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Developer/packaging notes for libabseil

This file documents how this build2 package is put together. It is intended for maintainers of the package, not its consumers.

Upstream source and layout

The upstream Abseil source code is tracked as a git submodule in upstream/ (see .gitmodules). The build2 package files (buildfiles, manifests, etc.) are kept separate and the upstream component directories under absl/ are overlaid into the libabseil/absl/ package directory using directory symlinks. For example:

libabseil/absl/base   -> ../../upstream/absl/base
libabseil/absl/strings -> ../../upstream/absl/strings
...

These directory symlinks are also declared in .gitattributes with symlink=dir so the package can be checked out from git directly on Windows.

Patches to upstream source

We patch upstream source code only when there is no other way, following the manual approach of the packaging guide (see "How do I patch upstream source code?"). Next to each patched file foo.cc there is foo.cc.orig, a symlink to the unmodified upstream file, and foo.cc.patch, the difference between the two.

Since the absl/ components are directory symlinks, patching a file requires turning the directories leading to it into real directories that contain symlinks to the remaining upstream entries. The directory symlinks inside such a tree are declared in .gitattributes.

absl/time/internal/cctz/src/zone_info_source.cc

With Clang targeting MSVC, cctz defines zone_info_source_factory with __attribute__((weak)), for which Clang emits a .weak.* helper symbol. The build2-generated DLL .def file exports this symbol and link.exe rejects it with LNK1242. The patch makes Clang use the same /alternatename approach as MSVC.

Upgrading to a new upstream version

  1. Update the upstream/ submodule to the new release tag and update the package version in libabseil/manifest.

  2. Re-create any symlinks for files/directories that upstream added or removed (compare against the upstream component layout).

  3. For each *.patch, make a fresh copy of the new upstream file and re-apply the patch, regenerating it if it no longer applies cleanly:

    $ cp -H foo.cc.orig foo.cc
$ patch <foo.cc.patch
$ diff -u foo.cc.orig foo.cc >foo.cc.patch

    Drop the patch (and restore the directory symlink) once upstream has addressed the issue.

  4. Build and run the tests in the native and MinGW cross configurations before releasing.