android P监听SD卡热插拔执行symlink软链接的实现

收到个android P项目的需求，要求监听SD卡hotplug热插拔，执行symlink软链接和unlink。

先看下运行效果：

拔SD卡时打印:

ja310_evb:/ # logcat |grep pis
08-05 10:08:00.386  2249  2263 W vold    : pis get kernel block event!!!
08-05 10:08:00.386  2249  2263 W vold    : pis VolumeManager->handleBlockEvent entry===
08-05 10:08:00.391  2249  2263 W vold    : pis get kernel block event!!!
08-05 10:08:00.391  2249  2263 W vold    : pis VolumeManager->handleBlockEvent entry===
08-05 10:08:00.392  2249  2263 W vold    : pis VolumeManager->handleBlockEvent remove action===
08-05 10:08:00.392  2249  2263 W vold    : **pis PublicVolume.cpp doUnmount entry**....
08-05 10:08:00.396  2541  3000 E StorageManagerService: pis StorageManagerService.ajva  onVolumeStateChanged...
08-05 10:08:01.222  2541  3000 E StorageManagerService: pis StorageManagerService.ajva  onVolumeStateChanged...
08-05 10:08:01.226  2541  3000 E StorageManagerService: pis StorageManagerService.ajva  onVolumeStateChanged...

再插入SD卡时打印：

08-05 10:08:49.965  2249  2263 W vold    : pis get kernel block event!!!
08-05 10:08:49.965  2249  2263 W vold    : pis VolumeManager->handleBlockEvent entry===
08-05 10:08:49.965  2249  2263 W vold    : pis VolumeManager->handleBlockEvent add action===
08-05 10:08:50.004  2249  2263 W vold    : pis get kernel block event!!!
08-05 10:08:50.004  2249  2263 W vold    : pis VolumeManager->handleBlockEvent entry===
08-05 10:08:50.004  2541  3000 E StorageManagerService: pis StorageManagerService.ajva  onVolumeStateChanged...
08-05 10:08:50.005  2249  2294 W vold    : **pis PublicVolume.cpp doMount entry**....
08-05 10:08:50.005  2541  3000 E StorageManagerService: pis StorageManagerService.ajva  onVolumeStateChanged...
08-05 10:08:50.336  2249  2294 E vold    : pis Linking /storage/sdcard0 to /storage/Tfcard start...
08-05 10:08:50.336  2249  2294 E vold    : pis Linking /storage/sdcard0 to /storage/Tfcard success
08-05 10:08:50.337  2541  2552 E StorageManagerService: pis StorageManagerService.ajva  onVolumeStateChanged...

软链接效果：

# ls -l /storage/Tfcard
lrwxrwxrwx 1 root reserved_disk 16 2021-08-05 10:08 /storage/Tfcard -> /storage/sdcard0

好了，以上这些logcat打印日志，全是手动添加上去的，修改记录如下：

framework侧的修改

diff --git a/services/core/java/com/android/server/StorageManagerService.java b/services/core/java/com/android/server/StorageManagerService.java
index 183be9b..82d3189 100644
--- a/services/core/java/com/android/server/StorageManagerService.java
+++ b/services/core/java/com/android/server/StorageManagerService.java
@@ -1064,6 +1064,7 @@ class StorageManagerService extends IStorageManager.Stub@Overridepublic void onVolumeStateChanged(String volId, int state) {+                       Slog.e(TAG, "pis StorageManagerService.ajva  onVolumeStateChanged...");synchronized (mLock) {final VolumeInfo vol = mVolumes.get(volId);if (vol != null) {

system/vold修改：

system/vold$ git diff .
diff --git a/NetlinkHandler.cpp b/NetlinkHandler.cpp
index 92131e9..0c9b6ce 100644
--- a/NetlinkHandler.cpp
+++ b/NetlinkHandler.cpp
@@ -50,6 +50,7 @@ void NetlinkHandler::onEvent(NetlinkEvent *evt) {}if (std::string(subsys) == "block") {+        LOG(WARNING) << "pis get kernel block event!!!";vm->handleBlockEvent(evt);}}
diff --git a/VolumeManager.cpp b/VolumeManager.cpp
index 8c32587..b3aea43 100644
--- a/VolumeManager.cpp
+++ b/VolumeManager.cpp
@@ -175,6 +175,7 @@ int VolumeManager::stop() {void VolumeManager::handleBlockEvent(NetlinkEvent *evt) {std::lock_guard<std::mutex> lock(mLock);
+    LOG(WARNING) << "pis VolumeManager->handleBlockEvent entry===";if (mDebug) {LOG(VERBOSE) << "----------------";
@@ -193,6 +194,7 @@ void VolumeManager::handleBlockEvent(NetlinkEvent *evt) {switch (evt->getAction()) {case NetlinkEvent::Action::kAdd: {+        LOG(WARNING) << "pis VolumeManager->handleBlockEvent add action===";for (const auto& source : mDiskSources) {if (source->matches(eventPath)) {// For now, assume that MMC and virtio-blk (the latter is
@@ -217,11 +219,13 @@ void VolumeManager::handleBlockEvent(NetlinkEvent *evt) {break;}case NetlinkEvent::Action::kChange: {+        LOG(WARNING) << "pis VolumeManager->handleBlockEvent changed action===";LOG(DEBUG) << "Disk at " << major << ":" << minor << " changed";handleDiskChanged(device);break;}case NetlinkEvent::Action::kRemove: {+        LOG(WARNING) << "pis VolumeManager->handleBlockEvent remove action===";handleDiskRemoved(device);break;}
diff --git a/model/PublicVolume.cpp b/model/PublicVolume.cpp
index fc7e96f..438a7f6 100644
--- a/model/PublicVolume.cpp
+++ b/model/PublicVolume.cpp
@@ -94,6 +94,8 @@ status_t PublicVolume::doDestroy() {}status_t PublicVolume::doMount() {+    LOG(WARNING) << "pis PublicVolume.cpp doMount entry....";
+readMetadata();if (mFsType == "vfat" && vfat::IsSupported()) {@@ -214,6 +216,14 @@ status_t PublicVolume::doMount() {if (TEMP_FAILURE_RETRY(waitpid(mFusePid, nullptr, WNOHANG)) == mFusePid)mFusePid = 0;+    LOG(ERROR) << "pis Linking /storage/sdcard0 to /storage/Tfcard start...";
+    if (TEMP_FAILURE_RETRY(symlink("/storage/sdcard0", "/storage/Tfcard"))) {+        PLOG(WARNING) << "pis Failed to link";
+        return -errno;
+    } else {+        LOG(ERROR) << "pis Linking /storage/sdcard0 to /storage/Tfcard success";
+    }
+return OK;}@@ -222,6 +232,7 @@ status_t PublicVolume::doUnmount() {// the FUSE process first, most file system operations will return// ENOTCONN until the unmount completes. This is an exotic and unusual// error code and might cause broken behaviour in applications.
+    LOG(WARNING) << "pis PublicVolume.cpp doUnmount entry....";KillProcessesUsingPath(getPath());ForceUnmount(kAsecPath);
@@ -247,6 +258,10 @@ status_t PublicVolume::doUnmount() {mFuseWrite.clear();mRawPath.clear();+    if (TEMP_FAILURE_RETRY(unlink("/storage/Tfcard"))) {+        LOG(ERROR) << "pis unlink /storage/sdcard0 to /storage/Tfcard error";
+    }
+return OK;}

以上是所有的修改，关于SD卡的插拔思路，CSDN上有很多文章都介绍过了，我这里就不做过多的介绍，主要的思想就是，监听底层上报的netlinker事件，然后针对Kernel上报的消息内容，和fstab中SD卡的内容进行比对，找到合适的节点内容。

通过vold来做中间者，上报消息给framework，因为framework侧是通过注册消息，收到SD卡的状态变更消息时，就可以告知所有的注册者，至于其他app或者服务进程收到状态变更的广播，原理也是类似的，注册相关消息，storageManagerService收到变更消息，群发状态给所有的注册者，进行函数回调。

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