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Name

ngx.pipe - spawn and communicate with OS processes via stdin/stdout/stderr in a non-blocking fashion.

Table of Contents

Status

This Lua module is currently considered experimental.

Synopsis

location = /t {
    content_by_lua_block {
        local ngx_pipe = require "ngx.pipe"
        local select = select

        local function count_char(...)
            local proc = ngx_pipe.spawn({'wc', '-c'})
            local n = select('#', ...)
            for i = 1, n do
                local arg = select(i, ...)
                local bytes, err = proc:write(arg)
                if not bytes then
                    ngx.say(err)
                    return
                end
            end

            local ok, err = proc:shutdown('stdin')
            if not ok then
                ngx.say(err)
                return
            end

            local data, err = proc:stdout_read_line()
            if not data then
                ngx.say(err)
                return
            end

            ngx.say(data)
        end

        count_char(("1234"):rep(2048))
    }
}

This example counts characters (bytes) directly fed by OpenResty to the UNIX command wc.

You could not do this with either io.popen or os.execute because wc will not output the result until its stdin is closed.

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Description

This module does not support non-POSIX operating systems like Windows yet.

If you are not using the Nginx core shipped with OpenResty, you will need to apply the socket_cloexec patch to the standard Nginx core.

Under the hood, this module uses fork and execvp with the user-specified command, and communicate with such spawned processes via the POSIX pipe API, which contributes to the name of this module.

A signal handler for SIGCHLD is registered so that we can receive a notification once the spawned processes exited.

We combine the above implementation with Nginx's event mechanism and OpenResty's Lua coroutine scheduler, in order to ensure communication with the spawned processes is non-blocking.

The communication APIs do not work in phases which do not support yielding, such as init_worker_by_lua* or log_by_lua*, because there is no way to yield the current light thread to avoid blocking the OS thread when communicating with processes in those phases.

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Methods

spawn

syntax: proc, err = pipe_module.spawn(args, opts?)

context: all phases except init_by_lua*

Creates and returns a new sub-process instance we can communicate with later.

For example:

local ngx_pipe = require "ngx.pipe"
local proc, err = ngx_pipe.spawn({"sh", "-c", "sleep 0.1 && exit 2"})
if not proc then
    ngx.say(err)
    return
end

In case of failure, this function returns nil and a string describing the error.

The sub-process will be killed via SIGKILL if it is still alive when the instance is collected by the garbage collector.

Note that args should either be a single level array-like Lua table with string values, or just a single string.

Some more examples:

local proc, err = ngx_pipe.spawn({"ls", "-l"})

local proc, err = ngx_pipe.spawn({"perl", "-e", "print 'hello, wolrd'"})

If args is specified as a string, it will be executed by the operating system shell, just like os.execute. The above example could thus be rewritten as:

local ngx_pipe = require "ngx.pipe"
local proc, err = ngx_pipe.spawn("sleep 0.1 && exit 2")
if not proc then
    ngx.say(err)
    return
end

In the shell mode, you should be very careful about shell injection attacks when interpolating variables into command string, especially variables from untrusted sources. Please make sure that you escape those variables while assembling the command string. For this reason, it is highly recommended to use the multi-arguments form (args as a table) to specify each command-line argument explicitly.

Since by default, Nginx does not pass along the PATH system environment variable, you will need to configure the env PATH directive if you wish for it to be respected during the searching of sub-processes:

env PATH;
...
content_by_lua_block {
    local ngx_pipe = require "ngx.pipe"

    local proc = ngx_pipe.spawn({'ls'})
}

The optional table argument opts can be used to control the behavior of spawned processes. For instance:

local opts = {merge_stderr = true, buffer_size = 256}
local proc, err = ngx_pipe.spawn({"sh", "-c", ">&2 echo data"}, opts)
if not proc then
    ngx.say(err)
    return
end

The following options are supported:

  • merge_stderr: when set to true, the output to stderr will be redirected to stdout in the spawned process. This is similar to doing >&1 in a shell.
  • buffer_size: specifies the buffer size used by reading operations, in bytes. The default buffer size is 4096.

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set_timeouts

syntax: proc:set_timeouts(write_timeout?, stdout_read_timeout?, stderr_read_timeout?, wait_timeout?)

Respectively sets: the write timeout threshold, stdout read timeout threshold, stderr read timeout threshold, and wait timeout threshold. All timeouts are in milliseconds.

The default threshold for each timeout is 10 seconds.

If a specified timeout argument is nil, the corresponding timeout threshold will not be changed. For example:

local proc, err = ngx_pipe.spawn({"sleep", "10s"})

-- only change the wait_timeout to 0.1 second.
proc:set_timeouts(nil, nil, nil, 100)

-- only change the send_timeout to 0.1 second.
proc:set_timeouts(100)

If a specified timeout argument is 0, the corresponding operation will never time out.

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wait

syntax: ok, reason, status = proc:wait()

context: phases that support yielding

Waits until the current sub-process exits.

It is possible to control how long to wait via set_timeouts. The default timeout is 10 seconds.

If process exited with status code zero, the ok return value will be true.

If process exited abnormally, the ok return value will be false.

The second return value, reason, will be a string. Its values may be:

  • exit: the process exited by calling exit(3), _exit(2), or by returning from main(). In this case, status will be the exit code.
  • signal: the process was terminated by a signal. In this case, status will be the signal number.

Note that only one light thread can wait on a process at a time. If another light thread tries to wait on a process, the return values will be nil and the error string "pipe busy waiting".

If a thread tries to wait an exited process, the return values will be nil and the error string "exited".

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pid

syntax: pid = proc:pid()

Returns the pid number of the sub-process.

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kill

syntax: ok, err = proc:kill(signum)

Sends a signal to the sub-process.

Note that the signum argument should be signal's numerical value. If the specified signum is not a number, an error will be thrown.

You should use lua-resty-signal's signum() function to convert signal names to signal numbers in order to ensure portability of your application.

In case of success, this method returns true. Otherwise, it returns nil and a string describing the error.

Killing an exited sub-process will return nil and the error string "exited".

Sending an invalid signal to the process will return nil and the error string "invalid signal".

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shutdown

syntax: ok, err = proc:shutdown(direction)

Closes the specified direction of the current sub-process.

The direction argument should be one of these three values: stdin, stdout and stderr.

In case of success, this method returns true. Otherwise, it returns nil and a string describing the error.

If the merge_stderr option is specified in spawn, closing the stderr direction will return nil and the error string "merged to stdout".

Shutting down a direction when a light thread is waiting on it (such as during reading or writing) will return nil and the error string "pipe busy writing" (for stdin) or "pipe busy reading" (for the others).

Shutting down directions of an exited process will return nil and the error string "closed".

It is fine to shut down the same direction of the same stream multiple times; no side effects are to be expected.

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write

syntax: nbytes, err = proc:write(data)

context: phases that support yielding

Writes data to the current sub-process's stdin stream.

The data argument can be a string or a single level array-like Lua table with string values.

This method is a synchronous and non-blocking operation that will not return until all the data has been flushed to the sub-process's stdin buffer, or an error occurs.

In case of success, it returns the total number of bytes that have been sent. Otherwise, it returns nil and a string describing the error.

The timeout threshold of this write operation can be controlled by the set_timeouts method. The default timeout threshold is 10 seconds.

When a timeout occurs, the data may be partially written into the sub-process's stdin buffer and read by the sub-process.

Only one light thread is allowed to write to the sub-process at a time. If another light thread tries to write to it, this method will return nil and the error string "pipe busy writing".

Writing to an exited sub-process will return nil and the error string "closed".

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stderr_read_all

syntax: data, err, partial = proc:stderr_read_all()

context: phases that support yielding

Reads all data from the current sub-process's stderr stream until it is closed.

This method is a synchronous and non-blocking operation, just like the write method.

The timeout threshold of this reading operation can be controlled by set_timeouts. The default timeout is 10 seconds.

In case of success, it returns the data received. Otherwise, it returns three values: nil, a string describing the error, and, optionally, the partial data received so far.

When merge_stderr is specified in spawn, calling stderr_read_all will return nil and the error string "merged to stdout".

Only one light thread is allowed to read from a sub-process's stderr or stdout stream at a time. If another thread tries to read from the same stream, this method will return nil and the error string "pipe busy reading".

Streams for stdout and stderr are separated, so at most two light threads may be reading from a sub-process at a time (one for each stream).

The same way, a light thread may read from a stream while another light thread is writing to the sub-process stdin stream.

Reading from an exited process's stream will return nil and the error string "closed".

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stdout_read_all

syntax: data, err, partial = proc:stdout_read_all()

context: phases that support yielding

Similar to the stderr_read_all method, but reading from the stdout stream of the sub-process.

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stderr_read_line

syntax: data, err, partial = proc:stderr_read_line()

context: phases that support yielding

Reads from stderr like stderr_read_all, but only reads a single line of data.

When merge_stderr is specified in spawn, calling stderr_read_line will return nil plus the error string "merged to stdout".

When the data stream is truncated without a new-line character, it returns 3 values: nil, the error string "closed", and the partial data received so far.

The line should be terminated by a Line Feed (LF) character (ASCII 10), optionally preceded by a Carriage Return (CR) character (ASCII 13). The CR and LF characters are not included in the returned line data.

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stdout_read_line

syntax: data, err, partial = proc:stdout_read_line()

context: phases that support yielding

Similar to stderr_read_line, but reading from the stdout stream of the sub-process.

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stderr_read_bytes

syntax: data, err, partial = proc:stderr_read_bytes(len)

context: phases that support yielding

Reads from stderr like stderr_read_all, but only reads the specified number of bytes.

If merge_stderr is specified in spawn, calling stderr_read_bytes will return nil plus the error string "merged to stdout".

If the data stream is truncated (fewer bytes of data available than requested), this method returns 3 values: nil, the error string "closed", and the partial data string received so far.

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stdout_read_bytes

syntax: data, err, partial = proc:stdout_read_bytes(len)

context: phases that support yielding

Similar to stderr_read_bytes, but reading from the stdout stream of the sub-process.

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stderr_read_any

syntax: data, err = proc:stderr_read_any(max)

context: phases that support yielding

Reads from stderr like stderr_read_all, but returns immediately when any amount of data is received.

At most max bytes are received.

If merge_stderr is specified in spawn, calling stderr_read_any will return nil plus the error string "merged to stdout".

If the received data is more than max bytes, this method will return with exactly max bytes of data. The remaining data in the underlying receive buffer can be fetched with a subsequent reading operation.

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stdout_read_any

syntax: data, err = proc:stdout_read_any(max)

context: phases that support yielding

Similar to stderr_read_any, but reading from the stdout stream of the sub-process.

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Community

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English Mailing List

The openresty-en mailing list is for English speakers.

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Chinese Mailing List

The openresty mailing list is for Chinese speakers.

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Bugs and Patches

Please report bugs or submit patches by

  1. creating a ticket on the GitHub Issue Tracker,
  2. or posting to the OpenResty community.

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Copyright and License

This module is licensed under the BSD license.

Copyright (C) 2018, by OpenResty Inc.

All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

  • Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.

  • Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

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See Also

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