Add optional location expansion for the nixopts attribute

BUILD.bazel

@@ -0,0 +1,4 @@
+exports_files([
+    "nixpkgs.json",
+    "nixpkgs.nix",
+])

CHANGELOG.md

@@ -4,6 +4,20 @@ All notable changes to this project will be documented in this file.
 
 The format is based on [Keep a Changelog](https://keepachangelog.com/).
 
+## [Unreleased]
+
+[Unreleased]: https://github.com/tweag/rules_nixpkgs/compare/v0.7.0...HEAD
+
+### Changed
+
+- The values in the `nixopts` attribute to `nixpkgs_package` are now subject to
+  location expansion. Any instance of `$(location LABEL)` in the `nixopts`
+  attribute will be expanded to the file path of the file referenced by
+  `LABEL`. To pass a plain `$` to Nix it must be escaped as `$$`.
+  See [#132][#132].
+
+[#132]: https://github.com/tweag/rules_nixpkgs/pull/132
+
 ## [0.7.0] - 2020-04-20
 
 [0.7.0]: https://github.com/tweag/rules_nixpkgs/compare/v0.6.0...v0.7.0

README.md

@@ -303,7 +303,12 @@ filegroup(
       <td><code>nixopts</code></td>
       <td>
         <p><code>String list; optional</code></p>
-        <p>Extra flags to pass when calling Nix.</p>
+        <p>
+            Extra flags to pass when calling Nix. Subject to location
+            expansion, any instance of <code>$(location LABEL)</code> will be
+            replaced by the path to the file ferenced by <code>LABEL</code>
+            relative to the workspace root.
+        </p>
       </td>
     </tr>
     <tr>

WORKSPACE

@@ -16,6 +16,10 @@ load(
 
 # For tests
 
+load("@bazel_skylib//lib:unittest.bzl", "register_unittest_toolchains")
+
+register_unittest_toolchains()
+
 nixpkgs_git_repository(
     name = "remote_nixpkgs",
     remote = "https://github.com/NixOS/nixpkgs",
@@ -170,6 +174,30 @@ nixpkgs_package(
     repository = "@remote_nixpkgs",
 )
 
+local_repository(
+    name = "nixpkgs_location_expansion_test_file",
+    path = "tests/location_expansion/test_repo",
+)
+
+nixpkgs_package(
+    name = "nixpkgs_location_expansion_test",
+    build_file_content = "exports_files(glob(['out/**']))",
+    nix_file = "//tests:location_expansion.nix",
+    nix_file_deps = [
+        "//tests:location_expansion/test_file",
+        "@nixpkgs_location_expansion_test_file//:test_file",
+    ],
+    nixopts = [
+        "--arg",
+        "local_file",
+        "$(location //tests:location_expansion/test_file)",
+        "--arg",
+        "external_file",
+        "$(location @nixpkgs_location_expansion_test_file//:test_file)",
+    ],
+    repository = "@remote_nixpkgs",
+)
+
 load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
 
 http_archive(
@@ -202,12 +230,11 @@ http_archive(
 
 load(
     "//nixpkgs:toolchains/go.bzl",
-    "nixpkgs_go_configure"
+    "nixpkgs_go_configure",
 )
 
 nixpkgs_go_configure(repository = "@nixpkgs")
 
-load("@io_bazel_rules_go//go:deps.bzl", "go_rules_dependencies", "go_register_toolchains")
+load("@io_bazel_rules_go//go:deps.bzl", "go_register_toolchains", "go_rules_dependencies")
 
 go_rules_dependencies()
-

nixpkgs/BUILD.bazel

@@ -6,7 +6,11 @@ exports_files([
     "nixpkgs.bzl",
 ])
 
-filegroup(name = "srcs", srcs = glob(["**"]), visibility = ["//visibility:public"])
+filegroup(
+    name = "srcs",
+    srcs = glob(["**"]),
+    visibility = ["//visibility:public"],
+)
 
 # @bazel_tools//tools does not define a bzl_library itself, instead we are
 # supposed to define our own using the @bazel_tools//tools:bzl_srcs filegroup.
@@ -33,9 +37,11 @@ bzl_library(
     name = "nixpkgs",
     srcs = [
         "nixpkgs.bzl",
+        "private/location_expansion.bzl",
     ],
     visibility = ["//visibility:public"],
     deps = [
         ":bazel_tools",
+        "@bazel_skylib//lib:paths",
     ],
 )

nixpkgs/constraints/BUILD.bazel

@@ -3,7 +3,7 @@ package(default_visibility = ["//visibility:public"])
 constraint_setting(name = "nix")
 
 constraint_value(
-    name =  "support_nix",
+    name = "support_nix",
     constraint_setting = ":nix",
 )
 

nixpkgs/nixpkgs.bzl

@@ -2,11 +2,13 @@
 
 load("@bazel_tools//tools/cpp:cc_configure.bzl", "cc_autoconf_impl")
 load("@bazel_tools//tools/cpp:lib_cc_configure.bzl", "get_cpu_value")
+load(":private/location_expansion.bzl", "expand_location")
 
 def _nixpkgs_git_repository_impl(repository_ctx):
     repository_ctx.file(
         "BUILD",
-        content = 'filegroup(name = "srcs", srcs = glob(["**"]), visibility = ["//visibility:public"])')
+        content = 'filegroup(name = "srcs", srcs = glob(["**"]), visibility = ["//visibility:public"])',
+    )
 
     # Make "@nixpkgs" (syntactic sugar for "@nixpkgs//:nixpkgs") a valid
     # label for default.nix.
@@ -119,8 +121,9 @@ def _nixpkgs_package_impl(repository_ctx):
     else:
         expr_args = ["-E", "import <nixpkgs> { config = {}; overlays = []; }"]
 
+    nix_file_deps = {}
     for dep in repository_ctx.attr.nix_file_deps:
-        _cp(repository_ctx, dep)
+        nix_file_deps[dep] = _cp(repository_ctx, dep)
 
     expr_args.extend([
         "-A",
@@ -135,7 +138,15 @@ def _nixpkgs_package_impl(repository_ctx):
         "bazel-support/nix-out-link",
     ])
 
-    expr_args.extend(repository_ctx.attr.nixopts)
+    expr_args.extend([
+        expand_location(
+            repository_ctx = repository_ctx,
+            string = opt,
+            labels = nix_file_deps,
+            attr = "nixopts",
+        )
+        for opt in repository_ctx.attr.nixopts
+    ])
 
     for repo in repositories.keys():
         path = str(repository_ctx.path(repo).dirname) + "/nix-file-deps"
@@ -208,7 +219,7 @@ def _nixpkgs_package_impl(repository_ctx):
         if create_build_file_if_needed:
             p = repository_ctx.path("BUILD")
             if not p.exists:
-               repository_ctx.template("BUILD", Label("@io_tweag_rules_nixpkgs//nixpkgs:BUILD.pkg"))
+                repository_ctx.template("BUILD", Label("@io_tweag_rules_nixpkgs//nixpkgs:BUILD.pkg"))
 
 _nixpkgs_package = repository_rule(
     implementation = _nixpkgs_package_impl,

nixpkgs/private/location_expansion.bzl

@@ -0,0 +1,128 @@
+load("@bazel_skylib//lib:paths.bzl", "paths")
+
+def parse_expand_location(string):
+    """Parse a string that might contain location expansion commands.
+
+    Generates a list of pairs of command and argument.
+    The command can have the following values:
+    - `string`: argument is a string, append it to the result.
+    - `location`: argument is a label, append its location to the result.
+
+    Attrs:
+      string: string, The string to parse.
+
+    Returns:
+      (result, error):
+        result: The generated list of pairs of command and argument.
+        error: string or None, This is set if an error occurred.
+    """
+    result = []
+    offset = 0
+    len_string = len(string)
+
+    # Step through occurrences of `$`. This is bounded by the length of the string.
+    for _ in range(len_string):
+        # Find the position of the next `$`.
+        position = string.find("$", offset)
+        if position == -1:
+            position = len_string
+
+        # Append the in-between literal string.
+        if offset < position:
+            result.append(("string", string[offset:position]))
+
+        # Terminate at the end of the string.
+        if position == len_string:
+            break
+
+        # Parse the `$` command.
+        if string[position:].startswith("$$"):
+            # Insert verbatim '$'.
+            result.append(("string", "$"))
+            offset = position + 2
+        elif string[position:].startswith("$("):
+            # Expand a location command.
+            group_start = position + 2
+            group_end = string.find(")", group_start)
+            if group_end == -1:
+                return (None, "Unbalanced parentheses in location expansion for '{}'.".format(string[position:]))
+
+            group = string[group_start:group_end]
+            command = None
+            if group.startswith("location "):
+                label_str = group[len("location "):]
+                command = ("location", label_str)
+            else:
+                return (None, "Unrecognized location expansion '$({})'.".format(group))
+
+            result.append(command)
+            offset = group_end + 1
+        else:
+            return (None, "Unescaped '$' in location expansion at position {} of input.".format(position))
+
+    return (result, None)
+
+def resolve_label(label_str, labels):
+    """Find the label that corresponds to the given string.
+
+    Attr:
+      label_str: string, String representation of a label.
+      labels: dict from Label to path: Known label to path mappings.
+
+    Returns:
+      (path, error):
+        path: path, The path to the resolved label
+        error: string or None, This is set if an error occurred.
+    """
+    label_candidates = [
+        (lbl, path)
+        for (lbl, path) in labels.items()
+        if lbl.relative(label_str) == lbl
+    ]
+
+    if len(label_candidates) == 0:
+        return (None, "Unknown label '{}' in location expansion.".format(label_str))
+    elif len(label_candidates) > 1:
+        return (None, "Ambiguous label '{}' in location expansion. Candidates: {}".format(
+            label_str,
+            ", ".join([str(lbl) for (lbl, _) in label_candidates]),
+        ))
+
+    return (label_candidates[0][1], None)
+
+def expand_location(repository_ctx, string, labels, attr = None):
+    """Expand `$(location label)` to a path.
+
+    Raises an error on unexpected occurrences of `$`.
+    Use `$$` to insert a verbatim `$`.
+
+    Attrs:
+      repository_ctx: The repository rule context.
+      string: string, Replace instances of `$(location )` in this string.
+      labels: dict from label to path: Known label to path mappings.
+      attr: string, The rule attribute to use for error reporting.
+
+    Returns:
+      The string with all instances of `$(location )` replaced by paths.
+    """
+    (parsed, error) = parse_expand_location(string)
+    if error != None:
+        fail(error, attr)
+
+    result = ""
+    for (command, argument) in parsed:
+        if command == "string":
+            result += argument
+        elif command == "location":
+            (label, error) = resolve_label(argument, labels)
+            if error != None:
+                fail(error, attr)
+
+            result += paths.join(".", paths.relativize(
+                str(repository_ctx.path(label)),
+                str(repository_ctx.path(".")),
+            ))
+        else:
+            fail("Internal error: Unknown location expansion command '{}'.".format(command), attr)
+
+    return result

nixpkgs/toolchains/go.bzl

@@ -2,20 +2,19 @@ load(
     "@io_bazel_rules_go//go:deps.bzl",
     "go_wrap_sdk",
 )
-
 load(
     "//nixpkgs:nixpkgs.bzl",
-    "nixpkgs_package"
+    "nixpkgs_package",
 )
 
 def nixpkgs_go_configure(
-    sdk_name = "go_sdk",
-    repository = None,
-    repositories = {},
-    nix_file = None,
-    nix_file_deps = None,
-    nix_file_content = None,
-    nixopts = []):
+        sdk_name = "go_sdk",
+        repository = None,
+        repositories = {},
+        nix_file = None,
+        nix_file_deps = None,
+        nix_file_content = None,
+        nixopts = []):
     """
     Use go toolchain from Nixpkgs. Will fail if not a nix-based platform.
 
@@ -41,7 +40,6 @@ def nixpkgs_go_configure(
             }
         """
 
-
     nixpkgs_package(
         name = "nixpkgs_go_toolchain",
         repository = repository,