[proposal] wheel_build_tag hook for unique wheel file names

docs/proposals/index.rst

@@ -7,3 +7,4 @@ Fromager Enhancement Proposals
    new-patcher-config
    new-resolver-config
    release-cooldown
+   wheel-build-tag-hook

docs/proposals/wheel-build-tag-hook.md

@@ -0,0 +1,221 @@
+# Unique wheel file names with a `wheel_build_tag` hook
+
+- Author: Christian Heimes
+- Created: 2026-04-16
+- Status: Open
+- GitHub issue: [#1059](https://github.com/python-wheel-build/fromager/issues/1059)
+
+## What
+
+This enhancement proposes a new stevedore hook point, `wheel_build_tag`, in
+the existing `fromager.hooks` namespace. The hook lets downstream plugin
+packages inject custom suffixes into the
+[wheel build tag](https://packaging.python.org/en/latest/specifications/binary-distribution-format/),
+producing unique wheel file names that encode platform, accelerator stack,
+and dependency ABI information.
+
+## Why
+
+Fromager-built wheels are platform-specific and may depend on:
+
+- **OS / distribution version**, e.g. Fedora 43, RHEL 9.6, RHEL 10.1
+- **AI accelerator stack**, e.g. CUDA 13.1 vs CUDA 12.9, ROCm 7.1
+- **Torch ABI**, which is unstable across versions; a wheel compiled for
+  Torch 2.10.0 may have a different ABI than one compiled for Torch 2.11.0
+
+Currently, wheel filenames carry none of this information. A rebuild for a
+new Torch version does not produce a new filename, making it difficult to:
+
+1. Invalidate caches when the underlying platform or dependency stack changes.
+2. Distinguish wheels built for different accelerator stacks or OS versions.
+3. Replace outdated wheels with correctly-targeted rebuilds.
+4. Share wheels between indexes. Downstream maintains a separate index for
+   each accelerator version, but only a couple of dozen packages out of over
+   1,200 are CUDA/ROCm/Torch-specific. Wheels like `pillow` or `fromager`
+   are identical across accelerator indexes and could be shared if their
+   filenames clearly indicate they have no accelerator dependency. Sharing is
+   out of scope for this proposal but is a possibility for future
+   improvements in downstream.
+
+## Goals
+
+- introduce a `wheel_build_tag` hook point in the `fromager.hooks` stevedore
+  namespace
+- allow hooks to contribute ordered suffix segments to the wheel build tag
+- produce unique, deterministic wheel file names that reflect the build
+  environment
+
+## Non-goals
+
+- Filtering or selecting wheels by build tag at install time. `pip install`
+  and `uv pip install` only use the build tag for sorting, not for filtering.
+- Sharing wheels across indexes. While unique file names enable this in
+  principle, the mechanics of cross-index sharing are out of scope.
+- Accessing wheel content, the build environment, or ELF dependency info from
+  within the hook. The hook must work identically whether a wheel is freshly
+  built or retrieved from cache.
+- Validation of annotations such as "depends on libtorch". A validation
+  system for `build_wheel` may be added in the future.
+- Per-package plugin hooks (`overrides.find_and_invoke()`). A per-package
+  override can be added later when the need arises.
+
+## How
+
+### Wheel spec background
+
+The wheel filename format is:
+
+```
+{distribution}-{version}(-{build tag})?-{python tag}-{abi tag}-{platform tag}.whl
+```
+
+The build tag is optional, must start with a digit, and must not contain `-`.
+It is parsed as `tuple[int, str]`. Fromager already fills the `int` part from
+variant + package changelog and sets the `str` suffix to `""`. The build tag
+is also stored in the `{name}-{version}.dist-info/WHEEL` metadata file and
+shown in `pip list` output.
+
+This proposal extends that suffix with hook-provided segments (e.g.
+`_el9.6_rocm7.1_torch2.10.0`).
+
+### Hook signature
+
+```python
+def build_tag_hook(
+    *,
+    ctx: context.WorkContext,
+    req: Requirement,
+    version: Version,
+    wheel_tags: frozenset[Tag],
+) -> typing.Sequence[tuple[int, str]]:
+    ...
+```
+
+Each registered hook returns `typing.Sequence[tuple[int, str]]`, a sequence
+of (sort-order, suffix) pairs. This allows a single hook to contribute
+multiple suffix segments (e.g. both an OS tag and an accelerator tag). The
+runner collects all pairs from all hooks, sorts them in ascending order, and
+returns the suffix parts as a sequence. Neither `sort-order` nor `suffix`
+have to be unique. The first appearance of a `suffix` is used. Suffixes are
+are joined with `_` and appended to the existing build tag.
+
+`suffix` must only contain alphanumeric ASCII characters or dot
+(`[a-zA-Z0-9.]`). When a hook returns any other character or raises an
+exception, the build fails.
+
+Build tag hooks are registered for `fromager.hooks` entry point and name
+`wheel_build_tag`.
+
+### Example hook
+
+```python
+def example_hook(
+    *,
+    ctx: context.WorkContext,
+    req: Requirement,
+    version: Version,
+    wheel_tags: frozenset[Tag],
+) -> typing.Sequence[tuple[int, str]]:
+    result: list[tuple[int, str]] = []
+    platlib = any(tag.platform != "any" for tag in wheel_tags)
+    if platlib:
+        # fc43, el9.6, ...
+        result.append((1, get_distro_tag()))
+    pbi = ctx.package_build_info(req)
+
+    # example how to use anntoations and ctx.variant for custom flags
+    if pbi.annotations.get("example.accelerator-specific") == "true":
+        # cpu, cuda13.0, ...
+        if ctx.variant.startswith("cpu"):
+            result.append((2, "cpu"))
+        elif ctx.variant.startswith("cuda"):
+            cv = Version(os.environ["CUDA_VERSION"])
+            result.append((2, f"cuda{cv.major}.{cv.minor}"))
+        else:
+            raise NotImplementedError(ctx.variant)
+    return result
+```
+
+```python
+@functools.cache
+def get_distro_tag() -> str:
+    info = platform.freedesktop_os_release()
+    ids = [info["ID"]]  # always defined
+    if "ID_LIKE" in info:  # ids in precedence order
+        ids.extend(info["ID_LIKE"].split())
+    version_id = info.get("VERSION_ID", "")
+    for ident in ids:
+        if ident == "rhel":  # RHEL and CentOS
+            return f"el{version_id}"
+        elif ident == "fedora":
+            return f"fc{version_id}"
+    # other distros
+    return f"{ids[0]}{version_id}".replace("_", "").replace("-", "")
+```
+
+Registration:
+
+```toml
+[project.entry-points."fromager.hooks"]
+wheel_build_tag = "mypackage.hooks:example_hook"
+```
+
+### What hooks can access
+
+- **`ctx` + `req`**: for `ctx.variant` and package configuration like annotations
+- **`wheel_tags`**: detect whether a wheel is purelib or platlib
+  (platform/arch-specific).
+
+Hooks can also use other information like **`os.environ`** or the
+**`platform`** stdlib module. They should only use information that
+is immutable and identifies the build context, e.g.
+**`platform.freedesktop_os_release()`** to read distribution name and
+version from `/etc/os-release`.
+
+### What hooks cannot access
+
+The hook **does not** have access to wheel content, the `build_env`, or
+ELF dependency info. While this information exists during the build, it is
+not available when wheels are retrieved from cache servers or local cache. The
+hook must work identically in both paths.
+
+### Examples
+
+#### RHEL 9.6, ROCm 7.1, Torch 2.10.0
+
+| Wheel | Build tag |
+| -- | -- |
+| `flash_attn-2.8.3-8_el9.6_rocm7.1_torch2.10.0-cp312-cp312-linux_x86_64.whl` | `8_el9.6_rocm7.1_torch2.10.0` |
+| `torch-2.10.0-7_el9.6_rocm7.1-cp312-cp312-linux_x86_64.whl` | `7_el9.6_rocm7.1` |
+| `pillow-12.2.0-2_el9.6-cp312-cp312-linux_x86_64.whl` | `2_el9.6` |
+| `fromager-0.79.0-2-py3-none-any.whl` | `2` (pure-python, no suffix) |
+
+#### Fedora 43, CUDA 13.0, Torch 2.9.1
+
+| Wheel | Build tag |
+| -- | -- |
+| `flash_attn-2.8.3-8_fc43_cuda13.0_torch2.9.1-cp312-cp312-linux_x86_64.whl` | `8_fc43_cuda13.0_torch2.9.1` |
+| `torch-2.9.1-8_fc43_cuda13.0-cp312-cp312-linux_x86_64.whl` | `8_fc43_cuda13.0` |
+| `pillow-12.2.0-2_fc43-cp312-cp312-linux_x86_64.whl` | `2_fc43` |
+| `fromager-0.79.0-2-py3-none-any.whl` | `2` (pure-python, no suffix) |
+
+Note how pure-python wheels (`py3-none-any`) receive no suffix, while
+platlib wheels get progressively more specific tags based on their actual
+dependencies.
+
+## Limitations
+
+The current wheel standard does not support multiple accelerator variants in
+a single package index. A single index cannot contain both CUDA and ROCm
+builds of the same package because `pip install` and `uv pip install` only
+use the build tag for sorting, not for filtering. An index with both CUDA
+and ROCm wheels would result in the installer picking whichever has the
+highest build tag, not the correct accelerator.
+
+The upcoming [Wheel.Next](https://wheelnext.dev/) initiative and
+[PEP 817](https://peps.python.org/pep-0817/) /
+[PEP 825](https://peps.python.org/pep-0825/) aim to address this by
+introducing wheel variants, which will enable CUDA and ROCm wheels to
+coexist in the same index. Logic for Torch and CUDA/ROCm dependency
+selection in this proposal may be reused by wheel variants when the new
+standard becomes available.

docs/spelling_wordlist.txt

@@ -32,6 +32,7 @@ iteratively
 json
 lexicographically
 libcurl
+libtorch
 linter
 localhost
 matcher
@@ -57,6 +58,7 @@ setuptools
 statelessly
 stderr
 stdin
+stdlib
 stdout
 subcommands
 subdirectory