Revert of base: Replace PurgeAndTruncate with Shrink function. (patchset #5 id:80001 of https://codereview.chromium.org/1018743003/)

Reason for revert:
Suspecting that this might be related to crbug.com/471188.

Original issue's description:
> base: Replace PurgeAndTruncate with Shrink function.
>
> This cleans up the API a bit and allows the code to be unit tested.
>
> This also prevents ftruncate from being used on Android where
> shared memory is backed by Ashmem. Ashmem doesn't support ftruncate.
>
> BUG=466509
> TEST=base_unittests --gtest_filter=DiscardableSharedMemoryTest.Shrink
>
> Committed: https://crrev.com/2d15df9ef3f9b982aef650d0be0be64a1c5d34a5
> Cr-Commit-Position: refs/heads/master@{#322207}

TBR=avi@chromium.org,danakj@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=466509

Review URL: https://codereview.chromium.org/1057493005

Cr-Commit-Position: refs/heads/master@{#323240}

3 files changed, 22 insertions, 48 deletions

diff --git a/base/memory/discardable_shared_memory.cc b/base/memory/discardable_shared_memory.cc
index 830d6b9..49e93cd 100644
--- a/base/memory/discardable_shared_memory.cc
+++ b/base/memory/discardable_shared_memory.cc
@@ -325,6 +325,22 @@ bool DiscardableSharedMemory::Purge(Time current_time) {
   return true;
 }
 
+bool DiscardableSharedMemory::PurgeAndTruncate(Time current_time) {
+  if (!Purge(current_time))
+    return false;
+
+#if defined(OS_POSIX)
+  // Truncate shared memory to size of SharedState.
+  SharedMemoryHandle handle = shared_memory_.handle();
+  if (SharedMemory::IsHandleValid(handle)) {
+    if (HANDLE_EINTR(ftruncate(handle.fd, sizeof(SharedState))) != 0)
+      DPLOG(ERROR) << "ftruncate() failed";
+  }
+#endif
+
+  return true;
+}
+
 bool DiscardableSharedMemory::IsMemoryResident() const {
   DCHECK(shared_memory_.memory());
 
@@ -341,26 +357,6 @@ void DiscardableSharedMemory::Close() {
   mapped_size_ = 0;
 }
 
-#if defined(DISCARDABLE_SHARED_MEMORY_SHRINKING)
-void DiscardableSharedMemory::Shrink() {
-#if defined(OS_POSIX)
-  SharedMemoryHandle handle = shared_memory_.handle();
-  if (!SharedMemory::IsHandleValid(handle))
-    return;
-
-  // Truncate shared memory to size of SharedState.
-  if (HANDLE_EINTR(
-          ftruncate(handle.fd, AlignToPageSize(sizeof(SharedState)))) != 0) {
-    DPLOG(ERROR) << "ftruncate() failed";
-    return;
-  }
-  mapped_size_ = 0;
-#else
-  NOTIMPLEMENTED();
-#endif
-}
-#endif
-
 Time DiscardableSharedMemory::Now() const {
   return Time::Now();
 }
diff --git a/base/memory/discardable_shared_memory.h b/base/memory/discardable_shared_memory.h
index 892d556..e3b437c 100644
--- a/base/memory/discardable_shared_memory.h
+++ b/base/memory/discardable_shared_memory.h
@@ -15,12 +15,6 @@
 #include <set>
 #endif
 
-// Define DISCARDABLE_SHARED_MEMORY_SHRINKING if platform supports shrinking
-// of discardable shared memory segments.
-#if defined(OS_POSIX) && !defined(OS_ANDROID)
-#define DISCARDABLE_SHARED_MEMORY_SHRINKING
-#endif
-
 namespace base {
 
 // Platform abstraction for discardable shared memory.
@@ -99,6 +93,12 @@ class BASE_EXPORT DiscardableSharedMemory {
   // each call.
   bool Purge(Time current_time);
 
+  // Purge and release as much memory as possible to the OS.
+  // Note: The amount of memory that can be released to the OS is platform
+  // specific. Best case, all but one page is released. Worst case, nothing
+  // is released.
+  bool PurgeAndTruncate(Time current_time);
+
   // Returns true if memory is still resident.
   bool IsMemoryResident() const;
 
@@ -116,12 +116,6 @@ class BASE_EXPORT DiscardableSharedMemory {
     return shared_memory_.ShareToProcess(process_handle, new_handle);
   }
 
-#if defined(DISCARDABLE_SHARED_MEMORY_SHRINKING)
-  // Release as much memory as possible to the OS. The change in size will
-  // be reflected by the return value of mapped_size().
-  void Shrink();
-#endif
-
  private:
   // Virtual for tests.
   virtual Time Now() const;
diff --git a/base/memory/discardable_shared_memory_unittest.cc b/base/memory/discardable_shared_memory_unittest.cc
index ae7235d..74d19a6 100644
--- a/base/memory/discardable_shared_memory_unittest.cc
+++ b/base/memory/discardable_shared_memory_unittest.cc
@@ -311,21 +311,5 @@ TEST(DiscardableSharedMemoryTest, MappedSize) {
   EXPECT_EQ(0u, memory.mapped_size());
 }
 
-#if defined(DISCARDABLE_SHARED_MEMORY_SHRINKING)
-TEST(DiscardableSharedMemoryTest, Shrink) {
-  const uint32 kDataSize = 1024;
-
-  TestDiscardableSharedMemory memory;
-  bool rv = memory.CreateAndMap(kDataSize);
-  ASSERT_TRUE(rv);
-
-  EXPECT_NE(0u, memory.mapped_size());
-
-  // Mapped size should be 0 after shrinking memory segment.
-  memory.Shrink();
-  EXPECT_EQ(0u, memory.mapped_size());
-}
-#endif
-
 }  // namespace
 }  // namespace base