Get rid of unneeded allocations in Extract...() APIs

Both Extract...() functions don't need dynamic allocation
for the writers, as those are strictly scoped. This CL
changes heap allocation to stack allocation.

Test: zip_archive_test

Change-Id: Id727e4b9848235cd063cc67ecbe052d21ca21326
diff --git a/libziparchive/zip_archive.cc b/libziparchive/zip_archive.cc
index 5ccbcc2..f9f8c73 100644
--- a/libziparchive/zip_archive.cc
+++ b/libziparchive/zip_archive.cc
@@ -783,12 +783,12 @@
   // block device).
   //
   // Returns a valid FileWriter on success, |nullptr| if an error occurred.
-  static std::unique_ptr<FileWriter> Create(int fd, const ZipEntry* entry) {
+  static FileWriter Create(int fd, const ZipEntry* entry) {
     const uint32_t declared_length = entry->uncompressed_length;
     const off64_t current_offset = lseek64(fd, 0, SEEK_CUR);
     if (current_offset == -1) {
       ALOGW("Zip: unable to seek to current location on fd %d: %s", fd, strerror(errno));
-      return nullptr;
+      return FileWriter{};
     }
 
     int result = 0;
@@ -808,7 +808,7 @@
         ALOGW("Zip: unable to allocate %" PRId64 " bytes at offset %" PRId64 ": %s",
               static_cast<int64_t>(declared_length), static_cast<int64_t>(current_offset),
               strerror(errno));
-        return std::unique_ptr<FileWriter>(nullptr);
+        return FileWriter{};
       }
     }
 #endif  // __linux__
@@ -816,7 +816,7 @@
     struct stat sb;
     if (fstat(fd, &sb) == -1) {
       ALOGW("Zip: unable to fstat file: %s", strerror(errno));
-      return std::unique_ptr<FileWriter>(nullptr);
+      return FileWriter{};
     }
 
     // Block device doesn't support ftruncate(2).
@@ -825,13 +825,22 @@
       if (result == -1) {
         ALOGW("Zip: unable to truncate file to %" PRId64 ": %s",
               static_cast<int64_t>(declared_length + current_offset), strerror(errno));
-        return std::unique_ptr<FileWriter>(nullptr);
+        return FileWriter{};
       }
     }
 
-    return std::unique_ptr<FileWriter>(new FileWriter(fd, declared_length));
+    return FileWriter(fd, declared_length);
   }
 
+  FileWriter(FileWriter&& other)
+      : fd_(other.fd_),
+        declared_length_(other.declared_length_),
+        total_bytes_written_(other.total_bytes_written_) {
+    other.fd_ = -1;
+  }
+
+  bool IsValid() const { return fd_ != -1; }
+
   virtual bool Append(uint8_t* buf, size_t buf_size) override {
     if (total_bytes_written_ + buf_size > declared_length_) {
       ALOGW("Zip: Unexpected size " ZD " (declared) vs " ZD " (actual)", declared_length_,
@@ -850,10 +859,10 @@
   }
 
  private:
-  FileWriter(const int fd, const size_t declared_length)
+  explicit FileWriter(const int fd = -1, const size_t declared_length = 0)
       : Writer(), fd_(fd), declared_length_(declared_length), total_bytes_written_(0) {}
 
-  const int fd_;
+  int fd_;
   const size_t declared_length_;
   size_t total_bytes_written_;
 };
@@ -1066,17 +1075,17 @@
 }
 
 int32_t ExtractToMemory(ZipArchiveHandle handle, ZipEntry* entry, uint8_t* begin, uint32_t size) {
-  std::unique_ptr<zip_archive::Writer> writer(new MemoryWriter(begin, size));
-  return ExtractToWriter(handle, entry, writer.get());
+  MemoryWriter writer(begin, size);
+  return ExtractToWriter(handle, entry, &writer);
 }
 
 int32_t ExtractEntryToFile(ZipArchiveHandle handle, ZipEntry* entry, int fd) {
-  std::unique_ptr<zip_archive::Writer> writer(FileWriter::Create(fd, entry));
-  if (writer.get() == nullptr) {
+  auto writer = FileWriter::Create(fd, entry);
+  if (!writer.IsValid()) {
     return kIoError;
   }
 
-  return ExtractToWriter(handle, entry, writer.get());
+  return ExtractToWriter(handle, entry, &writer);
 }
 
 const char* ErrorCodeString(int32_t error_code) {