[base] Make MappedFile work with OS file handles

adb uses its own implementation of Windows HANDLE->int mapping,
and it doesn't play well with _get_osfhandle() function used in
MappedFile::FromFd().

This CL adds another function that accepts raw handle, so adb
can pass it directly

+ make constant functions 'const'
+ make the MappedFile movable, as nothing prevents it from being
  one

Test: libbase_test

Change-Id: Ifde4a4094b910e9c7b431126ecf3fef5aa3bb4a6

base/include/android-base/mapped_file.h

@@ -28,15 +28,17 @@
 #include <windows.h>
 #define PROT_READ 1
 #define PROT_WRITE 2
+using os_handle = HANDLE;
 #else
 #include <sys/mman.h>
+using os_handle = int;
 #endif
 
 namespace android {
 namespace base {
 
 /**
- * A region of a file mapped into memory, also known as MmapFile.
+ * A region of a file mapped into memory (for grepping: also known as MmapFile or file mapping).
  */
 class MappedFile {
  public:
@@ -48,17 +50,34 @@ class MappedFile {
   static std::unique_ptr<MappedFile> FromFd(borrowed_fd fd, off64_t offset, size_t length,
                                             int prot);
 
+  /**
+   * Same thing, but using the raw OS file handle instead of a CRT wrapper.
+   */
+  static MappedFile FromOsHandle(os_handle h, off64_t offset, size_t length, int prot);
+
   /**
    * Removes the mapping.
    */
   ~MappedFile();
 
-  char* data() { return base_ + offset_; }
-  size_t size() { return size_; }
+  /**
+   * Not copyable but movable.
+   */
+  MappedFile(MappedFile&& other);
+  MappedFile& operator=(MappedFile&& other);
+
+  char* data() const { return base_ + offset_; }
+  size_t size() const { return size_; }
+
+  bool isValid() const { return base_ != nullptr; }
+
+  explicit operator bool() const { return isValid(); }
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(MappedFile);
 
+  void Close();
+
   char* base_;
   size_t size_;
 

base/mapped_file.cpp

@@ -16,13 +16,15 @@
 
 #include "android-base/mapped_file.h"
 
-#include <errno.h>
+#include <utility>
 
-#include "android-base/unique_fd.h"
+#include <errno.h>
 
 namespace android {
 namespace base {
 
+static constexpr char kEmptyBuffer[] = {'0'};
+
 static off64_t InitPageSize() {
 #if defined(_WIN32)
   SYSTEM_INFO si;
@@ -35,51 +37,86 @@ static off64_t InitPageSize() {
 
 std::unique_ptr<MappedFile> MappedFile::FromFd(borrowed_fd fd, off64_t offset, size_t length,
                                                int prot) {
-  static off64_t page_size = InitPageSize();
+#if defined(_WIN32)
+  auto file =
+      FromOsHandle(reinterpret_cast<HANDLE>(_get_osfhandle(fd.get())), offset, length, prot);
+#else
+  auto file = FromOsHandle(fd.get(), offset, length, prot);
+#endif
+  return file ? std::make_unique<MappedFile>(std::move(file)) : std::unique_ptr<MappedFile>{};
+}
+
+MappedFile MappedFile::FromOsHandle(os_handle h, off64_t offset, size_t length, int prot) {
+  static const off64_t page_size = InitPageSize();
   size_t slop = offset % page_size;
   off64_t file_offset = offset - slop;
   off64_t file_length = length + slop;
 
 #if defined(_WIN32)
-  HANDLE handle =
-      CreateFileMapping(reinterpret_cast<HANDLE>(_get_osfhandle(fd.get())), nullptr,
-                        (prot & PROT_WRITE) ? PAGE_READWRITE : PAGE_READONLY, 0, 0, nullptr);
+  HANDLE handle = CreateFileMappingW(
+      h, nullptr, (prot & PROT_WRITE) ? PAGE_READWRITE : PAGE_READONLY, 0, 0, nullptr);
   if (handle == nullptr) {
     // http://b/119818070 "app crashes when reading asset of zero length".
     // Return a MappedFile that's only valid for reading the size.
-    if (length == 0) {
-      return std::unique_ptr<MappedFile>(new MappedFile{nullptr, 0, 0, nullptr});
+    if (length == 0 && ::GetLastError() == ERROR_FILE_INVALID) {
+      return MappedFile{const_cast<char*>(kEmptyBuffer), 0, 0, nullptr};
     }
-    return nullptr;
+    return MappedFile(nullptr, 0, 0, nullptr);
   }
   void* base = MapViewOfFile(handle, (prot & PROT_WRITE) ? FILE_MAP_ALL_ACCESS : FILE_MAP_READ, 0,
                              file_offset, file_length);
   if (base == nullptr) {
     CloseHandle(handle);
-    return nullptr;
+    return MappedFile(nullptr, 0, 0, nullptr);
   }
-  return std::unique_ptr<MappedFile>(
-      new MappedFile{static_cast<char*>(base), length, slop, handle});
+  return MappedFile{static_cast<char*>(base), length, slop, handle};
 #else
-  void* base = mmap(nullptr, file_length, prot, MAP_SHARED, fd.get(), file_offset);
+  void* base = mmap(nullptr, file_length, prot, MAP_SHARED, h, file_offset);
   if (base == MAP_FAILED) {
     // http://b/119818070 "app crashes when reading asset of zero length".
     // mmap fails with EINVAL for a zero length region.
     if (errno == EINVAL && length == 0) {
-      return std::unique_ptr<MappedFile>(new MappedFile{nullptr, 0, 0});
+      return MappedFile{const_cast<char*>(kEmptyBuffer), 0, 0};
     }
-    return nullptr;
+    return MappedFile(nullptr, 0, 0);
   }
-  return std::unique_ptr<MappedFile>(new MappedFile{static_cast<char*>(base), length, slop});
+  return MappedFile{static_cast<char*>(base), length, slop};
 #endif
 }
 
+MappedFile::MappedFile(MappedFile&& other)
+    : base_(std::exchange(other.base_, nullptr)),
+      size_(std::exchange(other.size_, 0)),
+      offset_(std::exchange(other.offset_, 0))
+#ifdef _WIN32
+      ,
+      handle_(std::exchange(other.handle_, nullptr))
+#endif
+{
+}
+
+MappedFile& MappedFile::operator=(MappedFile&& other) {
+  Close();
+  base_ = std::exchange(other.base_, nullptr);
+  size_ = std::exchange(other.size_, 0);
+  offset_ = std::exchange(other.offset_, 0);
+#ifdef _WIN32
+  handle_ = std::exchange(other.handle_, nullptr);
+#endif
+  return *this;
+}
+
 MappedFile::~MappedFile() {
+  Close();
+}
+
+void MappedFile::Close() {
 #if defined(_WIN32)
-  if (base_ != nullptr) UnmapViewOfFile(base_);
+  if (base_ != nullptr && size_ != 0) UnmapViewOfFile(base_);
   if (handle_ != nullptr) CloseHandle(handle_);
+  handle_ = nullptr;
 #else
-  if (base_ != nullptr) munmap(base_, size_ + offset_);
+  if (base_ != nullptr && size_ != 0) munmap(base_, size_ + offset_);
 #endif
 
   base_ = nullptr;

base/mapped_file_test.cpp

@@ -44,5 +44,8 @@ TEST(mapped_file, zero_length_mapping) {
   ASSERT_TRUE(tf.fd != -1);
 
   auto m = android::base::MappedFile::FromFd(tf.fd, 4096, 0, PROT_READ);
-  ASSERT_EQ(0u, m->size());
+  ASSERT_NE(nullptr, m);
+  EXPECT_TRUE((bool)*m);
+  EXPECT_EQ(0u, m->size());
+  EXPECT_NE(nullptr, m->data());
 }