Create an IO type for reading composite disk images

Bug: b/495556334

Author: Databean

base/cvd/cuttlefish/host/libs/image_aggregator/BUILD.bazel

@@ -33,6 +33,21 @@ cf_cc_library(
     ],
 )
 
+cf_cc_library(
+    name = "composite_io",
+    srcs = ["composite_io.cc"],
+    hdrs = ["composite_io.h"],
+    deps = [
+        "//cuttlefish/host/libs/image_aggregator:cdisk_spec_cc_proto",
+        "//cuttlefish/host/libs/image_aggregator:composite_disk",
+        "//cuttlefish/io",
+        "//cuttlefish/io:fake_seek",
+        "//cuttlefish/io:filesystem",
+        "//cuttlefish/result:expect",
+        "//cuttlefish/result:result_type",
+    ],
+)
+
 cf_cc_library(
     name = "disk_image",
     hdrs = ["disk_image.h"],

base/cvd/cuttlefish/host/libs/image_aggregator/composite_io.cc

@@ -0,0 +1,65 @@
+/*
+ * Copyright (C) 2026 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+#include "cuttlefish/host/libs/image_aggregator/composite_io.h"
+
+#include <stdint.h>
+
+#include "cuttlefish/host/libs/image_aggregator/composite_disk.h"
+#include "cuttlefish/io/fake_seek.h"
+#include "cuttlefish/io/filesystem.h"
+#include "cuttlefish/result/expect.h"
+#include "cuttlefish/result/result_type.h"
+
+namespace cuttlefish {
+
+/* static */ Result<CompositeDiskReaderIo> CompositeDiskReaderIo::Create(
+    CompositeDiskImage composite, ReadFilesystem& filesystem) {
+  std::map<uint64_t, std::unique_ptr<ReaderSeeker>> offset_to_file;
+  for (const ComponentDisk& member : composite.GetCompositeDisk().component_disks()) {
+    std::unique_ptr<ReaderSeeker> file =
+        CF_EXPECT(filesystem.OpenReadOnly(member.file_path()));
+    CF_EXPECT(file.get());
+    offset_to_file[member.offset()] = std::move(file);
+  }
+  CF_EXPECT_EQ(offset_to_file.count(0), 1, "No initial member");
+  return CompositeDiskReaderIo(std::move(composite), std::move(offset_to_file));
+}
+
+CompositeDiskReaderIo::CompositeDiskReaderIo(
+    CompositeDiskImage composite, std::map<uint64_t, std::unique_ptr<ReaderSeeker>> offset_to_file)
+    : ReaderFakeSeeker(composite.GetCompositeDisk().length()),
+      composite_(std::move(composite)),
+      offset_to_file_(std::move(offset_to_file)) {}
+
+Result<uint64_t> CompositeDiskReaderIo::PRead(void* buf, uint64_t count, uint64_t offset) const {
+  const uint64_t full_length = composite_.GetCompositeDisk().length();
+  auto it = offset_to_file_.upper_bound(offset);
+  uint64_t read_end = it == offset_to_file_.end() ? full_length : it->first;
+  CF_EXPECT(it != offset_to_file_.begin());
+  it--;
+  CF_EXPECT(it != offset_to_file_.end());
+  CF_EXPECT_GE(offset, it->first);
+  if (read_end < offset) {
+    return 0;
+  }
+  if (offset + count > read_end) {
+    count = read_end - offset;
+  }
+  CF_EXPECT(it->second.get());
+  return CF_EXPECT(it->second->PRead(buf, count, offset - it->first));
+}
+
+}  // namespace cuttlefish

base/cvd/cuttlefish/host/libs/image_aggregator/composite_io.h

@@ -0,0 +1,43 @@
+/*
+ * Copyright (C) 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+#pragma once
+
+#include <stdint.h>
+
+#include "cuttlefish/host/libs/image_aggregator/composite_disk.h"
+#include "cuttlefish/io/fake_seek.h"
+#include "cuttlefish/io/filesystem.h"
+#include "cuttlefish/result/result_type.h"
+
+namespace cuttlefish {
+
+class CompositeDiskReaderIo : public ReaderFakeSeeker {
+ public:
+  static Result<CompositeDiskReaderIo> Create(CompositeDiskImage, ReadFilesystem&);
+
+  CompositeDiskReaderIo(CompositeDiskReaderIo&&) = default;
+  ~CompositeDiskReaderIo() = default;
+  CompositeDiskReaderIo& operator=(CompositeDiskReaderIo&&) = default;
+
+  Result<uint64_t> PRead(void* buf, uint64_t count, uint64_t offset) const override;
+ private:
+  CompositeDiskReaderIo(CompositeDiskImage, std::map<uint64_t, std::unique_ptr<ReaderSeeker>>);
+
+  CompositeDiskImage composite_;
+  std::map<uint64_t, std::unique_ptr<ReaderSeeker>> offset_to_file_;
+};
+
+}  // namespace cuttlefish