Unhelpful error message for missing BUILD file: looks like referenced file is missing instead

[alexeagle]

User of rules_nodejs hits a problem in bazel-contrib/rules_nodejs#628

In a repository rule, they reference //:yarn.lock

load("@build_bazel_rules_nodejs//:defs.bzl", "yarn_install")
yarn_install(
    name = "npm",
    package_json = "//:package.json",
    yarn_lock = "//:yarn.lock",
)

But there is no BUILD file in the root of the WORKSPACE.

The error ought to say sth like "you referenced a file in /path/to/workspace but there is no BUILD.bazel file there. Fix by creating an empty file there".

Instead the error is

ERROR: error loading package '': Encountered error while reading extension file 'install_bazel_dependencies.bzl': no such package '@npm//': Traceback (most recent call last):
          File "/private/var/tmp/_bazel_justinmah/684a432d68d5cb4f45acf4255d4873d0/external/build_bazel_rules_nodejs/internal/npm_install/npm_install.bzl", line 237
                          repository_ctx.template("yarn.lock", repository_ctx.path(r...), {})
                                  File "/private/var/tmp/_bazel_justinmah/684a432d68d5cb4f45acf4255d4873d0/external/build_bazel_rules_nodejs/internal/npm_install/npm_install.bzl", line 239, in repository_ctx.template
                                                  repository_ctx.path(repository_ctx.attr.yarn_lock)
  Unable to load package for //:yarn.lock: not found.

The last line is very misleading because it seems like "I couldn't load package" (users may not know what Bazel means by a "package") and the reason is that the yarn.lock file is not found. The user sees that the yarn.lock file is there, so can't understand how to fix this.

[aehlig]

I believe this is fixed by now. With bazel at head, the error message currently reads as follows.

aehlig@aehlig1:/tmp/issue-7854$ cat WORKSPACE 
load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
http_archive(
    name = "build_bazel_rules_nodejs",
    sha256 = "4c702ffeeab2d24dd4101601b6d27cf582d2e0d4cdc3abefddd4834664669b6b",
    urls = ["https://github.com/bazelbuild/rules_nodejs/releases/download/0.28.0/rules_nodejs-0.28.0.tar.gz"],
)

load("@build_bazel_rules_nodejs//:defs.bzl", "yarn_install")
yarn_install(
    name = "npm",
    package_json = "//:package.json",
    yarn_lock = "//:yarn.lock",
)

load("@npm//:install_bazel_dependencies.bzl", "install_bazel_dependencies")
install_bazel_dependencies()
aehlig@aehlig1:/tmp/issue-7854$ ls -al
total 300
drwxr-xr-x  2 aehlig primarygroup   4096 Apr 29 11:35 .
drwxrwxrwt 52 root   root         294912 Apr 29 11:36 ..
-rw-r--r--  1 aehlig primarygroup    588 Apr 29 11:35 WORKSPACE
aehlig@aehlig1:/tmp/issue-7854$ bazel build ...
Starting local Bazel server and connecting to it...
(11:36:58) INFO: Invocation ID: 084d748e-6771-4d83-b39e-d9632b684245
(11:36:58) INFO: Current date is 2019-04-29
(11:37:03) INFO: Call stack for the definition of repository 'npm' which is a yarn_install (rule definition at /usr/local/google/home/aehlig/.cache/bazel/_bazel_aehlig/f75f4eb183e8028dc1c9c396d5868f94/external/build_bazel_rules_nodejs/internal/npm_install/npm_install.bzl:298:16):
 - /usr/local/google/home/aehlig/.cache/bazel/_bazel_aehlig/f75f4eb183e8028dc1c9c396d5868f94/external/build_bazel_rules_nodejs/defs.bzl:82:5
 - /tmp/issue-7854/WORKSPACE:9:1
(11:37:03) ERROR: An error occurred during the fetch of repository 'npm':
   Traceback (most recent call last):
	File "/usr/local/google/home/aehlig/.cache/bazel/_bazel_aehlig/f75f4eb183e8028dc1c9c396d5868f94/external/build_bazel_rules_nodejs/internal/npm_install/npm_install.bzl", line 243
		repository_ctx.template("yarn.lock", repository_ctx.path(r...), {})
	File "/usr/local/google/home/aehlig/.cache/bazel/_bazel_aehlig/f75f4eb183e8028dc1c9c396d5868f94/external/build_bazel_rules_nodejs/internal/npm_install/npm_install.bzl", line 245, in repository_ctx.template
		repository_ctx.path(repository_ctx.attr.yarn_lock)
Unable to load package for //:yarn.lock: BUILD file not found in any of the following directories.
 - /tmp/issue-7854
(11:37:03) ERROR: error loading package '': Encountered error while reading extension file 'install_bazel_dependencies.bzl': no such package '@npm//': Traceback (most recent call last):
	File "/usr/local/google/home/aehlig/.cache/bazel/_bazel_aehlig/f75f4eb183e8028dc1c9c396d5868f94/external/build_bazel_rules_nodejs/internal/npm_install/npm_install.bzl", line 243
		repository_ctx.template("yarn.lock", repository_ctx.path(r...), {})
	File "/usr/local/google/home/aehlig/.cache/bazel/_bazel_aehlig/f75f4eb183e8028dc1c9c396d5868f94/external/build_bazel_rules_nodejs/internal/npm_install/npm_install.bzl", line 245, in repository_ctx.template
		repository_ctx.path(repository_ctx.attr.yarn_lock)
Unable to load package for //:yarn.lock: BUILD file not found in any of the following directories.
 - /tmp/issue-7854
(11:37:03) ERROR: error loading package '': Encountered error while reading extension file 'install_bazel_dependencies.bzl': no such package '@npm//': Traceback (most recent call last):
	File "/usr/local/google/home/aehlig/.cache/bazel/_bazel_aehlig/f75f4eb183e8028dc1c9c396d5868f94/external/build_bazel_rules_nodejs/internal/npm_install/npm_install.bzl", line 243
		repository_ctx.template("yarn.lock", repository_ctx.path(r...), {})
	File "/usr/local/google/home/aehlig/.cache/bazel/_bazel_aehlig/f75f4eb183e8028dc1c9c396d5868f94/external/build_bazel_rules_nodejs/internal/npm_install/npm_install.bzl", line 245, in repository_ctx.template
		repository_ctx.path(repository_ctx.attr.yarn_lock)
Unable to load package for //:yarn.lock: BUILD file not found in any of the following directories.
 - /tmp/issue-7854
(11:37:03) INFO: Elapsed time: 5.842s
(11:37:03) INFO: 0 processes.
(11:37:03) FAILED: Build did NOT complete successfully (0 packages loaded)
aehlig@aehlig1:/tmp/issue-7854$