snapuserd: Move GetNumSectors call to snapuserd_server.

Calling this in snapuserd_core fails when the base path is a regular
file. Since the value is only read once, just call it from
snapuserd_server instead, which also means we don't have to add an
S_ISBLK check here.

Bug: 288273605
Test: snapuserd_test
Change-Id: Ic26bf807b24611f2d97829d1b4eb1d0ede2feb6a

fs_mgr/libsnapshot/snapuserd/include/snapuserd/snapuserd_kernel.h

@@ -40,6 +40,7 @@ static constexpr uint32_t SNAPSHOT_VALID = 1;
  * multiple of 512 bytes. Hence these two constants.
  */
 static constexpr uint32_t SECTOR_SHIFT = 9;
+static constexpr uint64_t SECTOR_SIZE = (1ULL << SECTOR_SHIFT);
 
 static constexpr size_t BLOCK_SZ = 4096;
 static constexpr size_t BLOCK_SHIFT = (__builtin_ffs(BLOCK_SZ) - 1);

fs_mgr/libsnapshot/snapuserd/user-space-merge/snapuserd_core.cpp

@@ -280,20 +280,6 @@ bool SnapshotHandler::InitCowDevice() {
         return false;
     }
 
-    unique_fd fd(TEMP_FAILURE_RETRY(open(base_path_merge_.c_str(), O_RDONLY | O_CLOEXEC)));
-    if (fd < 0) {
-        SNAP_LOG(ERROR) << "Cannot open block device";
-        return false;
-    }
-
-    uint64_t dev_sz = get_block_device_size(fd.get());
-    if (!dev_sz) {
-        SNAP_LOG(ERROR) << "Failed to find block device size: " << base_path_merge_;
-        return false;
-    }
-
-    num_sectors_ = dev_sz >> SECTOR_SHIFT;
-
     return ReadMetadata();
 }
 
@@ -460,5 +446,21 @@ void SnapshotHandler::FreeResources() {
     merge_thread_ = nullptr;
 }
 
+uint64_t SnapshotHandler::GetNumSectors() const {
+    unique_fd fd(TEMP_FAILURE_RETRY(open(base_path_merge_.c_str(), O_RDONLY | O_CLOEXEC)));
+    if (fd < 0) {
+        SNAP_LOG(ERROR) << "Cannot open base path: " << base_path_merge_;
+        return false;
+    }
+
+    uint64_t dev_sz = get_block_device_size(fd.get());
+    if (!dev_sz) {
+        SNAP_LOG(ERROR) << "Failed to find block device size: " << base_path_merge_;
+        return false;
+    }
+
+    return dev_sz / SECTOR_SIZE;
+}
+
 }  // namespace snapshot
 }  // namespace android

fs_mgr/libsnapshot/snapuserd/user-space-merge/snapuserd_core.h

@@ -109,7 +109,7 @@ class SnapshotHandler : public std::enable_shared_from_this<SnapshotHandler> {
 
     const std::string& GetControlDevicePath() { return control_device_; }
     const std::string& GetMiscName() { return misc_name_; }
-    const uint64_t& GetNumSectors() { return num_sectors_; }
+    uint64_t GetNumSectors() const;
     const bool& IsAttached() const { return attached_; }
     void AttachControlDevice() { attached_ = true; }
 
@@ -202,8 +202,6 @@ class SnapshotHandler : public std::enable_shared_from_this<SnapshotHandler> {
 
     unique_fd cow_fd_;
 
-    uint64_t num_sectors_;
-
     std::unique_ptr<CowReader> reader_;
 
     // chunk_vec stores the pseudo mapping of sector

fs_mgr/libsnapshot/snapuserd/user-space-merge/snapuserd_server.cpp

@@ -130,7 +130,12 @@ bool UserSnapshotServer::Receivemsg(android::base::borrowed_fd fd, const std::st
             return Sendmsg(fd, "fail");
         }
 
-        auto retval = "success," + std::to_string(handler->snapuserd()->GetNumSectors());
+        auto num_sectors = handler->snapuserd()->GetNumSectors();
+        if (!num_sectors) {
+            return Sendmsg(fd, "fail");
+        }
+
+        auto retval = "success," + std::to_string(num_sectors);
         return Sendmsg(fd, retval);
     } else if (cmd == "start") {
         // Message format:

fs_mgr/libsnapshot/snapuserd/user-space-merge/snapuserd_test.cpp

@@ -540,7 +540,9 @@ void SnapuserdTest::InitCowDevice() {
                                  base_loop_->device(), 1, use_iouring, false);
     ASSERT_NE(handler, nullptr);
     ASSERT_NE(handler->snapuserd(), nullptr);
+#ifdef __ANDROID__
     ASSERT_NE(handler->snapuserd()->GetNumSectors(), 0);
+#endif
 }
 
 void SnapuserdTest::SetDeviceControlName() {