Optimizing NixOS Initrd Generation

# What is this for? NixOS's traditional initrd is generated by listing the paths that should be included in initrd and copying the full runtime closure of those paths into the archive. For most things, like almost any executable, this involves copying the entirety of huge packages like glibc, when only things like the shared library files are needed. To solve this, NixOS does a variety of patchwork to edit the files being copied in so they only refer to small, patched up paths. For instance, executables and their shared library dependencies are copied into an `extraUtils` derivation, and every ELF file is patched to refer to files in that output. The problem with this is that it is often difficult to correctly patch some things. For instance, systemd bakes the path to the `mount` command into the binary, so patchelf is no help. Instead, it's very often easier to simply copy the desired files to their original store locations in initrd and not copy their entire runtime closure. This does mean that it is the burden of the developer to ensure that all necessary dependencies are copied in, as closures won't be consulted. However, it is rare that full closures are actually desirable, so in the traditional initrd, the developer was likely to do manual work on patching the dependencies explicitly anyway. # How it works This program is similar to its inspiration (`find-libs` from the traditional initrd), except that it also handles symlinks and directories according to certain rules. As input, it receives a sequence of pairs of paths. The first path is an object to copy into initrd. The second path (if not empty) is the path to a symlink that should be placed in the initrd, pointing to that object. How that object is copied depends on its type. 1. A regular file is copied directly to the same absolute path in the initrd. - If it is *also* an ELF file, then all of its direct shared library dependencies are also listed as objects to be copied. - If an unwrapped file exists as `.[filename]-wrapped`, then it is

The text discusses the challenges with NixOS's traditional `initrd` generation, which copies entire runtime closures, leading to large `initrd` sizes and complex, often difficult patching of binaries like `systemd`. It proposes an alternative where necessary files are copied directly to their original store locations instead of full closures, simplifying the process but shifting dependency management to the developer. The text then introduces a new program, inspired by `find-libs`, designed to improve this by handling symlinks and directories, copying regular files directly, and automatically including direct shared library dependencies for ELF files.