fetchTree: Allow fetching plain files

Add a new `file` fetcher type, which will fetch a plain file over
http(s), or from the local file.

Because plain `http(s)://` or `file://` urls can already correspond to
`tarball` inputs (if the path ends-up with a know archive extension),
the URL parsing logic is a bit convuluted in that:

- {http,https,file}:// urls will be interpreted as either a tarball or a
  file input, depending on the extensions of the path part (so
  `https://foo.com/bar` will be a `file` input and
  `https://foo.com/bar.tar.gz` as a `tarball` input)
- `file+{something}://` urls will be interpreted as `file` urls (with
  the `file+` part removed)
- `tarball+{something}://` urls will be interpreted as `tarball` urls (with
  the `tarball+` part removed)

Fix #3785

Co-Authored-By: Tony Olagbaiye <me@fron.io>

tests/fetchTree-file.sh

@@ -0,0 +1,105 @@
+source common.sh
+
+clearStore
+
+cd "$TEST_ROOT"
+
+test_fetch_file () {
+    echo foo > test_input
+
+    input_hash="$(nix hash path test_input)"
+
+    nix eval --impure --file - <<EOF
+    let
+        tree = builtins.fetchTree { type = "file"; url = "file://$PWD/test_input"; };
+    in
+    assert (tree.narHash == "$input_hash");
+    tree
+EOF
+}
+
+# Make sure that `http(s)` and `file` flake inputs are properly extracted when
+# they should be, and treated as opaque files when they should be
+test_file_flake_input () {
+    rm -fr "$TEST_ROOT/testFlake";
+    mkdir "$TEST_ROOT/testFlake";
+    pushd testFlake
+
+    mkdir inputs
+    echo foo > inputs/test_input_file
+    tar cfa test_input.tar.gz inputs
+    cp test_input.tar.gz test_input_no_ext
+    input_tarball_hash="$(nix hash path test_input.tar.gz)"
+    input_directory_hash="$(nix hash path inputs)"
+
+    cat <<EOF > flake.nix
+    {
+        inputs.no_ext_default_no_unpack = {
+            url = "file://$PWD/test_input_no_ext";
+            flake = false;
+        };
+        inputs.no_ext_explicit_unpack = {
+            url = "tarball+file://$PWD/test_input_no_ext";
+            flake = false;
+        };
+        inputs.tarball_default_unpack = {
+            url = "file://$PWD/test_input.tar.gz";
+            flake = false;
+        };
+        inputs.tarball_explicit_no_unpack = {
+            url = "file+file://$PWD/test_input.tar.gz";
+            flake = false;
+        };
+        outputs = { ... }: {};
+    }
+EOF
+
+    nix flake update
+    nix eval --file - <<EOF
+    with (builtins.fromJSON (builtins.readFile ./flake.lock));
+
+    # Url inputs whose extension doesn’t match a know archive format should
+    # not be unpacked by default
+    assert (nodes.no_ext_default_no_unpack.locked.type == "file");
+    assert (nodes.no_ext_default_no_unpack.locked.unpack or false == false);
+    assert (nodes.no_ext_default_no_unpack.locked.narHash == "$input_tarball_hash");
+
+    # For backwards compatibility, flake inputs that correspond to the
+    # old 'tarball' fetcher should still have their type set to 'tarball'
+    assert (nodes.tarball_default_unpack.locked.type == "tarball");
+    # Unless explicitely specified, the 'unpack' parameter shouldn’t appear here
+    # because that would break older Nix versions
+    assert (!nodes.tarball_default_unpack.locked ? unpack);
+    assert (nodes.tarball_default_unpack.locked.narHash == "$input_directory_hash");
+
+    # Explicitely passing the unpack parameter should enforce the desired behavior
+    assert (nodes.no_ext_explicit_unpack.locked.narHash == nodes.tarball_default_unpack.locked.narHash);
+    assert (nodes.tarball_explicit_no_unpack.locked.narHash == nodes.no_ext_default_no_unpack.locked.narHash);
+    true
+EOF
+    popd
+
+    [[ -z "${NIX_DAEMON_PACKAGE}" ]] && return 0
+
+    # Ensure that a lockfile generated by the current Nix for tarball inputs
+    # can still be read by an older Nix
+
+    cat <<EOF > flake.nix
+    {
+        inputs.tarball = {
+            url = "file://$PWD/test_input.tar.gz";
+            flake = false;
+        };
+        outputs = { self, tarball }: {
+            foo = builtins.readFile "${tarball}/test_input_file";
+        };
+    }
+    nix flake update
+
+    clearStore
+    "$NIX_DAEMON_PACKAGE/bin/nix" eval .#foo
+EOF
+}
+
+test_fetch_file
+test_file_flake_input