[devfs] Move walk method out of Devnode into test

Devnode::walk was only used in the one test file; the function is
brittle (see https://fxbug.dev/112401 for instance) and we would
like to avoid accidental future dependencies on it.

Change-Id: I5da1b2af748087a7c01ee5df0a7b05f55d92a75f
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/773664
Reviewed-by: David Gilhooley <[email protected]>
Commit-Queue: Auto-Submit <[email protected]>
Fuchsia-Auto-Submit: Ian Fisher <[email protected]>
Reviewed-by: Tamir Duberstein <[email protected]>

Author: Ian Fisher

src/devices/bin/driver_manager/devfs/devfs.cc

@@ -88,30 +88,6 @@ void Devnode::advertise_modified() {
   parent_->Notify(name(), fio::wire::WatchEvent::kAdded);
 }
 
-zx::result<Devnode*> Devnode::walk(std::string_view path) {
-  Devnode* dn = this;
-
-  while (!path.empty()) {
-    const size_t i = path.find('/');
-    if (i == 0) {
-      return zx::error(ZX_ERR_BAD_PATH);
-    }
-    std::string_view name = path;
-    if (i != std::string::npos) {
-      name = path.substr(0, i);
-      path = path.substr(i + 1);
-    } else {
-      path = {};
-    }
-    fbl::RefPtr<fs::Vnode> out;
-    if (const zx_status_t status = dn->children().Lookup(name, &out); status != ZX_OK) {
-      return zx::error(status);
-    }
-    dn = &fbl::RefPtr<Devnode::VnodeImpl>::Downcast(out)->holder_;
-  }
-  return zx::ok(dn);
-}
-
 Devnode::VnodeImpl::VnodeImpl(Devnode& holder, Target target)
     : holder_(holder), target_(std::move(target)) {}
 

src/devices/bin/driver_manager/devfs/devfs.h

@@ -59,8 +59,6 @@ class Devnode {
   Devnode(Devnode&&) = delete;
   Devnode& operator=(Devnode&&) = delete;
 
-  zx::result<Devnode*> walk(std::string_view path);
-
   // Add a child to this devnode with a given `name`, `protocol`, and `remote.
   // The child will be constructed in `out_child`.
   zx_status_t add_child(std::string_view name, uint32_t protocol, Remote remote,

src/devices/bin/driver_manager/tests/composite_device_tests.cc

@@ -393,6 +393,36 @@ void CompositeAddOrderTestCase::ExecuteTest(AddLocation add) {
                                                  std::size(device_indexes), fragment_device_indexes,
                                                  &composite));
 }
+
+zx::result<Devnode*> resolve_path(Devnode* dn, std::string_view path) {
+  // This function does not work with /dev/class entries due to the downcast to Devnode::VnodeImpl.
+  if (cpp20::starts_with(path, "class/")) {
+    ADD_FAILURE("resolve_path does not work with /dev/class entry: %.*s",
+                static_cast<int>(path.size()), path.data());
+    return zx::error(ZX_ERR_INVALID_ARGS);
+  }
+
+  while (!path.empty()) {
+    const size_t i = path.find('/');
+    if (i == 0) {
+      return zx::error(ZX_ERR_BAD_PATH);
+    }
+    std::string_view name = path;
+    if (i != std::string::npos) {
+      name = path.substr(0, i);
+      path = path.substr(i + 1);
+    } else {
+      path = {};
+    }
+    fbl::RefPtr<fs::Vnode> out;
+    if (const zx_status_t status = dn->children().Lookup(name, &out); status != ZX_OK) {
+      return zx::error(status);
+    }
+    dn = &fbl::RefPtr<Devnode::VnodeImpl>::Downcast(out)->holder_;
+  }
+  return zx::ok(dn);
+}
+
 TEST_F(CompositeAddOrderTestCase, DefineBeforeDevices) {
   ASSERT_NO_FATAL_FAILURE(ExecuteTest(AddLocation::BEFORE));
 }
@@ -933,9 +963,9 @@ TEST_F(CompositeTestCase, Topology) {
     ASSERT_OK(path.status_value());
 
     const std::string_view parent_topological_path{path.value().c_str() + kDev.size()};
-    zx::result parent = root.walk(parent_topological_path);
+    zx::result parent = resolve_path(&root, parent_topological_path);
     ASSERT_OK(parent);
-    zx::result child = parent.value()->walk(kCompositeDevName);
+    zx::result child = resolve_path(parent.value(), kCompositeDevName);
     if (i != 0) {
       // This isn't the primary parent; the composite device isn't a child node.
       ASSERT_STATUS(child.status_value(), ZX_ERR_NOT_FOUND);