2.  They are lazy because
    we don't want to create them in all programs that happen to link with async. *)
val stdout : t Lazy.t
val stderr : t Lazy.t


(** [create ?buf_len ?syscall ?buffer_age_limit fd] creates a new writer.  The file
    descriptor fd should not be in use for writing by anything else.

    By default, a write system call occurs at the end of a cycle in which bytes were
    written.  One can supply ~syscall:(`Periodic span) to get better performance.  This
    batches writes together, doing the write system call periodically according to the
    supplied span.

    A writer can asynchronously fail if the underlying write syscall returns an error,
    e.g. EBADF, EPIPE, ....

    [buffer_age_limit] specifies how backed up you can get before raising an exception.
    Default is 2 minutes.  You can supply [None] to turn off buffer-age checks.

    [raise_epipe] specifies whether the writer should raise an exception when it receives
    an EPIPE or silently drop all further writes.  In the case of the latter, the writer
    will eventually fail with Writer_buffer_older_than if the application remains open
    long enough. *)
val create :
  ?buf_len:int
  -> ?syscall:[ `Per_cycle | `Periodic of Time.Span.t ]
  -> ?buffer_age_limit:[ `At_most of Time.Span.t | `Unlimited ]
  -> ?raise_epipe:bool
  -> Fd.t
  -> t

(** [set_raise_epipe t bool] sets the [raise_epipe] flag of [t], which determies how
    [t] responds to a write system call raising EPIPE (see [create]). *)
val set_raise_epipe : t -> bool -> unit

(** [got_epipe t] returns a deferred that becomes determined when [t] attempts to write
    to a broken pipe. *)
val got_epipe : t -> unit Deferred.t

val stop_permanently : t -> unit

val of_out_channel : out_channel -> Fd.Kind.t -> t

(** [open_file ?append file] opens [file] for writing and returns a writer for it.  It
    uses [Async_unix.open_write] to open the file.  *)
val open_file : ?append:bool -> string -> t Deferred.t

(** [with_file ~file f] opens [file] for writing, creates a writer [t], and runs [f t] to
    obtain a deferred [d].  When [d] becomes determined, the writer is closed.  When the
    close completes, the result of [with_file] becomes determined with the value of [d].

    There is no need to call [Writer.flushed] to ensure that [with_file] waits for the
    writer to be flushed before closing it.  [Writer.close] will already wait for the
    flush. *)
val with_file :
  ?append:bool
  -> ?exclusive:bool
  -> string
  -> f:(t -> 'a Deferred.t)
  -> 'a Deferred.t

val write_file : file:string -> string -> unit Deferred.t

(** [id t] @return an id for this writer that is unique among all other writers *)
val id : t -> Id.t

(** [fd t] @return the Fd.t used to create this writer *)
val fd : t -> Fd.t

(** [set_fd t fd] sets the fd used by [t] for its underlying system calls.  It first waits
    until everything being sent to the current fd is flushed.  Of course, one must
    understand how the writer works and what one is doing to use this. *)
val set_fd : t -> Fd.t -> unit Deferred.t

(** [write ?pos ?len t s] adds a job to the writer's queue of pending writes.  The
    contents of the string are copied to an internal buffer before write returns, so
    clients can do whatever they want with [s] after that. *)
val write : ?pos:int -> ?len:int -> t -> string -> unit

val writef : t -> ('a, unit, string, unit) format4 -> 'a

(** [to_formatter t] @return an OCaml-formatter that one can print to using
    {!Format.fprintf}.  Note that flushing the formatter will only submit all buffered
    data to the writer, but does _not_ guarantee flushing to the operating system. *)
val to_formatter : t -> Format.formatter

val write_substring : t -> Substring.t -> unit

val write_bigsubstring : t -> Bigsubstring.t -> unit

(** [write_char t c] writes the character *)
val write_char : t -> char -> unit

val newline : t -> unit

(** [write_byte t i] writes one 8-bit integer (as the single character with that code).
    The given integer is taken modulo 256. *)
val write_byte : t -> int -> unit

val write_sexp : ?hum:bool -> t -> Sexp.t -> unit

(** [write_bin_prot] writes out a value using its bin_prot sizer/writer pair.  The format
    is the "size-prefixed binary protocol", in which the length of the data is written
    before the data itself.  This is the format that Reader.read_bin_prot reads. *)
val write_bin_prot : t -> 'a Bin_prot.Type_class.writer -> 'a -> unit

(** serialize data using marshal and write it to the writer

    The [Or_error] is because it may not be implemented on some platforms.
    The [unit -> ] is to allow polymorphism. *)
val write_marshal
  : unit -> (t -> flags:Marshal.extern_flags list -> 'a -> unit) Or_error.t

(** Unlike the [write_] functions, all functions starting with [schedule_] require
    flushing or closing of the writer after returning before it is safe to modify the
    bigstrings which were directly or indirectly passed to these functions.  The reason is
    that these bigstrings will be read from directly when writing, their contents is not
    copied to internal buffers.

    This is important if users need to send the same large data string to a huge number of
    clients simultaneously (e.g. on a cluster), because these functions then avoid
    needlessly exhausting memory by sharing the data. *)

(** [schedule_bigstring t bstr] schedules a write of bigstring [bstr].
    It is not safe to change the bigstring until the writer has been
    successfully flushed or closed after this operation. *)
val schedule_bigstring : t -> ?pos:int -> ?len:int -> Bigstring.t -> unit

(** [schedule_iovec t iovec] schedules a write of I/O-vector [iovec].  It is not safe to
    change the bigstrings underlying the I/O-vector until the writer has been successfully
    flushed or closed after this operation. *)
val schedule_iovec : t -> Bigstring.t Unix.IOVec.t -> unit

(** [schedule_iovecs t iovecs] like {!schedule_iovec}, but takes a whole queue [iovecs] of
    I/O-vectors as argument.  The queue is guaranteed to be empty when this function
    returns and can be modified.  It is not safe to change the bigstrings underlying the
    I/O-vectors until the writer has been successfully flushed or closed after this
    operation. *)
val schedule_iovecs : t -> Bigstring.t Unix.IOVec.t Queue.t -> unit

(** [flushed t] returns a deferred that will become determined when all prior writes
    complete (i.e. the write() system call returns).  If a prior write fails, then the
    deferred will never become determined. *)
val flushed      : t ->   unit Deferred.t
val flushed_time : t -> Time.t Deferred.t

val fsync : t -> unit Deferred.t
val fdatasync : (t -> unit Deferred.t) Or_error.t

(** [send t s] writes a string to the channel that can be read back
    using Reader.recv *)
val send : t -> string -> unit

(** [monitor t] returns the writer's monitor. *)
val monitor : t -> Monitor.t

(** [close ?force_close t] waits for the writer to be flushed, and then calls [Unix.close]
    on the underlying file descriptor.  [force_close] causes the [Unix.close] to happen
    even if the flush hangs.  By default [force_close] is [Deferred.never ()] for files
    and [after (sec 5)] for other types of file descriptors (e.g. sockets).  If the close
    is forced, data in the writer's buffer may not be written to the file descriptor.  You
    can check this by calling [bytes_to_write] after [close] finishes.

    [close] will raise an exception if the [Unix.close] on the underlying file descriptor
    fails.

    It is required to call [close] on a writer in order to close the underlying file
    descriptor.  Not doing so will cause a file descriptor leak.  It also will cause a
    space leak, because until the writer is closed, it is held on to in order to flush the
    writer on shutdown.

    It is an error to call other writer operations after [close] has been called, except
    that subsequent calls to [close] will return the same deferred. *)
val close : ?force_close:unit Deferred.t -> t -> unit Deferred.t

val close_finished : t -> unit Deferred.t
val close_was_started : t -> bool

(* [is_open t] is [not (close_was_called t)] *)
val is_open : t -> bool

(** [bytes_to_write t] returns how many bytes have been requested to write but have not
    yet been written. *)
val bytes_to_write : t -> int

(** [bytes_written t] returns how many bytes have been written. *)
val bytes_written : t -> Int63.t

(** [save ?temp_prefix ?perm ?fsync file ~contents] atomically replaces contents of [file]
    with [contents].  The intermediate temporary file will be prefixed by [temp_prefix]
    (prefix may refer to a different directory!) if given, and suffixed by a unique random
    sequence of six characters.  It may need to be removed in case of a crash so it may be
    prudent to choose a [temp_prefix] that can be easily found by cleanup tools.

    If [fsync] is [true], the temporary file will be flushed to disk before it takes the
    place of the target file, thus guaranteeing that the target file will always be in a
    sound state, even after a machine crash.  Since synchronization is extremely slow,
    this is not the default.  Think carefully about the event of machine crashes and
    whether you may need this option!

    We intend for [save] to preserve the behavior of the [open] system call, so if [file]
    does not exist, we will apply the umask to [perm].  If it does exist, [perm] will
    default to the file's current permissions rather than 0o666.

    @param temp_prefix default = no prefix
    @param perm default = [0o666]
    @param fsync default = [false]
*)
val save :
  ?temp_prefix:string
  -> ?perm:Unix.file_perm
  -> ?fsync:bool
  -> string
  -> contents:string
  -> unit Deferred.t

val save_sexp :
  ?temp_prefix:string
  -> ?perm:Unix.file_perm
  -> ?fsync:bool
  -> ?hum:bool
  -> string
  -> Sexp.t
  -> unit Deferred.t

(** [pipe t] returns the writing end of a pipe attached to [t] that pushes back when [t]
    cannot keep up with the data being pushed in.  Closing the pipe will close [t]. *)
val pipe : t -> string Pipe.Writer.t

(** [attach_pipe t pipe_r f] repeatedly pulls values from [pipe_r], and feeds them to [f],
    which should in turn write them to [t].  It provides pushback to [pipe_r] by not
    reading when [t] cannot keep up with the data being pushed in.  Unless [close_on_eof]
    is set to false, [t] will be closed when [pipe_r] reaches its Eof. *)
val attach_pipe :
  ?close_on_eof:bool
  -> t
  -> 'a Pipe.Reader.t
  -> ('a -> unit)
  -> unit
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