Add parser for EROFS file data

This CL parses file names and compression info from input EROFS image,
so that OTA generation can use it later.

Test: th
Test: Generate an incremental OTA with erofs images, make sure we see
file lists.
Bug: 206729162

Change-Id: I70536d7762acc19a6977460738065d5557338532
diff --git a/payload_generator/erofs_iterate.h b/payload_generator/erofs_iterate.h
new file mode 100644
index 0000000..ac9d67e
--- /dev/null
+++ b/payload_generator/erofs_iterate.h
@@ -0,0 +1,76 @@
+//
+// Copyright (C) 2021 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 <string>
+
+#include <erofs/dir.h>
+
+#include "update_engine/common/utils.h"
+
+//  The only way to pass extra information to callback function is to use a
+//  wrapper type for erofs_dir_context. So here we go
+struct erofs_iterate_dir_context {
+  struct erofs_dir_context ctx;
+  std::string path;
+  void* arg;
+};
+
+// Dear compiler, please don't reoder fields inside erofs_iterate_dir_context.
+// Because EROFS expects us to pass a wrapper type. So |ctx| member of
+// erofs_iterate_dir_context must be put at 0 offset.
+static_assert(offsetof(erofs_iterate_dir_context, ctx) == 0);
+
+// Callable shold be a functor like
+// std::function<int(struct erofs_inode_info *)>
+template <typename Callable>
+int erofs_iterate_root_dir(const struct erofs_sb_info* sbi, Callable cb) {
+  struct erofs_inode dir {
+    .nid = sbi->root_nid
+  };
+  int err = erofs_read_inode_from_disk(&dir);
+  if (err) {
+    LOG(ERROR) << "Failed to read inode " << sbi->root_nid << " from disk";
+    return err;
+  }
+  struct erofs_iterate_dir_context param {
+    .ctx.dir = &dir, .ctx.pnid = sbi->root_nid,
+    .ctx.cb = [](struct erofs_dir_context* arg) -> int {
+      auto ctx = reinterpret_cast<erofs_iterate_dir_context*>(arg);
+      auto& path = ctx->path;
+      const auto len = path.size();
+      path.push_back('/');
+      path.insert(
+          path.end(), ctx->ctx.dname, ctx->ctx.dname + ctx->ctx.de_namelen);
+      auto cb = static_cast<Callable*>(ctx->arg);
+      const auto err = (*cb)(ctx);
+      if (!err && !ctx->ctx.dot_dotdot && ctx->ctx.de_ftype == EROFS_FT_DIR) {
+        // recursively walk into subdirectories
+        erofs_inode dir{.nid = ctx->ctx.de_nid};
+        if (const int err = erofs_read_inode_from_disk(&dir); err) {
+          return err;
+        }
+        ctx->ctx.dir = &dir;
+        if (const auto err = erofs_iterate_dir(&ctx->ctx, false); err) {
+          return err;
+        }
+      }
+      path.resize(len);
+      return err;
+    },
+    .arg = &cb,
+  };
+  return erofs_iterate_dir(&param.ctx, false);
+}