Use `--disk_cache` to share the bazel cache between multiple derivations

bazel-caching.nix

# This file uses the `--disk_cache` argument to share bazel caches
# between separate nix builds. Proof-of-work
# The second build succeds (after analyze phase) with:
# `INFO: 3 processes: 3 remote cache hit.`
with import <nixpkgs> {};

let
  bazel-example = { name, buildPhase, caches }: stdenv.mkDerivation {
    inherit name;

    buildInputs = [ unzip ];
    # we use the cpp hello examples in the bazel source distribution
    src = bazel.src;
    sourceRoot = ".";

    phases = [ "unpackPhase" "buildPhase" ];

    buildPhase = ''
      export HOME=$(mktemp -d)

      # out is used as storage for the cache
      mkdir $out

      # link all inputs of caches into a combined, writable cache in $out
      ${lib.concatMapStringsSep "\n"
         (c: "ln -s -t $out ${c}/*") caches}
      echo "NEW cache contents in $out"
      ls $out

      ${buildPhase}
    '';

  };

  lib-cache = { name, caches }: bazel-example {
    name = "hello-lib-cache";
    inherit caches;

    buildPhase = ''
      ${bazel}/bin/bazel build \
        --disk_cache=$out //examples/cpp:hello-lib
    '';
  };

  # first time around, there is no cache and lib gets built
  first-build = lib-cache {
    name = "hello-lib-1";
    caches = [];
  };

  # second time around, everything is a cache hit
  # (gives 3 remote cache hits)
  second-build = lib-cache {
    name = "hello-lib-2";
    caches = [ first-build ];
  };

  # since hello uses hello-lib, we can use the cache for those targets
  # (gives 1 remote cache hit)
  hello-build = bazel-example {
    name = "hello-world";
    caches = [second-build];

    buildPhase = ''
      ${bazel}/bin/bazel build \
        --disk_cache=$out //examples/cpp:hello-world
    '';
  };


in hello-build

output

> nix-build default.nix 
these derivations will be built:
  /nix/store/94rprhg2pnk2wv4nqh1jf0scvd3y2nib-hello-lib-cache.drv
  /nix/store/987jwyv7k10dcgx802c9ik5dfn8x5z5c-hello-lib-cache.drv
  /nix/store/k3mwc1b5mslhrw05zcwv5mzwnhx9b52d-hello-world.drv
building '/nix/store/94rprhg2pnk2wv4nqh1jf0scvd3y2nib-hello-lib-cache.drv'...
unpacking sources
unpacking source archive /nix/store/62081xrk8ipgcj9a0xm8dvcpk657sjfg-bazel-0.15.1-dist.zip
source root is .
setting SOURCE_DATE_EPOCH to timestamp 315532800 of file ./tools/zip/BUILD.tools
building
NEW cache contents in /nix/store/17sm3q14wi3x14z98f0q833bhw6hmrak-hello-lib-cache
Extracting Bazel installation...
Starting local Bazel server and connecting to it...
.................
Loading:
Loading: 0 packages loaded
Analyzing: target //examples/cpp:hello-lib (1 packages loaded)
Analyzing: target //examples/cpp:hello-lib (2 packages loaded)
Analyzing: target //examples/cpp:hello-lib (4 packages loaded)
INFO: Analysed target //examples/cpp:hello-lib (7 packages loaded).
INFO: Found 1 target...
[0 / 6] [-----] Writing file examples/cpp/libhello-lib.so-2.params
Target //examples/cpp:hello-lib up-to-date:
  bazel-bin/examples/cpp/libhello-lib.a
  bazel-bin/examples/cpp/libhello-lib.so
INFO: Elapsed time: 5.601s, Critical Path: 0.56s
INFO: 3 processes: 3 processwrapper-sandbox.
INFO: Build completed successfully, 6 total actions
INFO: Build completed successfully, 6 total actions
building '/nix/store/987jwyv7k10dcgx802c9ik5dfn8x5z5c-hello-lib-cache.drv'...
unpacking sources
unpacking source archive /nix/store/62081xrk8ipgcj9a0xm8dvcpk657sjfg-bazel-0.15.1-dist.zip
source root is .
setting SOURCE_DATE_EPOCH to timestamp 315532800 of file ./tools/zip/BUILD.tools
building
NEW cache contents in /nix/store/r5bng0l0i1wm9ds1kaf5vfjp43vw1sjy-hello-lib-cache
11491c374ddaf658b0c20d9e621d3f104ad8a52a69df9d5f977baef3d684d1a8
5b2377ad14d73890b72ce6b4156f5759c2b317b36afdcc16958c5618eaa15f88
75e50b4429d950c101093e7507ee87ceacce15eaa17f8808746c7698883685cf
8e411653e08fdbb03ca45a0f4f2ad72bf0f1cdf7b1b6e08bc393cb7a7f0fb2eb
8fbafb4819ed2c5dc3a5be3029d2b1ff0bd82737de8dc4c1cfc45196ad3d80bf
a92b16a8210086d028a5b8ce1d57b8b3d328854447e55d9833714173a90e7817
c768625f80761de32e48fcb60bb54ddadabdeb33fadb562d2eacc62384303eac
Extracting Bazel installation...
Starting local Bazel server and connecting to it...
................
Loading:
Loading: 0 packages loaded
Analyzing: target //examples/cpp:hello-lib (1 packages loaded)
Analyzing: target //examples/cpp:hello-lib (2 packages loaded)
Analyzing: target //examples/cpp:hello-lib (3 packages loaded)
INFO: Analysed target //examples/cpp:hello-lib (7 packages loaded).
INFO: Found 1 target...
[0 / 5] [-----] BazelWorkspaceStatusAction stable-status.txt
Target //examples/cpp:hello-lib up-to-date:
  bazel-bin/examples/cpp/libhello-lib.a
  bazel-bin/examples/cpp/libhello-lib.so
INFO: Elapsed time: 5.321s, Critical Path: 0.11s
INFO: 3 processes: 3 remote cache hit.
INFO: Build completed successfully, 6 total actions
INFO: Build completed successfully, 6 total actions
building '/nix/store/k3mwc1b5mslhrw05zcwv5mzwnhx9b52d-hello-world.drv'...
unpacking sources
unpacking source archive /nix/store/62081xrk8ipgcj9a0xm8dvcpk657sjfg-bazel-0.15.1-dist.zip
source root is .
setting SOURCE_DATE_EPOCH to timestamp 315532800 of file ./tools/zip/BUILD.tools
building
NEW cache contents in /nix/store/96rj8rzirw7w85zvmhxk0m2pa40y2agy-hello-world
11491c374ddaf658b0c20d9e621d3f104ad8a52a69df9d5f977baef3d684d1a8
5b2377ad14d73890b72ce6b4156f5759c2b317b36afdcc16958c5618eaa15f88
75e50b4429d950c101093e7507ee87ceacce15eaa17f8808746c7698883685cf
8e411653e08fdbb03ca45a0f4f2ad72bf0f1cdf7b1b6e08bc393cb7a7f0fb2eb
8fbafb4819ed2c5dc3a5be3029d2b1ff0bd82737de8dc4c1cfc45196ad3d80bf
a92b16a8210086d028a5b8ce1d57b8b3d328854447e55d9833714173a90e7817
c768625f80761de32e48fcb60bb54ddadabdeb33fadb562d2eacc62384303eac
Extracting Bazel installation...
Starting local Bazel server and connecting to it...
................
Loading:
Loading: 0 packages loaded
Analyzing: target //examples/cpp:hello-world (1 packages loaded)
Analyzing: target //examples/cpp:hello-world (2 packages loaded)
Analyzing: target //examples/cpp:hello-world (3 packages loaded)
INFO: Analysed target //examples/cpp:hello-world (7 packages loaded).
INFO: Found 1 target...
[0 / 6] [-----] Creating source manifest for //examples/cpp:hello-world
Target //examples/cpp:hello-world up-to-date:
  bazel-bin/examples/cpp/hello-world
INFO: Elapsed time: 5.670s, Critical Path: 0.50s
INFO: 3 processes: 1 remote cache hit, 2 processwrapper-sandbox.
INFO: Build completed successfully, 7 total actions
INFO: Build completed successfully, 7 total actions
/nix/store/96rj8rzirw7w85zvmhxk0m2pa40y2agy-hello-world