From 96b21cc589349d346e4c587d92867ddee66f45dd Mon Sep 17 00:00:00 2001
From: Bart Van Assche <bvanassche@google.com>
Date: Thu, 12 Aug 2021 10:56:44 -0700
Subject: [PATCH] libdm: Make ExtractBlockDeviceName() return its result

From https://engdoc.corp.google.com/eng/doc/devguide/cpp/styleguide.md:
"Prefer using return values over output parameters: they improve
readability, and often provide the same or better performance (see the C++
Primer)." Implement this advice for ExtractBlockDeviceName(). This patch
does not change any functionality.

Bug: 194450129
Test: mm libfs_mgr libdm_test
Merged-In: I6363781163eba08e6128507b403200f472f68a59
Change-Id: I7d340b33281ebccded0836cd0b5a293e941f4043
Ignore-AOSP-First: Already in AOSP.
Signed-off-by: Bart Van Assche <bvanassche@google.com>
---
 fs_mgr/libdm/dm.cpp | 30 +++++++++++++-----------------
 1 file changed, 13 insertions(+), 17 deletions(-)

diff --git a/fs_mgr/libdm/dm.cpp b/fs_mgr/libdm/dm.cpp
index c4874b8d1..9a518c696 100644
--- a/fs_mgr/libdm/dm.cpp
+++ b/fs_mgr/libdm/dm.cpp
@@ -560,34 +560,30 @@ std::string DeviceMapper::GetTargetType(const struct dm_target_spec& spec) {
     return std::string{spec.target_type, sizeof(spec.target_type)};
 }
 
-static bool ExtractBlockDeviceName(const std::string& path, std::string* name) {
+static std::optional<std::string> ExtractBlockDeviceName(const std::string& path) {
     static constexpr std::string_view kDevBlockPrefix("/dev/block/");
     if (android::base::StartsWith(path, kDevBlockPrefix)) {
-        *name = path.substr(kDevBlockPrefix.length());
-        return true;
+        return path.substr(kDevBlockPrefix.length());
     }
-    return false;
+    return {};
 }
 
 bool DeviceMapper::IsDmBlockDevice(const std::string& path) {
-    std::string name;
-    if (!ExtractBlockDeviceName(path, &name)) {
-        return false;
-    }
-    return android::base::StartsWith(name, "dm-");
+    std::optional<std::string> name = ExtractBlockDeviceName(path);
+    return name && android::base::StartsWith(*name, "dm-");
 }
 
 std::optional<std::string> DeviceMapper::GetDmDeviceNameByPath(const std::string& path) {
-    std::string name;
-    if (!ExtractBlockDeviceName(path, &name)) {
+    std::optional<std::string> name = ExtractBlockDeviceName(path);
+    if (!name) {
         LOG(WARNING) << path << " is not a block device";
         return std::nullopt;
     }
-    if (!android::base::StartsWith(name, "dm-")) {
+    if (!android::base::StartsWith(*name, "dm-")) {
         LOG(WARNING) << path << " is not a dm device";
         return std::nullopt;
     }
-    std::string dm_name_file = "/sys/block/" + name + "/dm/name";
+    std::string dm_name_file = "/sys/block/" + *name + "/dm/name";
     std::string dm_name;
     if (!android::base::ReadFileToString(dm_name_file, &dm_name)) {
         PLOG(ERROR) << "Failed to read file " << dm_name_file;
@@ -598,16 +594,16 @@ std::optional<std::string> DeviceMapper::GetDmDeviceNameByPath(const std::string
 }
 
 std::optional<std::string> DeviceMapper::GetParentBlockDeviceByPath(const std::string& path) {
-    std::string name;
-    if (!ExtractBlockDeviceName(path, &name)) {
+    std::optional<std::string> name = ExtractBlockDeviceName(path);
+    if (!name) {
         LOG(WARNING) << path << " is not a block device";
         return std::nullopt;
     }
-    if (!android::base::StartsWith(name, "dm-")) {
+    if (!android::base::StartsWith(*name, "dm-")) {
         // Reached bottom of the device mapper stack.
         return std::nullopt;
     }
-    auto slaves_dir = "/sys/block/" + name + "/slaves";
+    auto slaves_dir = "/sys/block/" + *name + "/slaves";
     auto dir = std::unique_ptr<DIR, decltype(&closedir)>(opendir(slaves_dir.c_str()), closedir);
     if (dir == nullptr) {
         PLOG(ERROR) << "Failed to open: " << slaves_dir;