[engine] Resolve relative paths correctly

When running `shac check --root <other-dir>`, relative paths of
executables run by `ctx.os.exec()` should be resolved relative to the
root directory, not the current working directory (which may differ from
the root directory).

Change-Id: I8ddf2169b7a51a975890852aa7292dc30d93a8b0
Reviewed-on: https://fuchsia-review.googlesource.com/c/shac-project/shac/+/909854
Reviewed-by: Marc-Antoine Ruel <[email protected]>
Fuchsia-Auto-Submit: Oliver Newman <[email protected]>
Commit-Queue: Auto-Submit <[email protected]>

internal/engine/run_test.go

@@ -1664,6 +1664,10 @@ func TestTestDataPrint(t *testing.T) {
 			"ctx-os-exec-parallel.star",
 			strings.Repeat("[//ctx-os-exec-parallel.star:27] Hello, world\n", 10),
 		},
+		{
+			"ctx-os-exec-relpath.star",
+			"[//ctx-os-exec-relpath.star:17] Hello from a nested file\n\n",
+		},
 		{
 			"ctx-os-exec-stdin.star",
 			"[//ctx-os-exec-stdin.star:30] stdout given NoneType for stdin:\n" +

internal/engine/runtime_ctx_os.go

@@ -286,13 +286,24 @@ func ctxOsExec(ctx context.Context, s *shacState, name string, args starlark.Tup
 		exeParts[0] = s.root
 		fullCmd[0] = strings.Join(exeParts, string(os.PathSeparator))
 	} else {
-		// nsjail doesn't do $PATH-based resolution of the command it's given, so it
-		// must either be an absolute or relative path. Do this resolution
-		// unconditionally for consistency across platforms even though it's not
-		// necessary when not using nsjail.
-		fullCmd[0], err = exec.LookPath(fullCmd[0])
-		if err != nil && !errors.Is(err, exec.ErrDot) {
-			return nil, err
+		// nsjail doesn't do $PATH-based resolution of the command it's
+		// given, so it must either be an absolute or relative path. Do this
+		// resolution unconditionally for consistency across platforms even
+		// though it's not necessary when not using nsjail.
+		if strings.Contains(fullCmd[0], "/") && !filepath.IsAbs(fullCmd[0]) {
+			// Make the path absolute if it's relative and contains slashes.
+			// We can't use exec.LookPath in this case because it tries to
+			// evaluate the path relative to the cwd, but we want to evaluate it
+			// relative to the shac.star file being evaluated.
+			//
+			// filepath.Join ignores empty elements, so s.subdir can be included
+			// unconditionally.
+			fullCmd[0] = filepath.Join(s.root, s.subdir, fullCmd[0])
+		} else {
+			fullCmd[0], err = exec.LookPath(fullCmd[0])
+			if err != nil && !errors.Is(err, exec.ErrDot) {
+				return nil, err
+			}
 		}
 	}
 

internal/engine/testdata/print/ctx-os-exec-relpath.star

@@ -0,0 +1,23 @@
+# Copyright 2023 The Shac Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+def cb(ctx):
+  ext = "bat" if ctx.platform.os == "windows" else "sh"
+  print(ctx.os.exec(
+    # Implicit relative path should be evaluated relative to the calling
+    # Starlark file.
+    ["dir/nested_hello_world." + ext]
+  ).wait().stdout)
+
+shac.register_check(cb)

internal/engine/testdata/print/dir/nested_hello_world.bat

@@ -0,0 +1,16 @@
+@echo off
+:: Copyright 2023 The Shac Authors
+::
+:: Licensed under the Apache License, Version 2.0 (the "License");
+:: you may not use this file except in compliance with the License.
+:: You may obtain a copy of the License at
+::
+::     http://www.apache.org/licenses/LICENSE-2.0
+::
+:: Unless required by applicable law or agreed to in writing, software
+:: distributed under the License is distributed on an "AS IS" BASIS,
+:: WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+:: See the License for the specific language governing permissions and
+:: limitations under the License.
+
+echo Hello from a nested file

internal/engine/testdata/print/dir/nested_hello_world.sh

@@ -0,0 +1,16 @@
+#!/bin/sh
+# Copyright 2023 The Shac Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+echo "Hello from a nested file"