wptserve Pipes

Pipes are designed to allow simple manipulation of the way that static files are sent without requiring any custom code. They are also useful for cross-origin tests because they can be used to activate a substitution mechanism which can fill in details of ports and server names in the setup on which the tests are being run.


Pipes are functions that may be used when serving files to alter parts of the response. These are invoked by adding a pipe= query parameter taking a | separated list of pipe functions and parameters. The pipe functions are applied to the response from left to right. For example:

GET /sample.txt?pipe=slice(1,200)|status(404).

This would serve bytes 1 to 199, inclusive, of foo.txt with the HTTP status code 404.

Note: If you write directly to the response socket using ResponseWriter, or when using the asis handler, only the trickle pipe will affect the response.

There are several built-in pipe functions, and it is possible to add more using the @pipe decorator on a function, if required.

Note: Because of the way pipes compose, using some pipe functions prevents the content-length of the response from being known in advance. In these cases the server will close the connection to indicate the end of the response, preventing the use of HTTP 1.1 keepalive.

Built-In Pipes


Used to substitute variables from the server environment, or from the request into the response. A typical use case is for testing cross-domain since the exact domain name and ports of the servers are generally unknown.

Substitutions are marked in a file using a block delimited by {{ and }}. Inside the block the following variables are available:

  • {{host}} - The host name of the server excluding any subdomain part.

  • {{domains[]}} - The domain name of a particular subdomain e.g. {{domains[www]}} for the www subdomain.

  • {{hosts[][]}} - The domain name of a particular subdomain for a particular host. The first key may be empty (designating the “default” host) or the value alt; i.e., {{hosts[alt][]}} (designating the alternate host).

  • {{ports[][]}} - The port number of servers, by protocol e.g. {{ports[http][0]}} for the first (and, depending on setup, possibly only) http server

  • {{headers[]}} The HTTP headers in the request e.g. {{headers[X-Test]}} for a hypothetical X-Test header.

  • {{header_or_default(header, default)}} The value of an HTTP header, or a default value if it is absent. e.g. {{header_or_default(X-Test, test-header-absent)}}

  • {{GET[]}} The query parameters for the request e.g. {{GET[id]}} for an id parameter sent with the request.

So, for example, to write a JavaScript file called xhr.js that depends on the host name of the server, without hardcoding, one might write:

var server_url = http://{{host}}:{{ports[http][0]}}/path/to/resource;
//Create the actual XHR and so on

The file would then be included as:

<script src="xhr.js?pipe=sub"></script>

This pipe can also be enabled by using a filename *.sub.ext, e.g. the file above could be called xhr.sub.js.


Used to set the HTTP status of the response, for example:



Used to add or replace http headers in the response. Takes two or three arguments; the header name, the header value and whether to append the header rather than replace an existing header (default: False). So, for example, a request for:


causes example.html to be returned with a text/plain content type whereas:


Will cause example.html to be returned with both text/html and text/plain content-type headers.

If the comma (,) or closing parenthesis ()) characters appear in the header value, those characters must be escaped with a backslash (\):


(Note that the programming environment from which the request is issued may require that the backslash character itself be escaped.)


Used to send only part of a response body. Takes the start and, optionally, end bytes as arguments, although either can be null to indicate the start or end of the file, respectively. So for example:


Would result in a response with a body containing 10 bytes of example.txt including byte 10 but excluding byte 20.


Would cause all bytes from byte 10 of example.txt to be sent, but:


Would send the first 20 bytes of example.txt.


Note: Using this function will force a connection close.

Used to send the body of a response in chunks with delays. Takes a single argument that is a microsyntax consisting of colon-separated commands. There are three types of commands:

  • Bare numbers represent a number of bytes to send

  • Numbers prefixed d indicate a delay in seconds

  • Numbers prefixed r must only appear at the end of the command, and indicate that the preceding N items must be repeated until there is no more content to send. The number of items to repeat must be even.

In the absence of a repetition command, the entire remainder of the content is sent at once when the command list is exhausted. So for example:


causes a 1s delay before sending the entirety of example.txt.


causes 100 bytes of example.txt to be sent, followed by a 1s delay, and then the remainder of the file to be sent. On the other hand:


Will cause the file to be sent in 100 byte chunks separated by a 1s delay until the whole content has been sent.