libsnapshot: Implement CowWriterV3::EmitzeroBlocks.

Add zero block operation to writer v3. We currently write this operation
to after the scratch space. Once resume point support is added we may
need to change this to write after the resume point. This CL only
supports zero blocks, so writeoperation does not take in data yet.

Bug: 307452468
Test: cow_api_test
Change-Id: I659b2e2e4f6e0d96e374ed29012318cc34b4158d
diff --git a/fs_mgr/libsnapshot/include/libsnapshot/cow_reader.h b/fs_mgr/libsnapshot/include/libsnapshot/cow_reader.h
index 4cbcab1..755a3af 100644
--- a/fs_mgr/libsnapshot/include/libsnapshot/cow_reader.h
+++ b/fs_mgr/libsnapshot/include/libsnapshot/cow_reader.h
@@ -145,6 +145,7 @@
                      size_t ignore_bytes = 0) override;
 
     CowHeader& GetHeader() override { return header_; }
+    const CowHeaderV3& header_v3() const { return header_; }
 
     bool GetRawBytes(const CowOperation* op, void* buffer, size_t len, size_t* read);
     bool GetRawBytes(uint64_t offset, void* buffer, size_t len, size_t* read);
diff --git a/fs_mgr/libsnapshot/include/libsnapshot/cow_writer.h b/fs_mgr/libsnapshot/include/libsnapshot/cow_writer.h
index 3016e93..5b1e56c 100644
--- a/fs_mgr/libsnapshot/include/libsnapshot/cow_writer.h
+++ b/fs_mgr/libsnapshot/include/libsnapshot/cow_writer.h
@@ -54,6 +54,9 @@
 
     // Batch write cluster ops
     bool batch_write = false;
+
+    // Size of the cow operation buffer; used in v3 only.
+    uint32_t op_count_max = 0;
 };
 
 // Interface for writing to a snapuserd COW. All operations are ordered; merges
diff --git a/fs_mgr/libsnapshot/libsnapshot_cow/test_v3.cpp b/fs_mgr/libsnapshot/libsnapshot_cow/test_v3.cpp
index a07ced5..07c1d4f 100644
--- a/fs_mgr/libsnapshot/libsnapshot_cow/test_v3.cpp
+++ b/fs_mgr/libsnapshot/libsnapshot_cow/test_v3.cpp
@@ -89,5 +89,36 @@
     ASSERT_EQ(header.cluster_ops, 0);
 }
 
+TEST_F(CowTestV3, ZeroOp) {
+    CowOptions options;
+    options.op_count_max = 20;
+    auto writer = CreateCowWriter(3, options, GetCowFd());
+    ASSERT_TRUE(writer->AddZeroBlocks(1, 2));
+    ASSERT_TRUE(writer->Finalize());
+
+    CowReader reader;
+    ASSERT_TRUE(reader.Parse(cow_->fd));
+    ASSERT_EQ(reader.header_v3().op_count, 2);
+
+    auto iter = reader.GetOpIter();
+    ASSERT_NE(iter, nullptr);
+    ASSERT_FALSE(iter->AtEnd());
+
+    auto op = iter->Get();
+    ASSERT_EQ(op->type, kCowZeroOp);
+    ASSERT_EQ(op->data_length, 0);
+    ASSERT_EQ(op->new_block, 1);
+    ASSERT_EQ(op->source_info, 0);
+
+    iter->Next();
+    ASSERT_FALSE(iter->AtEnd());
+    op = iter->Get();
+
+    ASSERT_EQ(op->type, kCowZeroOp);
+    ASSERT_EQ(op->data_length, 0);
+    ASSERT_EQ(op->new_block, 2);
+    ASSERT_EQ(op->source_info, 0);
+}
+
 }  // namespace snapshot
 }  // namespace android
diff --git a/fs_mgr/libsnapshot/libsnapshot_cow/writer_base.h b/fs_mgr/libsnapshot/libsnapshot_cow/writer_base.h
index 709b248..5274456 100644
--- a/fs_mgr/libsnapshot/libsnapshot_cow/writer_base.h
+++ b/fs_mgr/libsnapshot/libsnapshot_cow/writer_base.h
@@ -62,6 +62,8 @@
     bool InitFd();
     bool ValidateNewBlock(uint64_t new_block);
 
+    bool IsEstimating() const { return is_dev_null_; }
+
     CowOptions options_;
 
     android::base::unique_fd fd_;
diff --git a/fs_mgr/libsnapshot/libsnapshot_cow/writer_v3.cpp b/fs_mgr/libsnapshot/libsnapshot_cow/writer_v3.cpp
index d69254f..ecbf97e 100644
--- a/fs_mgr/libsnapshot/libsnapshot_cow/writer_v3.cpp
+++ b/fs_mgr/libsnapshot/libsnapshot_cow/writer_v3.cpp
@@ -155,6 +155,7 @@
             return false;
         }
     }
+    header_.op_count_max = options_.op_count_max;
 
     if (!Sync()) {
         LOG(ERROR) << "Header sync failed";
@@ -184,9 +185,17 @@
 }
 
 bool CowWriterV3::EmitZeroBlocks(uint64_t new_block_start, uint64_t num_blocks) {
-    LOG(ERROR) << __LINE__ << " " << __FILE__ << " <- function here should never be called";
-    if (new_block_start && num_blocks) return false;
-    return false;
+    for (uint64_t i = 0; i < num_blocks; i++) {
+        CowOperationV3 op;
+        op.type = kCowZeroOp;
+        op.data_length = 0;
+        op.new_block = new_block_start + i;
+        op.source_info = 0;
+        if (!WriteOperation(op)) {
+            return false;
+        }
+    }
+    return true;
 }
 
 bool CowWriterV3::EmitLabel(uint64_t label) {
@@ -201,7 +210,33 @@
     return false;
 }
 
+bool CowWriterV3::WriteOperation(const CowOperationV3& op) {
+    if (IsEstimating()) {
+        header_.op_count++;
+        header_.op_count_max++;
+        return true;
+    }
+
+    if (header_.op_count + 1 > header_.op_count_max) {
+        LOG(ERROR) << "Maximum number of ops reached: " << header_.op_count_max;
+        return false;
+    }
+
+    const off_t offset = GetOpOffset(header_.op_count);
+    if (!android::base::WriteFullyAtOffset(fd_, &op, sizeof(op), offset)) {
+        return false;
+    }
+
+    header_.op_count++;
+    return true;
+}
+
 bool CowWriterV3::Finalize() {
+    CHECK_GE(header_.prefix.header_size, sizeof(CowHeaderV3));
+    CHECK_LE(header_.prefix.header_size, sizeof(header_));
+    if (!android::base::WriteFullyAtOffset(fd_, &header_, header_.prefix.header_size, 0)) {
+        return false;
+    }
     return Sync();
 }
 
diff --git a/fs_mgr/libsnapshot/libsnapshot_cow/writer_v3.h b/fs_mgr/libsnapshot/libsnapshot_cow/writer_v3.h
index a0d7ab9..8717c01 100644
--- a/fs_mgr/libsnapshot/libsnapshot_cow/writer_v3.h
+++ b/fs_mgr/libsnapshot/libsnapshot_cow/writer_v3.h
@@ -14,7 +14,8 @@
 
 #pragma once
 
-#include <future>
+#include <android-base/logging.h>
+
 #include "writer_base.h"
 
 namespace android {
@@ -42,6 +43,13 @@
     void SetupHeaders();
     bool ParseOptions();
     bool OpenForWrite();
+    bool WriteOperation(const CowOperationV3& op);
+
+    off_t GetOpOffset(uint32_t op_index) const {
+        CHECK_LT(op_index, header_.op_count_max);
+        return header_.prefix.header_size + header_.buffer_size +
+               (op_index * sizeof(CowOperationV3));
+    }
 
   private:
     CowHeaderV3 header_{};