Module Async_unix.Writer

Overall IO statistics for all writers.

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.

When either stdout or stderr is created, they both are created. Furthermore, if they point to the same inode, then they will be the same writer to Fd.stdout. This can be confusing, because fd (force stderr) will be Fd.stdout, not Fd.stderr. And subsequent modifications of Fd.stderr will have no effect on Writer.stderr.

Unfortunately, the sharing is necessary because Async uses OS threads to do write() syscalls using the writer buffer. When calling a program that redirects stdout and stderr to the same file, as in:

      foo.exe >/tmp/z.file 2>&1

if Writer.stdout and Writer.stderr weren't the same writer, then they could have threads simultaneously writing to the same file, which could easily cause data loss.

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, ECONNRESET, ....

buffer_age_limit specifies how backed up you can get before raising an exception. The default is `Unlimited for files, and 2 minutes for other kinds of file descriptors. You can supply `Unlimited to turn off buffer-age checks.

raise_when_consumer_leaves specifies whether the writer should raise an exception when the consumer receiving bytes from the writer leaves, i.e., in Unix, the write syscall returns EPIPE or ECONNRESET. If not raise_when_consumer_leaves, then the writer will silently drop all writes after the consumer leaves, and the writer will eventually fail with a writer-buffer-older-than error if the application remains open long enough.

line_ending determines how newline and write_line terminate lines by default. If line_ending = Unix then end of line is "\n"; if line_ending = Dos then end of line is "\r\n". Note that line_ending = Dos is not equivalent to opening the file in text mode because any "\n" characters being printed by other means (e.g., write "\n") are still written verbatim (in Unix style).

time_source is useful in tests to trigger buffer_age_limit-related conditions, or simply to have the result of (for example) flushed_time_ns agree with your test's synthetic time. It is also used to schedule the `Periodic syscalls.

set_raise_when_consumer_leaves t bool sets the raise_when_consumer_leaves flag of t, which determies how t responds to a write system call raising EPIPE and ECONNRESET (see create).

set_buffer_age_limit t buffer_age_limit replaces the existing buffer age limit with the new one. This is useful for stdout and stderr, which are lazily created in a context that does not allow applications to specify buffer_age_limit.

consumer_left t returns a deferred that becomes determined when t attempts to write to a pipe that broke because the consumer on the other side left.

open_file file opens file for writing and returns a writer for it. It uses Unix_syscalls.openfile to open the file.

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.

id returns an id for this writer that is unique among all other writers.

fd returns the Fd.t used to create this writer.

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.

write_gen t a writes a to writer t, with length specifying the number of bytes needed and blit_to_bigstring blitting a directly into the t's buffer. If one has a type that has length and blit_to_bigstring functions, like:

module A : sig
  type t
  val length : t -> int
  val blit_to_bigstring : (t, Bigstring.t) Blit.blit
end 

then one can use write_gen to implement a custom analog of Writer.write, like:

module Write_a : sig
  val write : ?pos:int -> ?len:int -> A.t -> Writer.t -> unit
end = struct
  let write ?pos ?len a writer =
    Writer.write_gen
      ~length:A.length
      ~blit_to_bigstring:A.blit_to_bigstring
      ?pos ?len writer a
end 

In some cases it may be difficult to write only part of a value:

module B : sig
  type t
  val length : t -> int
  val blit_to_bigstring : t -> Bigstring.t -> pos:int -> unit
end 

In these cases, use write_gen_whole instead. It never requires writing only part of a value, although it is potentially less space-efficient. It may waste portions of previously-allocated write buffers if they are too small.

module Write_b : sig
  val write : B.t -> Writer.t -> unit
end = struct
  let write b writer =
    Writer.write_gen_whole
      ~length:B.length
      ~blit_to_bigstring:B.blit_to_bigstring
      writer b
end 

Note: write_gen and write_gen_whole give you access to the writer's internal buffer. You should not capture it; doing so might lead to errors of the segfault kind.

write_direct t ~f gives t's internal buffer to f. pos and len define the portion of the buffer that can be filled. f must return a pair (x, written) where written is the number of bytes written to the buffer at pos. write_direct raises if written < 0 || written > len. write_direct returns Some x, or None if the writer is stopped. By using write_direct only, one can ensure that the writer's internal buffer never grows. Look at the write_direct expect tests for an example of how this can be used to construct a write_string like function that never grows the internal buffer.

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.

to_formatter returns 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.

write_char t c writes the character.

newline t writes the end-of-line terminator. line_ending can override t's line_ending.

write_line t s ?line_ending is write t s; newline t ?line_ending.

write_byte t i writes one 8-bit integer (as the single character with that code). The given integer is taken modulo 256.

write_sexp t sexp writes to t the string representation of sexp, possibly followed by a terminating character as per Terminate_with. With ~terminate_with:Newline, the terminating character is a newline. With ~terminate_with:Space_if_needed, if a space is needed to ensure that the sexp reader knows that it has reached the end of the sexp, then the terminating character will be a space; otherwise, no terminating character is added. A terminating space is needed if the string representation doesn't end in ')' or '"'.

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.

Writes out a value using its bin_prot writer. Unlike write_bin_prot, this doesn't prefix the output with the size of the bin_prot blob. size is the expected size. This function will raise if the bin_prot writer writes an amount other than size bytes.

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.

schedule_iobuf_peek is like schedule_bigstring, but for an iobuf. It is not safe to change the iobuf until the writer has been successfully flushed or closed after this operation.

schedule_iobuf_consume is like schedule_iobuf_peek, and additionally advances the iobuf beyond the portion that has been written. Until the result is determined, it is not safe to assume whether the iobuf has been advanced yet or not.

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.

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.

flushed_or_failed_with_result t returns a deferred that will become determined when all prior writes complete (i.e. the write() system call returns), or when any of them fail.

Handling the Error case can be tricky due to the following race: the result gets determined concurrently with the exception propagation through the writer's monitor. The caller needs to make sure that the program behavior does not depend on which signal propagates first.

flushed_or_failed_unit t returns a deferred that will become determined when all prior writes complete, or when any of them fail.

Unlike flushed_or_failed_with_result, its return value gives you no indication of which happened. In the Error case, the result will be determined in parallel with the error propagating to the writer's monitor. The caller should robustly handle either side winning that race.

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.

It is OK to call flushed t after t has been closed.

send writes a string to the writer that can be read back using Reader.recv.

monitor t returns the writer's monitor.

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.

WARNING: force_close will not reliably stop any write that is in progress. If there are any in-flight system calls, it will wait for them to finish, which includes writev, which can legitimately block forever.

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

You must 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 operations on t after close t has been called, except that calls of close subsequent to the original call to close will return the same deferred as the original call.

close_started t becomes determined as soon as close is called.

close_finished t becomes determined after t's underlying file descriptor has been closed, i.e., it is the same as the result of close. close_finished differs from close in that it does not have the side effect of initiating a close.

is_closed t returns true iff close t has been called.

is_open t is not (is_closed t)

with_close t ~f runs f (), and closes t after f finishes or raises.

can_write t returns true if calls to write* functions on t are allowed. If is_open t then can_write t. But one can have is_closed t and can_write t, during the time after close t before closing has finished.

Errors raised within the writer can stop the background job that flushes out the writer's buffers. is_stopped_permanently returns true when the background job has stopped. stopped_permanently becomes determined when the background job has stopped.

In addition to flushing its internal buffer prior to closing, a writer keeps track of producers that are feeding it data, so that when Writer.close is called, it does the following:

1. requests that the writer's producers flush their data to it
2. flushes the writer's internal buffer
3. calls Unix.close on the writer's underlying file descriptor

with_flushed_at_close t ~flushed ~f calls f and adds flushed to the set of producers that should be flushed-at-close, for the duration of f.

bytes_to_write t returns how many bytes have been requested to write but have not yet been written.

bytes_written t returns how many bytes have been written.

bytes_received t returns how many bytes have been received by the writer. As long as the writer is running, bytes_received = bytes_written + bytes_to_write.

with_file_atomic ?temp_file ?perm ?fsync file ~f creates a writer to a temp file, feeds that writer to f, and when the result of f becomes determined, atomically moves (using Unix.rename) the temp file to file. If file currently exists, it will be replaced, even if it is read-only. The temp file will be file (or temp_file if supplied) suffixed by a unique random sequence of six characters. The temp file may need to be removed in case of a crash so it may be prudent to choose a temp file that can be easily found by cleanup tools.

If fsync is true, the temp 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 with_file_atomic to mimic the behavior of the open system call, so if file does not exist, we will apply the current umask to perm (the effective permissions become perm land lnot umask, see man 2 open). However, if file does exist and perm is specified, we do something different from open system call: we override the permission with perm, ignoring the umask. This means that if you create and then immediately overwrite the file with with_file_atomic ~perm, then the umask will be honored the first time and ignored the second time. If perm is not specified, then any existing file permissions are preserved.

If f closes the writer passed to it, with_file_atomic raises and does not create file.

save is a special case of with_file_atomic that atomically writes the given string to the specified file.

save_lines file lines writes all lines in lines to file, with each line followed by a newline.

save_sexp is a special case of with_file_atomic that atomically writes the given sexp to the specified file.

save_sexp t sexp writes sexp to t, followed by a newline. To read a file produced using save_sexp, one would typically use Reader.load_sexp, which deals with the additional whitespace and works nicely with converting the sexp to a value.

save_sexps works similarly to save_sexp, but saves a sequence of sexps instead, separated by newlines. There is a corresponding Reader.load_sexps for reading back in.

save_bin_prot t bin_writer 'a is a special case of with_file_atomic that writes 'a to t using its bin_writer, in the size-prefixed format, like write_bin_prot. To read a file produced using save_bin_prot, one would typically use Reader.load_bin_prot.

transfer' t pipe_r f repeatedly reads 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.

By default, each read from pipe_r reads all the values in pipe_r. One can supply max_num_values_per_read to limit the number of values per read.

The transfer' stops and the result becomes determined when stop becomes determined, when pipe_r reaches its EOF, when t is closed, or when t's consumer leaves. In the latter two cases, transfer' closes pipe_r.

transfer' causes Pipe.flushed on pipe_r's writer to ensure that the bytes have been flushed to t before returning. It also waits on Pipe.upstream_flushed at shutdown.

transfer t pipe_r f is equivalent to:

transfer' t pipe_r (fun q -> Queue.iter q ~f; return ()) 

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 does not close t.

of_pipe info pipe_w returns a writer t such that data written to t will appear on pipe_w. If either t or pipe_w are closed, the other is closed as well.

of_pipe is implemented by attaching t to the write-end of a Unix pipe, and shuttling bytes from the read-end of the Unix pipe to pipe_w.

behave_nicely_in_pipeline ~writers () causes the program to silently exit with status 0 if any of the consumers of writers go away. It also sets the buffer age to unlimited, in case there is a human (e.g., using less) on the other side of the pipeline.

set_synchronous_out_channel t out_channel waits until byte_to_write t = 0, and then mutates t so that all future writes to t synchronously call Out_channel.output* functions to send data to the OS immediately.

set_synchronous_out_channel is used by expect tests to ensure that the interleaving between calls to Core.printf (and similar IO functions) and Async.printf generates output with the same interleaving. set_synchronous_out_channel is idempotent.

using_synchronous_backing_out_channel t = true if writes to t are being done synchronously, e.g., due to set_synchronous_out_channel, set_synchronous_backing_out_channel, use_synchronous_stdout_and_stderr.

clear_synchronous_out_channel t restores t to its normal state, with the background writer asynchronously feeding data to the OS. clear_synchronous_out_channel is idempotent.

use_synchronous_stdout_and_stderr () causes all subsequent writes to stdout and stderr to occur synchronously (after any pending writes have flushed).

This ensures printf-family writes happen immediately, which avoids two common sources of confusion:

• unexpected interleaving of Core.printf and Async.printf calls; and
• Async.printf calls that don't get flushed before an application exits

The disadvantages are:

• this makes writes blocking, which can delay unrelated asynchronous jobs until the consumer stops pushing back; and
• the errors raised by write are different and it won't respect behave_nicely_in_pipeline anymore

Backing_out_channel generalizes Out_channel to a narrow interface that can be used to collect strings, etc.