move lookupTaskBin to executor_linux, for os dependency clarity

drivers/shared/executor/executor_linux.go

@@ -17,6 +17,7 @@ import (
 	"github.com/hashicorp/consul-template/signals"
 	hclog "github.com/hashicorp/go-hclog"
 	multierror "github.com/hashicorp/go-multierror"
+	"github.com/hashicorp/nomad/client/allocdir"
 	"github.com/hashicorp/nomad/client/stats"
 	cstructs "github.com/hashicorp/nomad/client/structs"
 	shelpers "github.com/hashicorp/nomad/helper/stats"
@@ -770,3 +771,43 @@ func cmdMounts(mounts []*drivers.MountConfig) []*lconfigs.Mount {
 
 	return r
 }
+
+// lookupTaskBin finds the file `bin` in taskDir/local, taskDir in that order, then performs
+// a PATH search inside taskDir. It returns an absolute path. See also executor.lookupBin
+func lookupTaskBin(taskDir string, bin string) (string, error) {
+	// Check in the local directory
+	localDir := filepath.Join(taskDir, allocdir.TaskLocal)
+	local := filepath.Join(localDir, bin)
+	if _, err := os.Stat(local); err == nil {
+		return local, nil
+	}
+
+	// Check at the root of the task's directory
+	root := filepath.Join(taskDir, bin)
+	if _, err := os.Stat(root); err == nil {
+		return root, nil
+	}
+
+	// Check the host PATH anchored inside the taskDir
+	if !strings.Contains(bin, "/") {
+		envPath := os.Getenv("PATH")
+		for _, dir := range filepath.SplitList(envPath) {
+			if dir == "" {
+				// match unix shell behavior, empty path element == .
+				dir = "."
+			}
+			for _, root := range []string{localDir, taskDir} {
+				path := filepath.Join(root, dir, bin)
+				f, err := os.Stat(path)
+				if err != nil {
+					continue
+				}
+				if m := f.Mode(); !m.IsDir() {
+					return path, nil
+				}
+			}
+		}
+	}
+
+	return "", fmt.Errorf("file %s not found under path %s", bin, taskDir)
+}