build-support: Add fetchpijul function.

pkgs/build-support/fetchpijul/default.nix

@@ -0,0 +1,56 @@
+{ lib, stdenvNoCC, pijul }:
+
+lib.makeOverridable (
+{ url
+, hash ? ""
+, change ? null
+, state ? null
+, channel ? "main"
+, name ? "fetchpijul"
+, # TODO: Changes in pijul are unordered so there's many ways to end up with the same repository state.
+  # This makes leaveDotPijul unfeasible to implement until pijul CLI implements
+  # a way of reordering changes to sort them in a consistent and deterministic manner.
+  # leaveDotPijul ? false
+}:
+if change != null && state != null then
+  throw "Only one of 'change' or 'state' can be set"
+else
+  stdenvNoCC.mkDerivation {
+    inherit name;
+    nativeBuildInputs = [ pijul ];
+
+    dontUnpack = true;
+    dontConfigure = true;
+    dontBuild = true;
+
+    installPhase = ''
+      runHook preInstall
+
+      pijul clone \
+        ''${change:+--change "$change"} \
+        ''${state:+--state "$state"} \
+        --channel "$channel" \
+        "$url" \
+        "$out"
+
+      runHook postInstall
+    '';
+
+    fixupPhase = ''
+      runHook preFixup
+
+      rm -rf "$out/.pijul"
+
+      runHook postFixup
+    '';
+
+    outputHashAlgo = if hash != "" then null else "sha256";
+    outputHashMode = "recursive";
+    outputHash = if hash != "" then
+      hash
+    else
+      lib.fakeSha256;
+
+    inherit url change state channel;
+  }
+)

pkgs/top-level/all-packages.nix

@@ -899,6 +899,8 @@ with pkgs;
 
   fetchMavenArtifact = callPackage ../build-support/fetchmavenartifact { };
 
+  fetchpijul = callPackage ../build-support/fetchpijul { };
+
   inherit (callPackage ../build-support/node/fetch-yarn-deps { })
     prefetch-yarn-deps
     fetchYarnDeps;