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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 [character], length [unsigned int]) pair is sent before the actual data. For example:

o
1234
<data: 1234 bytes>

that is 1234 bytes sent on channel 'o', with the 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.

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)

server

client

notes

connected, waiting for capabilities

o: 24
capabilities: runcommand

server is waiting for a command

runcommand\n
7
summary

client talks to server on stdin

starts running command

o: 27
parent: 14571:17c0cb1045e5

o: 3
tip

o: 1
\n

o: 53
paper, coal: display diffstat on the changeset page\n

o: 16
branch: default\n

o: 16
commit: (clean)\n

o: 18
update: (current)\n

r: 4
0

server finished running command, writes ret on the 'r' channel to the client

closes server stdin

client disconnects

server exits

client waits for server to exit

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

(starting after client read capabilities)

server

client

notes

server is waiting for a command

runcommand\n
8
import\0
-

client talks to server on stdin

starts running command

o: 26
applying patch from stdin\n

l: 4096

server tells client to send it a line

21
# HG changeset patch\n

client responds with <length><line>

l: 4096

server processes line, asks for another one

...this goes on until the client has no more input

l: 4096

0

it responds with length=0

r: 4
0

server finished running command, writes ret on the 'r' channel to the client

closes server stdin

client disconnects

server exits

client waits for server to exit


CategoryDeveloper

CommandServer (last edited 2022-12-23 22:42:51 by gavenkoa)