nix: put each version in a separate file

pkgs/tools/package-management/nix/default.nix

@@ -119,72 +119,48 @@ let
     hash = "sha256-YrmFkVpwPreiig1/BsP+DInpTdQrPmS7bEY0WUGpw+c=";
   };
 
-in lib.makeExtensible (self: ({
-  nix_2_3 = (common rec {
-    version = "2.3.16";
-    src = fetchurl {
-      url = "https://nixos.org/releases/nix/nix-${version}/nix-${version}.tar.xz";
-      hash = "sha256-fuaBtp8FtSVJLSAsO+3Nne4ZYLuBj2JpD2xEk7fCqrw=";
-    };
-    patches = [
-      patch-monitorfdhup
-    ];
-  }).override { boehmgc = boehmgc-nix_2_3; };
-
-  nix_2_10 = common {
-    version = "2.10.3";
-    hash = "sha256-B9EyDUz/9tlcWwf24lwxCFmkxuPTVW7HFYvp0C4xGbc=";
-    patches = [
-      ./patches/flaky-tests.patch
-      patch-non-existing-output
-      patch-monitorfdhup
-      patch-sqlite-exception
-    ];
-  };
+in lib.makeExtensible (self: {
+
+  # Please keep the attrset containing these `throw`s *before* the
+  # `versions` attrset, so the `throw` only occurs if the file is
+  # missing.
+  #
+  # This allows developers to resurrect old versions, at their own
+  # risk, by reverting the deletion of the $major.$minor.nix file.
+  # Unlike a file edit, reverting a deletion won't cause a merge
+  # conflict with every subsequent rebase that touches nearby
+  # lines of code.
 
-  nix_2_11 = common {
-    version = "2.11.1";
-    hash = "sha256-qCV65kw09AG+EkdchDPq7RoeBznX0Q6Qa4yzPqobdOk=";
-    patches = [
-      ./patches/flaky-tests.patch
-      patch-non-existing-output
-      patch-monitorfdhup
-      patch-sqlite-exception
-    ];
-  };
+  nix_2_4 = throw "nixVersions.nix_2_4 has been removed";
 
-  nix_2_12 = common {
-    version = "2.12.1";
-    hash = "sha256-GmHKhq0uFtdOiJnuBwj2YwlZjvh6YTkfQZgeu4e0dLU=";
-    patches = [
-      ./patches/flaky-tests.patch
-      patch-monitorfdhup
-      patch-sqlite-exception
-    ];
-  };
+  nix_2_5 = throw "nixVersions.nix_2_5 has been removed";
 
-  nix_2_13 = common {
-    version = "2.13.3";
-    hash = "sha256-jUc2ccTR8f6MGY2pUKgujm+lxSPNGm/ZAP+toX+nMNc=";
-    patches = [
-      patch-fix-aarch64-darwin-static
-    ];
-  };
+  nix_2_6 = throw "nixVersions.nix_2_6 has been removed";
 
-  nix_2_14 = common {
-    version = "2.14.1";
-    hash = "sha256-5aCmGZbsFcLIckCDfvnPD4clGPQI7qYAqHYlttN/Wkg=";
-  };
+  nix_2_7 = throw "nixVersions.nix_2_7 has been removed";
 
-  nix_2_15 = common {
-    version = "2.15.1";
-    hash = "sha256-o7bxsNeq2LF6/dTl+lT2k50bSItkID80/uoZYVtlxro=";
-  };
+  nix_2_8 = throw "nixVersions.nix_2_8 has been removed";
 
-  nix_2_16 = common {
-    version = "2.16.1";
-    hash = "sha256-/XCWa2osNFIpPC5MkxlX6qTZf/DaTLwS3LWN0SRFiuU=";
-  };
+  nix_2_9 = throw "nixVersions.nix_2_9 has been removed";
+
+} //
+
+(let
+  # Given a version number, produce a derivation for that Nix version.
+  version_to_derivation = version:
+    callPackage (./versions + "/${version}") {
+      inherit common patch-sqlite-exception boehmgc-nix_2_3 patch-monitorfdhup patch-fix-aarch64-darwin-static;
+    };
+  versions = lib.attrNames (builtins.readDir ./versions);
+  version_to_nameValuePair_drv = version:
+    lib.nameValuePair
+      "nix_${lib.replaceStrings ["."] ["_"] version}"
+      (version_to_derivation version);
+
+in
+  lib.listToAttrs (map version_to_nameValuePair_drv versions))
+
+// {
 
   # The minimum Nix version supported by Nixpkgs
   # Note that some functionality *might* have been backported into this Nix version,
@@ -218,4 +194,4 @@ in lib.makeExtensible (self: ({
   nix_2_8 = throw "nixVersions.nix_2_8 has been removed";
 
   nix_2_9 = throw "nixVersions.nix_2_9 has been removed";
-}))
+})

pkgs/tools/package-management/nix/patches/install-nlohmann_json-headers.patch

@@ -0,0 +1,36 @@
+From 3884f7a69a57d8ecfcbcaae476ec2ff53ffbd549 Mon Sep 17 00:00:00 2001
+From: Robert Hensing <robert@roberthensing.nl>
+Date: Thu, 11 Nov 2021 11:03:21 +0100
+Subject: [PATCH] Install nlohmann_json headers
+
+These headers are included by the libexpr, libfetchers, libstore
+and libutil headers.
+Considering that these are vendored sources, Nix should expose them,
+as it is not a good idea for reverse dependencies to rely on a
+potentially different source that can go out of sync.
+---
+ Makefile              | 1 +
+ src/nlohmann/local.mk | 2 ++
+ 2 files changed, 3 insertions(+)
+ create mode 100644 src/nlohmann/local.mk
+
+diff --git a/Makefile b/Makefile
+index 5040d288485..e6ce50cbdb7 100644
+--- a/Makefile
++++ b/Makefile
+@@ -10,6 +10,7 @@ makefiles = \
+   src/libexpr/local.mk \
+   src/libcmd/local.mk \
+   src/nix/local.mk \
++  src/nlohmann/local.mk \
+   src/resolve-system-dependencies/local.mk \
+   scripts/local.mk \
+   misc/bash/local.mk \
+diff --git a/src/nlohmann/local.mk b/src/nlohmann/local.mk
+new file mode 100644
+index 00000000000..63c427e000e
+--- /dev/null
++++ b/src/nlohmann/local.mk
+@@ -0,0 +1,2 @@
++$(foreach i, $(wildcard src/nlohmann/*.hpp), \
++  $(eval $(call install-file-in, $(i), $(includedir)/nlohmann, 0644)))

pkgs/tools/package-management/nix/versions/2.10/default.nix

@@ -0,0 +1,22 @@
+{ common
+, fetchpatch
+, patch-sqlite-exception
+, patch-monitorfdhup
+, ...
+}:
+
+common {
+  version = "2.10.3";
+  hash = "sha256-B9EyDUz/9tlcWwf24lwxCFmkxuPTVW7HFYvp0C4xGbc=";
+  patches = [
+    ../../patches/flaky-tests.patch
+    (fetchpatch {
+      # https://github.com/NixOS/nix/pull/7283
+      name = "fix-requires-non-existing-output.patch";
+      url = "https://github.com/NixOS/nix/commit/3ade5f5d6026b825a80bdcc221058c4f14e10a27.patch";
+      hash = "sha256-s1ybRFCjQaSGj7LKu0Z5g7UiHqdJGeD+iPoQL0vaiS0=";
+    })
+    patch-sqlite-exception
+    patch-monitorfdhup
+  ];
+}

pkgs/tools/package-management/nix/versions/2.11/default.nix

@@ -0,0 +1,22 @@
+{ common
+, fetchpatch
+, patch-sqlite-exception
+, patch-monitorfdhup
+, ...
+}:
+
+common {
+  version = "2.11.1";
+  hash = "sha256-qCV65kw09AG+EkdchDPq7RoeBznX0Q6Qa4yzPqobdOk=";
+  patches = [
+    ../../patches/flaky-tests.patch
+    (fetchpatch {
+      # https://github.com/NixOS/nix/pull/7283
+      name = "fix-requires-non-existing-output.patch";
+      url = "https://github.com/NixOS/nix/commit/3ade5f5d6026b825a80bdcc221058c4f14e10a27.patch";
+      hash = "sha256-s1ybRFCjQaSGj7LKu0Z5g7UiHqdJGeD+iPoQL0vaiS0=";
+    })
+    patch-sqlite-exception
+    patch-monitorfdhup
+  ];
+}

pkgs/tools/package-management/nix/versions/2.12/default.nix

@@ -0,0 +1,15 @@
+{ common
+, patch-sqlite-exception
+, patch-monitorfdhup
+, ...
+}:
+
+common {
+  version = "2.12.0";
+  hash = "sha256-sQ9C101CL/eVN5JgH91ozHFWU4+bXr8/Fi/8NQk6xRI=";
+  patches = [
+    ../../patches/flaky-tests.patch
+    patch-sqlite-exception
+    patch-monitorfdhup
+  ];
+}

pkgs/tools/package-management/nix/versions/2.13/default.nix

@@ -0,0 +1,12 @@
+{ common
+, patch-fix-aarch64-darwin-static
+, ...
+}:
+
+common {
+  version = "2.13.1";
+  hash = "sha256-uXh4+xjJUHQSCg+LHh6+SSYtMdjKQiTXMZ4uZFwzdq4=";
+  patches = [
+    patch-fix-aarch64-darwin-static
+  ];
+}

pkgs/tools/package-management/nix/versions/2.14/default.nix

@@ -0,0 +1,8 @@
+{ common
+, ...
+}:
+
+common {
+  version = "2.14.1";
+  hash = "sha256-5aCmGZbsFcLIckCDfvnPD4clGPQI7qYAqHYlttN/Wkg=";
+}

pkgs/tools/package-management/nix/versions/2.15/default.nix

@@ -0,0 +1,8 @@
+{ common
+, ...
+}:
+
+common {
+  version = "2.15.1";
+  hash = "sha256-o7bxsNeq2LF6/dTl+lT2k50bSItkID80/uoZYVtlxro=";
+}

pkgs/tools/package-management/nix/versions/2.16/default.nix

@@ -0,0 +1,8 @@
+{ common
+, ...
+}:
+
+common {
+  version = "2.16.1";
+  hash = "sha256-/XCWa2osNFIpPC5MkxlX6qTZf/DaTLwS3LWN0SRFiuU=";
+}

pkgs/tools/package-management/nix/versions/2.3/default.nix

@@ -0,0 +1,21 @@
+{ boehmgc-nix_2_3
+, fetchurl
+, common
+, patch-monitorfdhup
+, ...
+}:
+
+(common rec {
+  version = "2.3.16";
+  src = fetchurl {
+    url = "https://nixos.org/releases/nix/nix-${version}/nix-${version}.tar.xz";
+    hash = "sha256-fuaBtp8FtSVJLSAsO+3Nne4ZYLuBj2JpD2xEk7fCqrw=";
+  };
+  patches = [
+    patch-monitorfdhup
+  ];
+})
+  .override {
+    boehmgc = boehmgc-nix_2_3;
+  }
+