fs_mgr: Add fs_mgr_overlayfs_is_setup()

Provide a means to determine if overlayfs is currently active with
fs_mgr_overlayfs_is_setup().  Search for current clues in the system
that the overrides are in play.  If /mnt/scratch/ is mounted, that
may be enough of a smoking gun because the resource itself may
interfere with operations.

Related to update_engine calling this function to reject performing
an OTA as its operations are incompatible with the overrides.

Test: adb-remount-test.sh
Bug: 120034852
Change-Id: I641163b1559c1a83fe7d0c336d04c10e9956ad3a

fs_mgr/README.overlayfs.md

@@ -89,6 +89,12 @@ Caveats
   if higher than 4.6.
 - *adb enable-verity* will free up overlayfs and as a bonus the
   device will be reverted pristine to before any content was updated.
+  Update engine does not take advantage of this, will perform a full OTA.
+- Update engine will not run if *fs_mgr_overlayfs_is_setup*() reports
+  true as adb remount overrides are incompatable with an OTA for
+  multiple reasons.
+  NB: This is not a problem for fastbootd or recovery as overrides are
+  disabled for those special boot scenarios.
 - If dynamic partitions runs out of space, resizing a logical
   partition larger may fail because of the scratch partition.
   If this happens, either fastboot flashall or adb enable-verity can

fs_mgr/fs_mgr_overlayfs.cpp

@@ -96,6 +96,10 @@ bool fs_mgr_overlayfs_teardown(const char*, bool* change) {
     return false;
 }
 
+bool fs_mgr_overlayfs_is_setup() {
+    return false;
+}
+
 #else  // ALLOW_ADBD_DISABLE_VERITY == 0
 
 namespace {
@@ -924,6 +928,17 @@ bool fs_mgr_overlayfs_teardown(const char* mount_point, bool* change) {
     return ret;
 }
 
+bool fs_mgr_overlayfs_is_setup() {
+    if (fs_mgr_overlayfs_already_mounted(kScratchMountPoint, false)) return true;
+    std::unique_ptr<fstab, decltype(&fs_mgr_free_fstab)> fstab(fs_mgr_read_fstab_default(),
+                                                               fs_mgr_free_fstab);
+    if (fs_mgr_overlayfs_invalid(fstab.get())) return false;
+    for (const auto& mount_point : fs_mgr_candidate_list(fstab.get())) {
+        if (fs_mgr_overlayfs_already_mounted(mount_point)) return true;
+    }
+    return false;
+}
+
 #endif  // ALLOW_ADBD_DISABLE_VERITY != 0
 
 bool fs_mgr_has_shared_blocks(const std::string& mount_point, const std::string& dev) {

fs_mgr/include/fs_mgr_overlayfs.h

@@ -29,6 +29,7 @@ std::vector<std::string> fs_mgr_overlayfs_required_devices(
 bool fs_mgr_overlayfs_setup(const char* backing = nullptr, const char* mount_point = nullptr,
                             bool* change = nullptr);
 bool fs_mgr_overlayfs_teardown(const char* mount_point = nullptr, bool* change = nullptr);
+bool fs_mgr_overlayfs_is_setup();
 bool fs_mgr_has_shared_blocks(const std::string& mount_point, const std::string& dev);
 std::string fs_mgr_get_context(const std::string& mount_point);