SOURCE_COPY operation: implement src == dst

Helper function to compare the source and destination extents of a
SOURCE_COPY InstallOperation.
The function returns true iff source and destination are identical with
the use of std::equal().

Bug: 141207436
Test: build
Change-Id: I146aeba1c8ede35f21cfef8e21d4af62274bda84
Signed-off-by: Alessio Balsini <balsini@google.com>

fs_mgr/libsnapshot/include/libsnapshot/snapshot.h

@@ -72,6 +72,8 @@ class SnapshotStatus;
 
 static constexpr const std::string_view kCowGroupName = "cow";
 
+bool SourceCopyOperationIsClone(const chromeos_update_engine::InstallOperation& operation);
+
 enum class UpdateState : unsigned int {
     // No update or merge is in progress.
     None,

fs_mgr/libsnapshot/partition_cow_creator.cpp

@@ -62,6 +62,19 @@ bool PartitionCowCreator::HasExtent(Partition* p, Extent* e) {
     return false;
 }
 
+bool SourceCopyOperationIsClone(const InstallOperation& operation) {
+    using ChromeOSExtent = chromeos_update_engine::Extent;
+    if (operation.src_extents().size() != operation.dst_extents().size()) {
+        return false;
+    }
+    return std::equal(operation.src_extents().begin(), operation.src_extents().end(),
+                      operation.dst_extents().begin(),
+                      [](const ChromeOSExtent& src, const ChromeOSExtent& dst) {
+                          return src.start_block() == dst.start_block() &&
+                                 src.num_blocks() == dst.num_blocks();
+                      });
+}
+
 void WriteExtent(DmSnapCowSizeCalculator* sc, const chromeos_update_engine::Extent& de,
                  unsigned int sectors_per_block) {
     const auto block_boundary = de.start_block() + de.num_blocks();
@@ -88,6 +101,11 @@ uint64_t PartitionCowCreator::GetCowSize() {
     if (operations == nullptr) return sc.cow_size_bytes();
 
     for (const auto& iop : *operations) {
+        // Do not allocate space for operations that are going to be skipped
+        // during OTA application.
+        if (iop.type() == InstallOperation::SOURCE_COPY && SourceCopyOperationIsClone(iop))
+            continue;
+
         for (const auto& de : iop.dst_extents()) {
             WriteExtent(&sc, de, sectors_per_block);
         }