feat: add lib.fetchurl function

tidepool/src/lib/default.nix

@@ -1,5 +1,6 @@
 {
   includes = [
+    ./fetchurl.nix
     ./options.nix
     ./packages.nix
     ./platforms.nix

tidepool/src/lib/fetchurl.nix

@@ -0,0 +1,73 @@
+{
+  config = {
+    # This function is a vendored version of <nix/fetchurl.nix>. It uses the (largely undocumented) builtin:fetchurl
+    # builder for derivations to give us a way to download files (eg. source tarballs) in the early bootstrap phase,
+    # before we have access to tools like curl. It functions almost identically to the builtins.fetchurl function,
+    # except it downloads files during the derivation's build, instead of during Nix evaluation, preventing evaluation
+    # from being blocked by downloads.
+    #
+    # Most packages should avoid using this function, and instead prefer using builders to download sources, such as
+    # (TODO: create these).
+    #
+    # Original source: https://git.lix.systems/lix-project/lix/src/commit/6599be1a9f76f0fba2a6905ea85235aeb7b7eae9/lix/libexpr/fetchurl.nix
+    lib.fetchurl =
+      {
+        # URL of file to download
+        url,
+        # SRI hash of downloaded file
+        hash ? "",
+
+        # Legacy, base32-encoded hash specifications
+        md5 ? "",
+        sha1 ? "",
+        sha256 ? "",
+        sha512 ? "",
+        outputHash ?
+          if hash != "" then
+            hash
+          else if sha512 != "" then
+            sha512
+          else if sha1 != "" then
+            sha1
+          else if md5 != "" then
+            md5
+          else
+            sha256,
+        outputHashAlgo ?
+          if hash != "" then
+            ""
+          else if sha512 != "" then
+            "sha512"
+          else if sha1 != "" then
+            "sha1"
+          else if md5 != "" then
+            "md5"
+          else
+            "sha256",
+
+        name ? baseNameOf url,
+        # Make the downloaded file executable (ie. chmod 755)
+        executable ? false,
+        # If the downloaded file is an archive, extract it before adding to the store
+        # NOTE: Only appears to unpack .xz archives?
+        unpack ? false,
+      }:
+      builtins.derivation {
+        inherit
+          name
+          url
+          executable
+          unpack
+          ;
+
+        builder = "builtin:fetchurl";
+        system = "builtin";
+        preferLocalBuild = true;
+
+        inherit outputHashAlgo outputHash;
+        outputHashMode = if unpack || executable then "recursive" else "flat";
+
+        urls = [ url ];
+      };
+  };
+}