vold: Allow pre-configured device pairs to switch mountpoints

The use-case for this are devices with internal extended storage
(pseudo-SDCard emmc) and an actual microSD card reader.
In CM, we choose to use the microSD as primary storage, and leave
the internal partition mostly unused; some users usually come up with
imaginative and data-destroying ways of working around that.

This allows us to specify a pair of mountpoints which can be
switched from a user preference. For this vold.fstab:

-----
dev_mount sdcard /mnt/sdcard ...<path-to-microSD-sysfs>
dev_mount emmc /mnt/emmc ...<path-to-emmc-partition>
-----

We can add to build.prop:
"ro.vold.switchablepair=/mnt/sdcard,/mnt/emmc"

A persistent toggle (persist.sys.vold.switchexternal), controllable
through CMParts, can be presented to the user to allow choosing
something other than our default as primary storage.

Change-Id: I9559fa442c833e3168287d820b7b8347736abb15

1 files changed, 24 insertions, 0 deletions

diff --git a/Volume.cpp b/Volume.cpp
index 624c3f0..b38f3b5 100644
--- a/Volume.cpp
+++ b/Volume.cpp
@@ -114,6 +114,7 @@ static const char *stateToStr(int state) {
 }
 
 Volume::Volume(VolumeManager *vm, const char *label, const char *mount_point) {
+    char switchable[PROPERTY_VALUE_MAX];
     mVm = vm;
     mDebug = false;
     mLabel = strdup(label);
@@ -122,6 +123,29 @@ Volume::Volume(VolumeManager *vm, const char *label, const char *mount_point) {
     mCurrentlyMountedKdev = -1;
     mPartIdx = -1;
     mRetryMount = false;
+
+    property_get("persist.sys.vold.switchexternal", switchable, "0");
+    if (!strcmp(switchable,"1")) {
+        char *first, *second = NULL;
+        const char *delim = ",";
+
+        property_get("ro.vold.switchablepair", switchable, "");
+
+        if (!(first = strtok(switchable, delim))) {
+            SLOGE("Mount switch requested, but no switchable mountpoints found");
+            return;
+        } else if (!(second = strtok(NULL, delim))) {
+            SLOGE("Mount switch requested, but bad switchable mountpoints found");
+            return;
+        }
+        if (!strcmp(mount_point,first)) {
+                free(mMountpoint);
+                mMountpoint = strdup(second);
+        } else if (!strcmp(mount_point,second)) {
+                free(mMountpoint);
+                mMountpoint = strdup(first);
+        }
+    }
 }
 
 Volume::~Volume() {