kleaf: no output_list for _kernel_build to support select

To support select() properly, the argument "outs" in
kernel_build macro cannot be parsed, and it needs to be
passed to _kernel_build directly. Hence, outs (and
for consistency, module_outs and implicit_outs) of _kernel_build
are changed to a string_list.

Then inside _kernel_build_impl, declare_file() of the proper
output file. Because _kernel_build_impl already explicitly
return DefaultInfo(), this is okay.

The downside is that the following becomes invalid:

  kernel_build(name = "kernel", out = ["vmlinux"])
  filegroup(name = "foo", srcs = ["kernel/vmlinux"])

This is because `kernel/vmlinux` is no longer in the explicit output
list.

See discussion thread:

  bazelbuild/bazel#281

Bug: 202075567
Bug: 202075499
Test: builds

Change-Id: I92d5c7f357d2d69c40039ef35b3e0a25cdb7214f

kleaf/kernel.bzl

@@ -78,7 +78,7 @@ def kernel_build(
         name,
         build_config,
         srcs,
-        outs,
+        outs = [],
         module_outs = [],
         generate_vmlinux_btf = False,
         deps = (),
@@ -220,9 +220,9 @@ def kernel_build(
         name = name,
         config = config_target_name,
         srcs = [sources_target_name],
-        outs = [name + "/" + out for out in outs],
-        module_outs = [name + "/" + module_out for module_out in module_outs],
-        implicit_outs = [name + "/" + out for out in _kernel_build_implicit_outs],
+        outs = outs,
+        module_outs = module_outs,
+        implicit_outs = _kernel_build_implicit_outs,
         deps = deps,
         archive_out_dir = archive_out_dir,
     )
@@ -534,14 +534,15 @@ def _kernel_build_impl(ctx):
     # kernel_build(name="kenrel", outs=["out"])
     # => _kernel_build(name="kernel", outs=["kernel/out"], implicit_outs=["kernel/Module.symvers", ...])
     # => all_output_names = ["foo", "Module.symvers", ...]
-    #    all_output_files = [File(...), File(...), ...]
-    all_output_files = []
+    #    all_output_files = {"out": [File(...)], "implicit_outs": [File(...)], ...}
+    all_output_files = {}
     for attr in ("outs", "module_outs", "implicit_outs"):
-        all_output_files += getattr(ctx.outputs, attr)
+        all_output_files[attr] = [ctx.actions.declare_file("{}/{}".format(ctx.label.name, name)) for name in getattr(ctx.attr, attr)]
     all_output_names = []
-    for out in all_output_files:
-        short_name = out.short_path[len(ruledir.short_path) + 1:]
-        all_output_names.append(short_name)
+    for l in all_output_files.values():
+        for out in l:
+            short_name = out.short_path[len(ruledir.short_path) + 1:]
+            all_output_names.append(short_name)
 
     modules_staging_archive = ctx.actions.declare_file(
         "{name}/modules_staging_dir.tar.gz".format(name = ctx.label.name),
@@ -552,14 +553,16 @@ def _kernel_build_impl(ctx):
     modules_staging_dir = modules_staging_archive.dirname + "/staging"
 
     # all outputs that |command| generates
-    command_outputs = all_output_files + [
+    command_outputs = [
         ruledir,
         modules_staging_archive,
         out_dir_kernel_headers_tar,
     ]
+    for l in all_output_files.values():
+        command_outputs += l
 
     # default outputs of this rule, aka output files that are emitted to DIST_DIR
-    default_outputs = ctx.outputs.outs + ctx.outputs.module_outs
+    default_outputs = all_output_files["outs"] + all_output_files["module_outs"]
 
     out_dir_tar_command = ""
     if ctx.attr.archive_out_dir:
@@ -622,8 +625,10 @@ def _kernel_build_impl(ctx):
 
     # Only outs and implicit_outs are needed. But for simplicity, copy the full {ruledir}
     # which includes module_outs too.
-    env_info_dependencies = ctx.attr.config[_KernelEnvInfo].dependencies + \
-                            all_output_files
+    env_info_dependencies = []
+    env_info_dependencies += ctx.attr.config[_KernelEnvInfo].dependencies
+    for l in all_output_files.values():
+        env_info_dependencies += l
     env_info_setup = ctx.attr.config[_KernelEnvInfo].setup + """
          # Restore kernel build outputs
            cp -R {ruledir}/* ${{OUT_DIR}}
@@ -648,7 +653,7 @@ def _kernel_build_impl(ctx):
             modules_staging_archive = modules_staging_archive,
             module_srcs = module_srcs,
             out_dir_kernel_headers_tar = out_dir_kernel_headers_tar,
-            outs = ctx.outputs.outs,
+            outs = all_output_files["outs"],
         ),
         DefaultInfo(files = depset(default_outputs)),
     ]
@@ -663,9 +668,9 @@ _kernel_build = rule(
             doc = "the kernel_config target",
         ),
         "srcs": attr.label_list(mandatory = True, doc = "kernel sources"),
-        "outs": attr.output_list(),
-        "module_outs": attr.output_list(doc = "output *.ko files"),
-        "implicit_outs": attr.output_list(doc = "Like `outs`, but not in dist"),
+        "outs": attr.string_list(),
+        "module_outs": attr.string_list(doc = "output *.ko files"),
+        "implicit_outs": attr.string_list(doc = "Like `outs`, but not in dist"),
         "_search_and_mv_output": attr.label(
             allow_single_file = True,
             default = Label("//build/kleaf:search_and_mv_output.py"),