feat(libgbm): add compat.libgbm 2026.08.29, GBM bound to the ecosystem's Mesa

docs/descriptor-examples.md

@@ -30,7 +30,8 @@ in the [root README](../README.md#reference-examples).
 | Header package filling a gap in the index | [`compat.glx-headers`](../pkgs/c/compat.glx-headers.lua) (libglvnd's `GL/glx.h`, absent from the Khronos registry and required by SDL's X11 backend) |
 | C++ application framework compat (dependencies reuse packages already in the index) | [`compat.eui-neo`](../pkgs/e/compat.eui-neo.lua) (upstream's `3rd/` ships 8 vendored dependencies; none of them is compiled here — all are redirected to the same-version `compat.*` packages in this index) |
 | Mutually exclusive backends (one of several inside one package) | [`compat.eui-neo`](../pkgs/e/compat.eui-neo.lua): `vulkan` / `sdl2` each **replace** the default OpenGL / GLFW, and the default backend is expressed by *naming no feature at all* — there is no `opengl`/`glfw` feature. A `default` feature cannot express exclusivity: its own `defines`/`sources`/`deps` have no effect whatsoever, while its `implies` always applies and cannot be overridden by a named feature (which is, conversely, exactly the solution for the "always-on interface define" row below). The workable answer is to read the `-DMCPP_FEATURE_<NAME>` mcpp passes anyway and decide up front in a force-included header. Note also that `cflags` only reaches C TUs — C++ needs `cxxflags`, so a backend define written only into `cflags` never reaches any `.cpp` |
-| Host runtime adaptation (drivers are not vendored) | [`compat.glx-runtime`](../pkgs/c/compat.glx-runtime.lua) · [`compat.vulkan-runtime`](../pkgs/c/compat.vulkan-runtime.lua) (mcpp binaries run against a bundled glibc, so a bare-soname `dlopen` never reaches the host drivers; a symlink farm plus `runtime.library_dirs` bridges that. Note the farm holds only versioned sonames — `library_dirs` also joins the link line) |
+| Host runtime adaptation (drivers are not vendored) | [`compat.glx-runtime`](../pkgs/c/compat.glx-runtime.lua) · [`compat.vulkan-runtime`](../pkgs/c/compat.vulkan-runtime.lua) (mcpp binaries run against a bundled glibc, so a bare-soname `dlopen` never reaches the host drivers; a symlink farm plus `runtime.library_dirs` bridges that. The farm holds only versioned sonames, so nothing there can shadow an index package. Note that `runtime.library_dirs` renders as `-Wl,-rpath` and **not** as `-L` — the `-L` key is `runtime.link_library_dirs`, which these two do not need because nothing links against their farms; see the row below for one that does) |
+| Ecosystem-stack binding (zero host) | [`compat.libgbm`](../pkgs/c/compat.libgbm.lua) (Mesa's GBM — buffer allocation out of a DRM device. The row above reaches the HOST; this one reaches the ECOSYSTEM and nothing else, and the distinction is the whole design. **Why not a source build**: libgbm is a build target *inside* Mesa, not a project — `src/gbm/meson.build` is `link_with: [libloader]`, and `libloader` wants `idep_mesautil`, the whole of Mesa's internal util library (~120 TUs plus Python-generated tables) for exactly one function, `loader_open_driver_lib`. Building it would make this index re-import libdrm + expat + xcb + a Mesa-util carve-out to duplicate what `xim:mesa` has already resolved hermetically. Contrast `compat.vulkan`, which *does* build the Khronos loader from source — Khronos releases that as a standalone project, Mesa releases no such thing for GBM. **Zero host, with no escape hatch**: unlike its two neighbours it has no `/usr/lib*` path and no `MCPP_HOST_*` override, because host libgbm is a leak the ecosystem already closed — `xim:nvidia-gl-host-link` names it directly ("the table … was missing libm, libdrm, **libgbm**, libgcc_s … all of which were therefore coming from the HOST, silently, which is the leak this package exists to close"). NVIDIA's own GBM backend, if ever needed, belongs in that host-link layer rather than here. The measured surface is 1 ecosystem package (`xim:mesa`, not `xim:graphics`'s twenty-two), zero index deps, and zero transitive burden — libgbm.so.1's own RUNPATH resolves entirely inside `xim-x-*`. **What it deliberately does NOT do**: set the backend search path. libgbm is a loader — `gbm_create_device()` dlopens `<path>/<driver>_gbm.so`, and Mesa's compiled-in `/usr/lib/gbm` is right on a distro and wrong the moment the payload is relocated. The mechanism to fix that is Mesa's own (`GBM_BACKENDS_PATH`) and the job belongs to the ENVIRONMENT, which is where every other relocated stack puts it — Valve's pressure-vessel answers the identical breakage with `GBM_BACKENDS_PATH=/run/host/usr/lib64/gbm` (steam-runtime#797), Nix and Conda set it at activation. In this ecosystem `xim:mesa` now declares it through the graphics discovery layer (openxlings/xim-pkgindex#713), so this package sets nothing, generates no TU and ships no header of its own. It briefly did carry a constructor that set the variable itself; that was a workaround for the missing declaration, and deleting it took the descriptor from 598 lines to 303. **Two directory keys, not one**: `library_dirs` renders as `-Wl,-rpath` and `link_library_dirs` as `-L`, so a package that is *linked against* (unlike glx-runtime/vulkan-runtime, whose farms are only dlopen'd) needs both — with `library_dirs` alone the farm is complete, the rpath correct, and the build still dies at `ld: cannot find -lgbm`. It ships **two** test binaries: `stock_usage.cpp` includes stock `<gbm.h>` and nothing else, which is the minimal consumer and the tripwire on the two things outside this repo the package now depends on — xim-pkgindex's DISCOVERY row and mcpp's subos-env injection · [`compat.libdrm`](../pkgs/c/compat.libdrm.lua) (the layer under GBM — `drmModeAddFB2`/`drmModeSetCrtc` turn an allocated buffer into a scanout. Passes the separable-unit test that libgbm fails — libdrm is an independent freedesktop project and Conan carries a real recipe — but is still a binding for the second reason: `xim:libdrm` exists, Mesa's own payload has DT_NEEDED on it, and two `libdrm.so.2` in one process means two DRM handle tables. **Two include roots**, which is the thing that bites: the public headers sit at the include root and the uapi headers they include sit under `libdrm/`, and `xf86drm.h` line 40 is a bare `#include <drm.h>` — expose one root and nothing compiles at all) · [`compat.egl`](../pkgs/c/compat.egl.lua) (what makes libgbm useful for RENDERING rather than only allocation: `eglGetPlatformDisplay(EGL_PLATFORM_GBM_KHR, gbm_device, NULL)`. Provider is libglvnd, not Mesa — EGL is a spec and the thing you link is a vendor-neutral dispatch library that must be the only one in the process. Ships **only** `EGL/` out of a payload that also carries `GL/`, `GLES2/`, `KHR/`: a third provider of `GL/` would make compat.glx-headers' documented two-provider race a three-way one, and `KHR/` comes from the index's existing `compat.khrplatform` instead — load-bearing, since `eglplatform.h` opens with `#include <KHR/khrplatform.h>`. X11 is deliberately NOT a dependency: that include is `USE_X11`-gated, and forcing Xorg on headless GBM users would be exactly wrong) · [`compat.wayland`](../pkgs/c/compat.wayland.lua) (client, server, cursor and EGL shim all harvested, but only `-lwayland-client` on `ldflags`. A dependency's ldflags reach the consumer's link line with no way to opt out, so forcing the server library on every client would be unfixable downstream; a compositor author adds `-lwayland-server` themselves and it resolves out of the farm. The test member does exactly that, so the documented escape hatch has a regression guarding it. `wayland-scanner` and the protocol XML are NOT here — that is a code generator plus a data package, the `compat.protobuf` `protoc` shape, and a separate package))
 | Always-on interface define | `CURL_STATICLIB` in [`compat.curl`](../pkgs/c/compat.curl.lua): `cflags` is always on but package-private, while a feature's `defines` reaches consumers yet has to be named — `default = { implies = … }` applies unconditionally and happens to give both |
 | Multiple majors in one package (shape switches with the version) | [`compat.catch2`](../pkgs/c/compat.catch2.lua) (3.x compiles `src/catch2/` into a static library; 2.x goes header-only through `single_include/`) |
 | External build system (`install()` builds from source) | [`compat.openblas`](../pkgs/c/compat.openblas.lua) (Make) · [`compat.openssl`](../pkgs/c/compat.openssl.lua) (Perl Configure + Make, static libssl/libcrypto) |

docs/package-types.md

@@ -19,6 +19,7 @@ combined as needed.
 | **F. Shared-library compat** | has to be the **only** copy of that `.so` in the process (third parties `dlopen` it) | the X11 family such as `pkgs/c/compat.x11.lua`, and `compat.vulkan.lua` (linux) | `targets = { kind = "shared", soname = … }` |
 | **G. Host runtime adaptation** | things that cannot be vendored, such as drivers — only a symlink farm plus metadata | `pkgs/c/compat.glx-runtime.lua`, `compat.vulkan-runtime.lua` | `runtime.library_dirs` / `capabilities` |
 | **H. Host tool provider** | the upstream tarball also holds a **code generator** consumers run at build time | `pkgs/c/compat.protobuf.lua` (`protoc`) | a `targets` entry with `kind = "bin"` + `main`, plus `required_features` |
+| **I. Ecosystem-stack binding** | the library is an internal build target of a project the **ecosystem already owns**, so vendoring it would fork that project | `pkgs/c/compat.libgbm.lua` (Mesa's GBM, via `xim:mesa`) | `xpm.<plat>.deps.runtime = { "xim:<pkg>" }` + a farm from `system.subos_sysrootdir()`, with `runtime.library_dirs` **and** `link_library_dirs` |
 
 For the complete sample index, see [Descriptor examples by shape](descriptor-examples.md).
 
@@ -193,10 +194,23 @@ every ICD manifest and yet cannot open a single driver.
 
 Two details that keep biting:
 
-- **Put only versioned sonames in the farm** (`lib*.so.*`). `runtime.library_dirs` also joins the **link line**, so a
-  bare `libxcb.so` shadows this repository's own `compat.xcb` and the link fails with
-  `undefined reference to XauDisposeAuth` (mcpp#304). Versioned names are invisible to the linker and are exactly what
-  `dlopen` asks for.
+- **Put only versioned sonames in the farm** (`lib*.so.*`). A bare `libxcb.so` shadowed this repository's own
+  `compat.xcb` and the link failed with `undefined reference to XauDisposeAuth` (mcpp#304). Versioned names are
+  invisible to the linker and are exactly what `dlopen` asks for, so this remains the rule.
+- **Know which directory key produces which flag** — they are not interchangeable, and the split is what makes the
+  point above version-dependent. Read off the emitted `build.ninja` on mcpp 2026.8.27.2:
+
+  | key | renders as |
+  |---|---|
+  | `runtime.library_dirs` | `-Wl,-rpath` |
+  | `runtime.link_library_dirs` | `-L` |
+  | `runtime.transitive_needed_dirs` | `-Wl,-rpath-link` |
+
+  So on this pin `library_dirs` alone does **not** put the farm on the link line (the separate `-L` key arrived in
+  2026.8.10.3; mcpp#304 predates it). Shape G packages want exactly that — nothing links against their farms, they
+  exist so a bare-soname `dlopen` resolves at run time. A package that *is* linked against needs
+  `link_library_dirs` too, or the build dies at `ld: cannot find -l<name>` with a farm that is perfectly correct;
+  see shape I.
 - **The closure has to be complete.** A farm holding `libxcb.so.1` but not the `libXau.so.6` it depends on shadows the
   host copy that would otherwise have resolved, and the executable simply fails to start.
 

docs/zh/descriptor-examples.md

@@ -28,7 +28,8 @@
 | 补索引空缺的头文件包 | [`compat.glx-headers`](../../pkgs/c/compat.glx-headers.lua)(libglvnd 的 `GL/glx.h`,Khronos registry 不含,SDL 的 X11 后端必需) |
 | C++ 应用框架 compat(依赖复用索引内既有包) | [`compat.eui-neo`](../../pkgs/e/compat.eui-neo.lua)(上游 `3rd/` 自带 8 个 vendored 依赖,此处一个不编,全部改指索引内同版本 `compat.*`) |
 | 互斥后端(同包多后端二选一) | [`compat.eui-neo`](../../pkgs/e/compat.eui-neo.lua):`vulkan` / `sdl2` 各自**替换**默认的 OpenGL / GLFW,默认后端由"不点名任何 feature"表达,并不存在 `opengl`/`glfw` feature。`default` feature 表达不了互斥 —— 它自带的 `defines`/`sources`/`deps` 完全不生效,而 `implies` 又恒生效、无法被点名的 feature 覆盖(后者反而正好是本表『恒开的 interface define』一行的解法)。可行解是读 mcpp 本就会传的 `-DMCPP_FEATURE_<NAME>`,在强制包含头里做前置判定。另注意 `cflags` 只作用于 C TU,C++ 需 `cxxflags` —— 只写进 `cflags` 的后端 define 到不了任何 `.cpp` |
-| 宿主运行时适配(不 vendor 驱动) | [`compat.glx-runtime`](../../pkgs/c/compat.glx-runtime.lua) · [`compat.vulkan-runtime`](../../pkgs/c/compat.vulkan-runtime.lua)(mcpp 产物跑在自带 glibc 下,裸 soname 的 `dlopen` 够不到宿主驱动;用符号链接农场 + `runtime.library_dirs` 打通。注意 farm 只放带版本号的 soname —— `library_dirs` 同时进链接行) |
+| 宿主运行时适配(不 vendor 驱动) | [`compat.glx-runtime`](../../pkgs/c/compat.glx-runtime.lua) · [`compat.vulkan-runtime`](../../pkgs/c/compat.vulkan-runtime.lua)(mcpp 产物跑在自带 glibc 下,裸 soname 的 `dlopen` 够不到宿主驱动;用符号链接农场 + `runtime.library_dirs` 打通。farm 只放带版本号的 soname,so 里面的东西遮蔽不了索引包。注意 `runtime.library_dirs` 渲染成 `-Wl,-rpath` 而**不是** `-L` —— `-L` 那个键叫 `runtime.link_library_dirs`;这两个包不需要它,因为没人链接它们的 farm,需要的例子见下一行) |
+| 生态栈绑定(零 host) | [`compat.libgbm`](../../pkgs/c/compat.libgbm.lua)(Mesa 的 GBM —— 从 DRM 设备分配 buffer。上一行伸手够的是 **host**,这一行够的是**生态**、且仅止于此,这个区别就是整个设计。**为什么不源码构建**:libgbm 是 Mesa **内部**的一个 build target,不是一个项目 —— `src/gbm/meson.build` 写着 `link_with: [libloader]`,而 `libloader` 又要 `idep_mesautil`(Mesa 整个内部 util 库,~120 个 TU 加 Python 生成的表),只为了其中一个函数 `loader_open_driver_lib`。真去建,就得让本索引把 libdrm + expat + xcb + 一块 Mesa-util 重新进口一遍,去复刻 `xim:mesa` 已经 hermetic 解析好的东西。对照 `compat.vulkan` —— 它**确实**从源码建 Khronos loader,因为 Khronos 把 loader 作为独立项目发布,而 Mesa 对 GBM 没有对应物。**零 host,且不留后门**:与相邻两个包不同,它没有 `/usr/lib*` 路径、也没有 `MCPP_HOST_*` 覆盖项 —— 因为 host libgbm 是生态**已经堵上**的泄漏,`xim:nvidia-gl-host-link` 里直接点了名("the table … was missing libm, libdrm, **libgbm**, libgcc_s … all of which were therefore coming from the HOST, silently, which is the leak this package exists to close")。NVIDIA 自家的 GBM 后端若将来需要,归那个 host-link 层管,不归这里。实测依赖面:生态 1 个包(`xim:mesa`,不是 `xim:graphics` 的 22 个)、索引依赖 0、传递负担 0 —— libgbm.so.1 自己的 RUNPATH 全落在 `xim-x-*` 内。**它刻意不做的事**:设置后端搜索路径。libgbm 是个 loader —— `gbm_create_device()` 会 dlopen `<path>/<driver>_gbm.so`,而 Mesa 编译进去的 `/usr/lib/gbm` 在发行版上是对的、payload 一被重定位就是错的。修它的机制是 **Mesa 自己的**(`GBM_BACKENDS_PATH`),而这件事归**环境**管 —— 所有被重定位的栈都是这么做的:Valve 的 pressure-vessel 用 `GBM_BACKENDS_PATH=/run/host/usr/lib64/gbm` 解决同一个 bug(steam-runtime#797),Nix/Conda 在环境激活时设。在本生态里,`xim:mesa` 现在通过 graphics discovery 层声明它(openxlings/xim-pkgindex#713),所以这个包**什么都不设**、不生成 TU、也不带自己的头文件。它一度自带一个 constructor 去设这个变量,那是声明缺失时的权宜之计;删掉它让描述符从 598 行降到 303 行。**两个目录键都要**:`library_dirs` 渲染成 `-Wl,-rpath`、`link_library_dirs` 渲染成 `-L`,所以一个**会被链接**的包(不像 glx-runtime/vulkan-runtime 的农场只被 dlopen)两个都得写 —— 只写 `library_dirs` 的话,农场完整、rpath 正确,构建照样死在 `ld: cannot find -lgbm`。它带**两个**测试二进制:`stock_usage.cpp` 只 include 原样的 `<gbm.h>`,是最小消费者,也是本仓之外那两件依赖的绊线 —— xim-pkgindex 的 DISCOVERY 行,以及 mcpp 的 subos-env 注入 · [`compat.libdrm`](../../pkgs/c/compat.libdrm.lua)(GBM 下面那一层 —— `drmModeAddFB2`/`drmModeSetCrtc` 把分配出来的 buffer 变成能扫描输出的画面。它**通过**了 libgbm 没通过的「可独立分发」判据(libdrm 是独立的 freedesktop 项目,Conan 也有真配方),但仍然做成绑定,理由是第二条:`xim:libdrm` 已存在、Mesa 自己的 payload 对它有 DT_NEEDED,一个进程里两份 `libdrm.so.2` 等于两张 DRM 句柄表。**两个 include 根**是真正会咬人的地方:公开头在 include 根、它们 include 的 uapi 头在 `libdrm/` 子目录,而 `xf86drm.h` 第 40 行就是裸的 `#include <drm.h>` —— 只暴露一个根的话什么都编不过) · [`compat.egl`](../../pkgs/c/compat.egl.lua)(让 libgbm 从「只能分配」变成「能渲染」的那一环:`eglGetPlatformDisplay(EGL_PLATFORM_GBM_KHR, gbm_device, NULL)`。提供方是 libglvnd 而不是 Mesa —— EGL 是规范,真正链接的是厂商中立的 dispatch 库,而它必须是进程里唯一的一份。payload 里还有 `GL/`、`GLES2/`、`KHR/`,但本包**只**暴露 `EGL/`:再出一个 `GL/` 提供方会把 compat.glx-headers 记录过的「两个提供方」竞争变成三方;`KHR/` 则交给索引已有的 `compat.khrplatform` —— 这条依赖是必需的,因为 `eglplatform.h` 开头就 `#include <KHR/khrplatform.h>`。X11 **刻意不**作为依赖:那个 include 被 `USE_X11` 门控,而给无显示的 headless GBM 用户强塞 Xorg 恰恰是错的) · [`compat.wayland`](../../pkgs/c/compat.wayland.lua)(client/server/cursor/egl 四个库全部收进 farm,但 `ldflags` 只放 `-lwayland-client`。依赖的 ldflags 会进消费者的链接行且无法退出,所以给每个客户端强塞 server 库是下游修不掉的;compositor 作者自己加 `-lwayland-server`,从 farm 里解析得到。测试成员就是这么写的,于是这条被文档化的退路有回归守着。`wayland-scanner` 与协议 XML **不在这里** —— 那是代码生成器加数据包,属于 `compat.protobuf` 的 `protoc` 形态,应当是另一个包))
 | 恒开的 interface define | [`compat.curl`](../../pkgs/c/compat.curl.lua) 的 `CURL_STATICLIB`:`cflags` 恒开但包私有,feature `defines` 可达消费端但需点名 —— `default = { implies = … }` 无条件生效,恰好两者兼得 |
 | 单包多 major(形态随版本切换) | [`compat.catch2`](../../pkgs/c/compat.catch2.lua)(3.x 编 `src/catch2/` 出静态库;2.x 走 `single_include/` header-only) |
 | 外部构建系统(`install()` 从源码构建) | [`compat.openblas`](../../pkgs/c/compat.openblas.lua)(Make) · [`compat.openssl`](../../pkgs/c/compat.openssl.lua)(Perl Configure + Make,静态 libssl/libcrypto) |