init: add MountHandler property handler

Set properties dev.mnt.blk.<mount_point>=<device_block_class> for mount
and umount operations by setting up an Epoll handler to catch
EPOLLERR or EPOLLPRI signals when /proc/mounts is changed.  Only
update properties associated with block devices.  For the mount
point of /, use the designation of /root instead.

Can use the properties in init rc expansion like:

on property dev.mnt.blk.root=*
    write /sys/block/${dev.mnt.blk.root}/queue/read_ahead_kb ${boot_read_ahead_kb:-2048}

on property dev.mnt.blk.data=*
    write /sys/block/${dev.mnt.blk.data}/queue/read_ahead_kb ${boot_read_ahead_kb:-2048}

on late-fs
    setprop boot_read_ahead_kb 128
    write /sys/block/${dev.mnt.blk.root}/queue/read_ahead_kb ${boot_read_ahead_kb}
    write /sys/block/${dev.mnt.blk.data}/queue/read_ahead_kb ${boot_read_ahead_kb}

Test: boot and inspect getprop results.
Bug: 124072565
Change-Id: I1b8aff44f922ba372cd926de2919c215c40ee874

init/Android.bp

@@ -110,6 +110,7 @@ cc_library_static {
         "init.cpp",
         "keychords.cpp",
         "modalias_handler.cpp",
+        "mount_handler.cpp",
         "mount_namespace.cpp",
         "parser.cpp",
         "persistent_properties.cpp",

init/README.md

@@ -660,12 +660,19 @@ The below pseudocode may explain this more clearly:
 
 Properties
 ----------
-Init provides information about the services that it is responsible
-for via the below properties.
+Init provides state information with the following properties.
 
 `init.svc.<name>`
 > State of a named service ("stopped", "stopping", "running", "restarting")
 
+`dev.mnt.blk.<mount_point>`
+> Block device base name associated with a *mount_point*.
+  The *mount_point* has / replaced by . and if referencing the root mount point
+  "/", it will use "/root", specifically `dev.mnt.blk.root`.
+  Meant for references to `/sys/device/block/${dev.mnt.blk.<mount_point>}/` and
+  `/sys/fs/ext4/${dev.mnt.blk.<mount_point>}/` to tune the block device
+  characteristics in a device agnostic manner.
+
 
 Boot timing
 -----------

init/init.cpp

@@ -57,6 +57,7 @@
 #include "first_stage_mount.h"
 #include "import_parser.h"
 #include "keychords.h"
+#include "mount_handler.h"
 #include "mount_namespace.h"
 #include "property_service.h"
 #include "reboot.h"
@@ -686,6 +687,7 @@ int SecondStageMain(int argc, char** argv) {
     fs_mgr_vendor_overlay_mount_all();
     export_oem_lock_status();
     StartPropertyService(&epoll);
+    MountHandler mount_handler(&epoll);
     set_usb_controller();
 
     const BuiltinFunctionMap function_map;

init/mount_handler.cpp

@@ -0,0 +1,140 @@
+/*
+ * Copyright (C) 2019 The Android Open Source Project
+ *
+ * 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.
+ */
+
+#include "mount_handler.h"
+
+#include <ctype.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/epoll.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#include <algorithm>
+#include <string>
+#include <utility>
+
+#include <android-base/file.h>
+#include <android-base/logging.h>
+#include <android-base/properties.h>
+#include <android-base/strings.h>
+#include <fs_mgr.h>
+#include <fstab/fstab.h>
+
+#include "epoll.h"
+#include "property_service.h"
+
+namespace android {
+namespace init {
+
+namespace {
+
+MountHandlerEntry ParseMount(const std::string& line) {
+    auto fields = android::base::Split(line, " ");
+    while (fields.size() < 3) fields.emplace_back("");
+    if (fields[0] == "/dev/root") {
+        if (android::fs_mgr::Fstab fstab; android::fs_mgr::ReadDefaultFstab(&fstab)) {
+            if (auto entry = GetEntryForMountPoint(&fstab, "/")) {
+                fields[0] = entry->blk_device;
+            }
+        }
+    }
+    if (android::base::StartsWith(fields[0], "/dev/")) {
+        if (std::string link; android::base::Readlink(fields[0], &link)) {
+            fields[0] = link;
+        }
+    }
+    return MountHandlerEntry(fields[0], fields[1], fields[2]);
+}
+
+void SetMountProperty(const MountHandlerEntry& entry, bool add) {
+    static constexpr char devblock[] = "/dev/block/";
+    if (!android::base::StartsWith(entry.blk_device, devblock)) return;
+    std::string value;
+    if (add) {
+        value = entry.blk_device.substr(strlen(devblock));
+        if (android::base::StartsWith(value, "sd")) {
+            // All sd partitions inherit their queue characteristics
+            // from the whole device reference.  Strip partition number.
+            auto it = std::find_if(value.begin(), value.end(), [](char c) { return isdigit(c); });
+            if (it != value.end()) value.erase(it, value.end());
+        }
+        auto queue = "/sys/block/" + value + "/queue";
+        struct stat sb;
+        if (stat(queue.c_str(), &sb) || !S_ISDIR(sb.st_mode)) value = "";
+        if (stat(entry.mount_point.c_str(), &sb) || !S_ISDIR(sb.st_mode)) value = "";
+        // Skip the noise associated with APEX until there is a need
+        if (android::base::StartsWith(value, "loop")) value = "";
+    }
+    std::string property =
+            "dev.mnt.blk" + ((entry.mount_point == "/") ? "/root" : entry.mount_point);
+    std::replace(property.begin(), property.end(), '/', '.');
+    if (value.empty() && android::base::GetProperty(property, "").empty()) return;
+    property_set(property, value);
+}
+
+}  // namespace
+
+MountHandlerEntry::MountHandlerEntry(const std::string& blk_device, const std::string& mount_point,
+                                     const std::string& fs_type)
+    : blk_device(blk_device), mount_point(mount_point), fs_type(fs_type) {}
+
+bool MountHandlerEntry::operator<(const MountHandlerEntry& r) const {
+    if (blk_device < r.blk_device) return true;
+    if (blk_device > r.blk_device) return false;
+    if (mount_point < r.mount_point) return true;
+    if (mount_point > r.mount_point) return false;
+    return fs_type < r.fs_type;
+}
+
+MountHandler::MountHandler(Epoll* epoll) : epoll_(epoll), fp_(fopen("/proc/mounts", "re"), fclose) {
+    if (!fp_) PLOG(FATAL) << "Could not open /proc/mounts";
+    auto result = epoll->RegisterHandler(
+            fileno(fp_.get()), [this]() { this->MountHandlerFunction(); }, EPOLLERR | EPOLLPRI);
+    if (!result) LOG(FATAL) << result.error();
+}
+
+MountHandler::~MountHandler() {
+    if (fp_) epoll_->UnregisterHandler(fileno(fp_.get())).IgnoreError();
+}
+
+void MountHandler::MountHandlerFunction() {
+    rewind(fp_.get());
+    char* buf = nullptr;
+    size_t len = 0;
+    auto untouched = mounts_;
+    while (getline(&buf, &len, fp_.get()) != -1) {
+        auto entry = ParseMount(std::string(buf, len));
+        auto match = untouched.find(entry);
+        if (match == untouched.end()) {
+            SetMountProperty(entry, true);
+            mounts_.emplace(std::move(entry));
+        } else {
+            untouched.erase(match);
+        }
+    }
+    free(buf);
+    for (auto entry : untouched) {
+        auto match = mounts_.find(entry);
+        if (match == mounts_.end()) continue;
+        mounts_.erase(match);
+        SetMountProperty(entry, false);
+    }
+}
+
+}  // namespace init
+}  // namespace android

init/mount_handler.h

@@ -0,0 +1,59 @@
+/*
+ * Copyright (C) 2019 The Android Open Source Project
+ *
+ * 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.
+ */
+
+#pragma once
+
+#include <stdio.h>
+
+#include <memory>
+#include <set>
+#include <string>
+
+#include "epoll.h"
+
+namespace android {
+namespace init {
+
+struct MountHandlerEntry {
+    MountHandlerEntry(const std::string& blk_device, const std::string& mount_point,
+                      const std::string& fs_type);
+
+    bool operator<(const MountHandlerEntry& r) const;
+
+    const std::string blk_device;
+    const std::string mount_point;
+    const std::string fs_type;
+};
+
+class MountHandler {
+  public:
+    explicit MountHandler(Epoll* epoll);
+    MountHandler(const MountHandler&) = delete;
+    MountHandler(MountHandler&&) = delete;
+    MountHandler& operator=(const MountHandler&) = delete;
+    MountHandler& operator=(MountHandler&&) = delete;
+    ~MountHandler();
+
+  private:
+    void MountHandlerFunction();
+
+    Epoll* epoll_;
+    std::unique_ptr<FILE, decltype(&fclose)> fp_;
+    std::set<MountHandlerEntry> mounts_;
+};
+
+}  // namespace init
+}  // namespace android