Command Server

A server that allows communication with Mercurial's API over a pipe.

1. Protocol

All communication with the server is done on stdin/stdout. The byte order used by the server is big-endian.

Data sent from the server is channel based, meaning a (channel, length) pair is sent before the actual data. The channel is a single character, while the length is an unsigned int. In the examples below, the length field is in plain text.

o
1234
<data: 1234 bytes>

that is 1234 bytes sent on channel 'o', with 1234 bytes of data following.

When starting the server, it will send a new-line separated list of capabilities (on the 'o' channel), in this format:

capabilities: capability1\n
capability2\n
...

At the most basic level, the server will support the 'runcommand' capability.

Following that, a list of required and optional channels is sent (also on the 'o' channel):

channels: il oedr

More on channels below.

1.1. Encoding

Strings are encoded in the local encoding. At the moment the encoding cannot be changed after server startup. To set it at startup, use HGENCODING. To query the servers encoding, see the 'getencoding' command.

1.2. Channels

There are currently 5 channels:

• 'o'utput channel: most of the communication happens on this channel. When running commands, output Mercurial writes to stdout is written to this channel.
• 'e'rror channel: when running commands, this correlates to stderr.
• 'i'nput channel: the length field here tells the client how many bytes to send.
• 'l'ine based input channel: the client should send a single line of input (trimmed to length). This channel is used when Mercurial interacts with the user or when iterating over stdin.

Input should be sent on stdin in the following format:

length
data

length = 0 sent by the client is interpreted as EOF by the server. The server will not ask for more than 4kb per request as to not fill up the pipe.

• 'r'esult channel: the server uses this channel to tell the client that a command finished by writing its return value (command specific).
• 'd'ebug channel: used when the server is started with logging to '-'.

1.3. Commands

The server is running on an endless loop (until stdin is closed) waiting for commands. A command request looks like this:

commandname\n
<command specific request>

If an unknown request is sent to the server it is ignored, without sending back any kind of response to the client. Clients are expected to check what commands are supported by the server by consulting the capabilities.

1.3.1. runcommand

Run the command specified by a list of \0-terminated strings. An unsigned int indicating the length of the arguments should be sent before the list. Example:

runcommand\n
8
log\0
-l\0
5

Which corresponds to running 'hg log -l 5'.

The server responds with input/output generated by Mercurial on the matching channels. When the command returns, the server writes the return code (signed integer) of the command to the 'r'esult channel.

1.3.2. getencoding

Returns the servers encoding on the result channel.

client:

getencoding\n

server responds with:

r
5
ascii

1.4. Examples

1.4.1. runcommand

Complete example of a client running 'hg summary', right after starting the server:

(text in the server column is <channel>: <length>, where length is really 4 byte unsigned ints, not plain text like below)

And another one with activity on the input channels too by running 'import -':

(starting after client read capabilities/channels)

CategoryDeveloper