Christopher M. Punches
17acad2b02
A module is built against the system-installed libdpm-core.so and is responsible for being correct against it. Where it needs to act on the version it is running under, dpm_core_version() reports that and the module decides for itself. dpm_module_core_min() is removed. It was a declaration handed to the library to enforce on the module's behalf, and enforcement of that kind belongs nowhere in a library that routes and hosts. The contract is now three reserved symbols and load validation is two steps: the reserved symbols resolve, and the version and description probes return well-formed values. compare_versions had no remaining caller and is removed; parse_version stays for the well-formedness probe. The core_too_new fixture went with the handshake it existed to exercise.
118 lines
6.5 KiB
Markdown
118 lines
6.5 KiB
Markdown
# Developing DPM Modules
|
|
|
|
A DPM module is one shared object in the module directory. libdpm-core.so loads it, validates it completely, and dispatches commands to it on behalf of whatever asked — the `dpm` binary, a build system, or another module. This document covers writing, building, testing, and installing a module.
|
|
|
|
## The module contract
|
|
|
|
A module includes `<dpm/core.h>`, links `-ldpm-core`, and exports the following symbols as extern "C". All returned strings must be non-NULL, static or module-owned, and valid for the lifetime of the loaded module; callers never free them.
|
|
|
|
**`int dpm_module_execute(dpm_ctx* ctx, const char* command, int argc, char** argv)`**
|
|
The command entry point, and the module's entire functional surface. `ctx` is the host context that dispatched the call; the module reaches every service (`dpm_log`, `dpm_config_get`, `dpm_module_path`, ...) through it, and reaches peer modules through it as well. `command` is the subcommand name, equal to argv[0] when argc > 0. NULL or empty `command` must behave as the module's help command. Returns 0 on success, nonzero on failure. It must be callable immediately after load with no other setup.
|
|
|
|
**`const char* dpm_module_version(void)`**
|
|
The module's own version as an X.Y.Z string. libdpm-core.so reports this value to consumers, and each consumer decides for itself whether the version suits it.
|
|
|
|
**`const char* dpm_module_description(void)`**
|
|
A one-line human-readable description, shown in module listings.
|
|
|
|
The `dpm_ctx` type and the service declarations all come from the installed public header:
|
|
|
|
```
|
|
#include <dpm/core.h>
|
|
```
|
|
|
|
## You determine your own compatibility with the library
|
|
|
|
Your module is built against the system-installed `libdpm-core.so` and is responsible for being correct against it. Where you need to act on what you are running under, `dpm_core_version()` reports the running version and you decide what to do:
|
|
|
|
```
|
|
const char* running = dpm_core_version();
|
|
```
|
|
|
|
Check it, proceed or fail on your own terms, and report through `dpm_log` and your return code.
|
|
|
|
## Your interface is your command vocabulary
|
|
|
|
A module publishes no headers, no struct layouts, and no symbols to anything that calls it. Everything it offers is reached through `dpm_module_execute`, addressed by command string, with arguments passed as an argument vector and a status returned as an int.
|
|
|
|
That is what a caller compiles against: a module name and a command name, both strings. Document your commands, their arguments, and their return codes — that documentation is your interface, and it is the only thing a consumer can depend on.
|
|
|
|
**Symbol naming**: every functional export is prefixed with the module's name (mymodule_\*). The dpm_ prefix is reserved for the contract symbols and for libdpm-core.so.
|
|
|
|
## Calling another module
|
|
|
|
A module reaches a peer by performing the same two steps its own caller performed — ask libdpm-core.so for the module by name, then ask libdpm-core.so to invoke it:
|
|
|
|
```
|
|
int dpm_module_execute(dpm_ctx* ctx, const char* command, int argc, char** argv)
|
|
{
|
|
dpm_module* peer = dpm_require(ctx, "othermodule");
|
|
if (!peer) {
|
|
dpm_log(ctx, DPM_LOG_ERROR, dpm_last_error(ctx));
|
|
return 1;
|
|
}
|
|
|
|
return dpm_execute(ctx, peer, "somecommand", argc, argv);
|
|
}
|
|
```
|
|
|
|
The `ctx` is the one handed to your entry point. Nothing else is needed to reach the library.
|
|
|
|
**Never link, include, or hardcode anything belonging to a peer.** No peer headers, no shared types, no peer symbols. Modules are loaded with `RTLD_LOCAL`, so a peer's symbols are not reachable from your module even if you tried — libdpm-core.so is the only path, and the only knowledge you hold about a peer is its name and the commands it documents.
|
|
|
|
A module that depends on a peer is the party that judges the peer's version. Require it, read its reported version with `dpm_module_info_of`, and decide whether it is suitable for the commands you intend to issue. libdpm-core.so reports; it does not rule.
|
|
|
|
## Validation at load
|
|
|
|
libdpm-core.so is the sole authority on module validity, and validation is all-or-nothing. Before a module is offered to anyone, it verifies that every reserved contract symbol resolves and that the version and description probes return well-formed values. A module failing either step is refused with an itemized reason, visible in the load-failure output. A module that loads is fully valid — consumers never defend against partial states.
|
|
|
|
## Building
|
|
|
|
A module repository builds with CMake:
|
|
|
|
```
|
|
cmake_minimum_required(VERSION 3.22)
|
|
project(mymodule)
|
|
|
|
set(CMAKE_CXX_STANDARD 20)
|
|
set(CMAKE_CXX_STANDARD_REQUIRED ON)
|
|
|
|
add_library(mymodule MODULE mymodule.cpp)
|
|
|
|
set_target_properties(mymodule PROPERTIES
|
|
PREFIX ""
|
|
SUFFIX ".so"
|
|
)
|
|
|
|
target_link_libraries(mymodule PRIVATE dpm-core)
|
|
|
|
install(TARGETS mymodule LIBRARY DESTINATION lib/dpm/modules)
|
|
```
|
|
|
|
```
|
|
cmake -B <build-dir>
|
|
cmake --build <build-dir>
|
|
```
|
|
|
|
libdpm-core.so is the only DPM link dependency a module ever has. Dependencies on other modules are runtime concerns, resolved by name through require and dispatch — a peer module is never linked, never included, and never needs to be present to build or to test.
|
|
|
|
## Running and testing locally
|
|
|
|
Load the freshly built module through a locally run `dpm` binary without installing anything:
|
|
|
|
```
|
|
dpm --module-path <build-dir> mymodule <command>
|
|
```
|
|
|
|
libdpm-core.so runs the full validation sequence on every load, so a contract mistake surfaces here, immediately and itemized, rather than after installation. The `--config-dir` flag points the module's configuration namespace at local files during development, and `--root` directs package operations at a scratch tree.
|
|
|
|
Where your module calls a peer, put a **stub module** in the fixture module path: a small .so exporting the three reserved symbols and answering the commands your module issues. libdpm-core.so validates and dispatches to it exactly as it would the real peer. Because a peer is addressed only by name and command string, the stub is a complete substitute — there is nothing else about the real peer your module could have depended on.
|
|
|
|
## Installing
|
|
|
|
Modules install to `lib/dpm/modules` under the install prefix (`/usr/lib/dpm/modules` on a distribution install). libdpm-core.so discovers the module on its next scan; no registration step exists beyond the file being present and valid.
|
|
|
|
## A working example
|
|
|
|
The info module bundled with the `dpm` binary and libdpm-core.so, at `src/bundled-modules/info/`, tests and demonstrates full DPM system functionality, and in doing so shows the contract, command routing, and this build structure in working form.
|
