Decrease Nix closure size

[neongreen]

Currently our closure takes about 700 MB:

$ storepath=$(nix-build)
$ nix-store --export $(nix-store -qR $storepath) > out-fencer
$ ls -lah out-fencer
-rw-r--r--  1 yom  staff   708M Sep 27 17:22 out-fencer

The biggest offenders are:

740 MB  └── nix
740 MB      └── store
393 MB          ├─⊕ i2nf6hslid0ak8fzgpj2m082sprihi92-clang-7.1.0
133 MB          ├─⊕ hfy6ibq1hwayv24yrh98f3avc47nmr00-llvm-7.1.0
 63 MB          ├─⊕ 0cifmxw0r6lijng796a3z3nwq67ma5b3-llvm-7.1.0-lib
 27 MB          ├─⊕ 5qk8vydayrvkq313s1srxhrnblhqvahk-grpc-1.2.0-e2cfe9d
 27 MB          ├─⊕ q959m66x0cfryybzapv7c4v0ki2jfr1a-clang-7.1.0-lib
 18 MB          ├─⊕ bhywddq18j79xr274n45byvqjb8fs52j-Libsystem-osx-10.12.6
 11 MB          ├─⊕ i7hz49am5vla34lmpkw5aqkjgd2b98hb-binutils-2.31.1
...

This primarily affects our Docker image size.

[neongreen]

Oh, this was on macOS. The situation on Linux is slightly better:

310 MB  └── nix
310 MB      └── store
132 MB          └─⊕ wr5dlcw1asw8dwgm4bwjmga8f52m5lfx-gcc-7.4.0
126 MB          ├─⊕ awmkcm3rk4d74lgm828phvslgb7drcij-grpc-1.2.0-e2cfe9d
 28 MB          ├─⊕ iykxb0bmfjmi7s53kfg6pjbfpd8jmza6-glibc-2.27
5.7 MB          ├─⊕ pjx3f50x0qziyivs7rbg5s12p99nn2np-gcc-7.4.0-lib
...

Still, it's unclear why gcc becomes a part of the closure.

[mdimjasevic]

As a side note, it seems that something affects the choice of a C compiler. On your macOS LLVM was chosen, while on a Linux-based system it was GCC. Probably there is a constraint along the lines depends-on: GCC | LLVM.

[neongreen]

Well, I didn't do a --pure build. In fact, I'm not even sure I can get a --pure build on macOS.

[neongreen]

Might be because stripping is disabled for grpc.

$ nix why-depends \
  /nix/store/xnndslwbim4zs06y1dw8dkqi9z1clak2-fencer-1.0.0 \
  /nix/store/i2nf6hslid0ak8fzgpj2m082sprihi92-clang-7.1.0

/nix/store/xnndslwbim4zs06y1dw8dkqi9z1clak2-fencer-1.0.0
╚═══bin/fencer: ….p.................../nix/store/pgisjgza96csd22mpxymc6qnw1bx8vjx-grpc-1
    => /nix/store/pgisjgza96csd22mpxymc6qnw1bx8vjx-grpc-1.2.0-e2cfe9d
    ╚═══lib/libgrpc++.a: …...................../nix/store/qvagarl5ghs07v39zhszm5x9rn08k2
        => /nix/store/qvagarl5ghs07v39zhszm5x9rn08k231-clang-wrapper-7.1.0
        ╚═══bin/clang: …k disable=SC2193.[[ "/nix/store/i2nf6hslid0ak8fzgpj2m082sprihi92
            => /nix/store/i2nf6hslid0ak8fzgpj2m082sprihi92-clang-7.1.0

[neongreen]

No, it's not disabled. And I tried strip-ping libgrpc++.a manually and didn't get anywhere.

[neongreen]

Asked at https://discourse.nixos.org/t/how-to-prevent-a-librarys-closure-from-including-the-compiler/4481.