Merge "nix: provide a python2 toolchain"

third_party/nix/BUILD

@@ -1,6 +1,6 @@
 load("@rules_python//python:defs.bzl", "py_runtime_pair")
 
-# Python3 toolchain definition that uses //third_party/nix:python.nix (via
+# Python toolchain definition that uses //third_party/nix:python.nix (via
 # external repository).
 
 py_runtime(
@@ -9,8 +9,16 @@ py_runtime(
     python_version = "PY3",
 )
 
+py_runtime(
+    name = "py2_runtime",
+    interpreter = "@hscloud_nix_python2//:python2",
+    python_version = "PY2",
+)
+
+
 py_runtime_pair(
     name = "py_runtime_pair",
+    py2_runtime = ":py2_runtime",
     py3_runtime = ":py3_runtime",
 )
 

third_party/nix/python.nix

@@ -8,26 +8,23 @@
 with import <nixpkgs> {};
 
 let
-  # Add cffi for import _cffi_backend in `cryptography` to work.
-  py = pkgs.python37.withPackages (ps: with ps; [ cffi ]);
-
-# We use mkDerivation instead of writeScript or writeScriptBin as we need a
-# derivation that both:
-# - has a directory structure (for rules_nixpkgs to be able to use it)
-# - has the Python interpreter directly in that structure and not in bin/, as
-#   rules_python's pip3_import interpreter_path requires a file target, and
-#   will not take an alias. Meanwhile, rules_nixpkgs only creates a BUILD file
-#   in the root path of the external repository (which is populated with a
-#   symlink tree from the nix derivation), so we can onlly directly reference
-#   file in the root of a Nix derivation.
-in stdenv.mkDerivation {
-  name = "py-wrapper";
-  version = "1.0";
-  src = ./.;
-  unpackPhase = "";
-  buildPhase = ''
-    mkdir -p $out
-    cat > $out/python3 <<EOF
+  # We use mkDerivation instead of writeScript or writeScriptBin as we need a
+  # derivation that both:
+  # - has a directory structure (for rules_nixpkgs to be able to use it)
+  # - has the Python interpreter directly in that structure and not in bin/, as
+  #   rules_python's pip3_import interpreter_path requires a file target, and
+  #   will not take an alias. Meanwhile, rules_nixpkgs only creates a BUILD file
+  #   in the root path of the external repository (which is populated with a
+  #   symlink tree from the nix derivation), so we can onlly directly reference
+  #   file in the root of a Nix derivation.
+  generic = package: binary:  stdenv.mkDerivation {
+    name = "${binary}-wrapper";
+    version = "1.0";
+    src = ./.;
+    unpackPhase = "";
+    buildPhase = ''
+      mkdir -p $out
+      cat > $out/${binary} <<EOF
 #!/bin/bash
 
 # pyscopg wants libpq, and uses pg_config to find paths. Inject pg_config into
@@ -37,10 +34,16 @@ export PATH="${pkgs.postgresql}/bin:\$PATH"
 # uWSGI has a truly cheese-grade build system, and this is the only way to let
 # it know where to find ncurses.
 export LDFLAGS="-L${pkgs.ncurses}/lib"
-exec ${py}/bin/python3 "\$@"
+exec ${package}/bin/${binary} "\$@"
 EOF
-  '';
-  installPhase = ''
-    chmod +x $out/python3
-  '';
+    '';
+    installPhase = ''
+      chmod +x $out/${binary}
+    '';
+  };
+
+in {
+  # Add cffi for import _cffi_backend in `cryptography` to work.
+  python2 = generic (pkgs.python27.withPackages (ps: with ps; [ cffi ])) "python2";
+  python3 = generic (pkgs.python37.withPackages (ps: with ps; [ cffi ])) "python3";
 }

third_party/nix/repository_rules.bzl

@@ -74,14 +74,25 @@ def hscloud_setup_nix(revision, sha256):
         sha256 = "8b64041bfb9760de9e797c0a985a4830880c21732489f397e217d877edd9a990",
     )
 
-    # Load python3 from nixpkgs. Python is a large source of non-hermiticity,
+    # Load python from nixpkgs. Python is a large source of non-hermiticity,
     # and loading it from nix vastly hermeticizes the build - well, at least to
     # also be dependent on this Nix store state. That's still better than just
     # grabbing whatever random system Python a user might have.
+    nixpkgs_package(
+        name = "hscloud_nix_python2",
+        repositories = { "nixpkgs": "@nixpkgs//:default.nix" },
+        nix_file = "//third_party/nix:python.nix",
+        attribute_path = "python2",
+        build_file_content = """
+package(default_visibility = ["//visibility:public"])
+exports_files(["python2"])
+        """,
+    )
     nixpkgs_package(
         name = "hscloud_nix_python3",
         repositories = { "nixpkgs": "@nixpkgs//:default.nix" },
         nix_file = "//third_party/nix:python.nix",
+        attribute_path = "python3",
         build_file_content = """
 package(default_visibility = ["//visibility:public"])
 exports_files(["python3"])