docs: howto: add auto_push_to_cachix

docs/howto/auto_push_to_cachix/cachix-package.nix

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+#MC # cachixPackage
+#MC
+#MC This file try to implement a package wrapper, which will automatically push `pkg` to cachix upon building.
+#MC However, this method seems not work, due to limited resources in nix-build environment.
+#MC For more details, see [here](./index.md).
+{ cachix
+, stdenv
+, writeShellScript
+}:
+
+{ pkg
+, sha256
+, cachix_dhall
+, cachix_name
+, name ? "cachixed"
+}:
+
+builtins.derivation {
+  inherit name;
+  system = builtins.currentSystem;
+  builder = writeShellScript "cachix-package-builder" ''
+    source ${stdenv}/setup
+    echo ${pkg} > $out
+    if [[ -f "${cachix_dhall}" ]]; then
+      ${cachix}/bin/cachix -c ${cachix_dhall} push ${cachix_name} ${pkg}
+      result=$?
+      echo result $result
+      exit $result
+    fi
+  '';
+
+  outputHashMode = "flat";
+  outputHashAlgo = "sha256";
+  outputHash = sha256;
+}

docs/howto/auto_push_to_cachix/index.md

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+# Auto Push Packages to Cachix
+
+**Objective**:
+Automatically push specific built packages to cachix during `home-manager switch`/`nixos-rebuild switch`/`nix-shell`.
+
+Although there are several existing ways to achieve this:
+
+* cachix's [watch-exec and watch-store](https://docs.cachix.org/pushing)
+* nix-build's [post-build-hook](https://nixos.org/manual/nix/stable/advanced-topics/post-build-hook)
+
+However, the granularity of these methods is coarse; they push all packages to cachix.
+Is there a way to allow users to control which packages are pushed?
+
+## Conclusion First
+
+Use **hooks** to push the selected packages to cachix:
+
+| Scenario      | Hook                       | Example                                                                                                            |
+|---------------|----------------------------|--------------------------------------------------------------------------------------------------------------------|
+| home-manager  | `home.activation`          | [modules/cachix.nix](../../../modules/cachix.nix.md), [usr/modules/cachix.nix](../../../usr/modules/cachix.nix.md) |
+| nixos-rebuild | `system.activationScripts` | [modules/cachix.nix](../../../modules/cachix.nix.md), [sys/modules/cachix.nix](../../../sys/modules/cachix.nix.md) |
+| nix-shell     | `shellHook`                | [openc910/shell.nix](https://github.com/xieby1/openc910/blob/main/shell.nix)                                       |
+
+---
+
+## My Explorations
+
+**Possible solutions**:
+Add a wrapper called `cachixPackages`, which recives the packages to be pushed and cachix information.
+This `cachixPackages` is a dummy package whose build stages will push the packages to cachix.
+However, normal nix packages are not allowed network access during building.
+To tackle this, like how fetch* series functions are implemented, the [fixed-output derivation](https://nixos.org/manual/nix/stable/language/advanced-attributes#adv-attr-outputHash) can be utilized to allow network access.
+
+However, the above method seems not work as below,
+because cachix needs accesses to some resources beyond nix-build process (such as nix-build's sandbox).
+
+```bash
+nix-build test.nix
+...
+cachix: CppStdException e "\ESC[31;1merror:\ESC[0m creating directory '\ESC[35;1m/nix/var\ESC[0m': \ESC[35;1mPermission denied\ESC[0m"(Just "nix::SysError")
+result 1
+...
+```
+
+Even though I disable the nix-build sandbox by using `--no-sandbox`,
+the cachix still does not satisfy as below.
+
+```bash
+$ nix-build test.nix --no-sandbox
+...
+cachix: CppStdException e "\ESC[31;1merror:\ESC[0m cannot open connection to remote store '\ESC[35;1mdaemon\ESC[0m': \ESC[35;1m\ESC[31;1merror:\ESC[0m reading from file: \ESC[35;1mConnection reset by peer\ESC[0m\ESC[0m"(Just "nix::Error")
+...
+```
+
+If you curious about my demo of `cachixPackages` and its test,
+see [cachix-package.nix](./cachix-package.nix.md) and [test.nix](./test.nix.md).

docs/howto/auto_push_to_cachix/test.nix

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+#MC # Test for cachixPackage
+#MC
+#MC To run the test:
+#MC
+#MC ```bash
+#MC # run with nix-build sandbox
+#MC nix-build test.nix
+#MC # run without nix-build sandbox
+#MC nix-build test.nix --no-sandbox
+#MC ```
+let
+  pkgs = import <nixpkgs> {};
+  cachixPackage = import ./cachix-package.nix {inherit (pkgs) cachix stdenv writeShellScript;};
+in cachixPackage {
+  pkg = pkgs.hello;
+  sha256 = "01vm275n169r0ly8ywgq0shgk8lrzg79d1aarshwybwxwffj4q0q";
+  cachix_dhall = /home/xieby1/Gist/Config/cachix.dhall;
+  cachix_name = "xieby1";
+}