libcmt io hook

[mpolitzer]

Mock implementation of "dehash" read operations.

The strategy employed was to delegate the IO to an external executables specified by an environment variable.
The user specified the mappings as a list of pairs: (reason, executable).
The request is written to stdin of the executable and its reply is read back via its stdout.
This frees the user to choose any programming language to handle the request.

As an example, assume:

• application is the C program below, linked against libcmt (mock version).
• echo.lua is the lua script below.

Invoked with: CMT_HOOKS=10:echo.lua ./application (echo.lua can be found on PATH).

application.c

#include<stdio.h>
#include<string.h>
#include "libcmt/rollup.h"

int main()
{
    cmt_rollup_t rollup;
    if (cmt_rollup_init(&rollup))
        return -1;

    cmt_buf_t tx[1] = {cmt_io_get_tx(rollup.io)};
    cmt_buf_t rx[1] = {cmt_io_get_rx(rollup.io)};

    int m = snprintf((char *)tx->begin, cmt_buf_length(tx), "request");
    struct cmt_io_yield req[1] = {{
        .dev    = CMT_IO_DEV,
        .cmd    = CMT_IO_CMD_MANUAL,
        .reason = 10,
        .data   = m,
    }};
    int rc = cmt_io_yield(rollup.io, req);
    printf("%d:%s \"%.*s\"\n", rc, strerror(rc), req->data, (char *)rx->begin);

    return 0;
}

echo.lua

io.stderr:write("got a request for reason: " .. arg[1] .. "\n")
local line = io.read()
io.write(line)

[mpolitzer]

We took a simpler approach to inject predefined binaries.