Support persistent system profiles.

components/metrics/persistent_system_profile.cc

Index: components/metrics/persistent_system_profile.cc
diff --git a/components/metrics/persistent_system_profile.cc b/components/metrics/persistent_system_profile.cc
new file mode 100644
index 0000000000000000000000000000000000000000..9f7c327a48dac5e0d31c9407e819cc51b3b0ae54
--- /dev/null
+++ b/components/metrics/persistent_system_profile.cc
@@ -0,0 +1,290 @@
+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "components/metrics/persistent_system_profile.h"
+
+#include "base/atomicops.h"
+#include "base/metrics/persistent_memory_allocator.h"
+#include "base/stl_util.h"
+
+namespace metrics {
+
+namespace {
+
+// To provide atomic addition of records so that there is no confusion between
+// writers and readers, all of the metadata about a record is contained in a
+// structure that can be stored as a single atomic 32-bit word.
+union RecordHeader {
+  struct {
+    unsigned continued : 1;  // Flag indicating if there is more after this.
+    unsigned type : 7;       // The type of this record.
+    unsigned amount : 24;    // The amount of data to follow.
+  } as_parts;
+  base::subtle::Atomic32 as_atomic;
+};
+
+constexpr uint32_t kTypeIdSystemProfile = 0x330A7150;  // SHA1(SystemProfile)
+constexpr size_t kSystemProfileAllocSize = 4 << 10;    // 4 KiB
+constexpr size_t kMaxRecordSize = (1 << 24) - sizeof(RecordHeader);
+
+static_assert(sizeof(RecordHeader) == sizeof(base::subtle::Atomic32),
+              "bad RecordHeader size");
+
+// Calculate the size of a record based on the amount of data. This adds room
+// for the record header and rounds up to the next multiple of the record-header
+// size.
+size_t CalcRecordSize(size_t data_amount) {

[Alexei Svitkine (slow), 2017/05/26 18:01:52]

Nit: Calculate

[bcwhite, 2017/05/29 18:32:26]

Done.

+  return (data_amount + sizeof(RecordHeader) + sizeof(RecordHeader) - 1) &
+         ~(sizeof(RecordHeader) - 1);
+}
+
+}  // namespace
+
+PersistentSystemProfile::RecordAllocator::RecordAllocator(
+    base::PersistentMemoryAllocator* memory_allocator,
+    size_t min_size)
+    : allocator_(memory_allocator), tail_reference_(0), end_offset_(0) {
+  AddSegment(min_size);
+}
+
+PersistentSystemProfile::RecordAllocator::RecordAllocator(
+    const base::PersistentMemoryAllocator* memory_allocator)
+    : allocator_(
+          const_cast<base::PersistentMemoryAllocator*>(memory_allocator)),
+      tail_reference_(0),
+      end_offset_(0) {}
+
+bool PersistentSystemProfile::RecordAllocator::Matches(
+    const base::PersistentMemoryAllocator* allocator) const {
+  return allocator_ == allocator;
+}
+
+void PersistentSystemProfile::RecordAllocator::Reset() {
+  // Clear the first word of all blocks so they're known to be "empty".
+  tail_reference_ = 0;
+  while (NextSegment()) {
+    base::subtle::Atomic32* block =
+        allocator_->GetAsArray<base::subtle::Atomic32>(tail_reference_,
+                                                       kTypeIdSystemProfile, 1);
+    DCHECK(block);
+    base::subtle::NoBarrier_Store(block, 0);
+  }
+
+  // Reset member variables.
+  tail_reference_ = 0;
+  end_offset_ = 0;
+}
+
+bool PersistentSystemProfile::RecordAllocator::Write(
+    RecordType type,
+    const std::string& record) {
+  const char* data = record.data();
+  size_t size = record.size();

[Alexei Svitkine (slow), 2017/05/26 18:01:52]

Nit: remaining_size

[bcwhite, 2017/05/29 18:32:26]

Done.

+
+  RecordHeader header;
+  header.as_atomic = 0;
+  header.as_parts.type = type;
+
+  // Make sure there is a place to write. Failure will cause a nullptr |block|
+  // when accessed below and will then return false.
+  if (tail_reference_ == 0)
+    AddSegment(size);
+
+  // Write records until everything has been stored.
+  do {
+    char* block =
+        allocator_->GetAsArray<char>(tail_reference_, kTypeIdSystemProfile,
+                                     base::PersistentMemoryAllocator::kSizeAny);
+    if (!block)
+      return false;
+
+    // The size of the allocation limits how big a record can be stored.
+    const size_t alloc_size =
+        allocator_->GetAllocSize(tail_reference_) & ~(sizeof(RecordHeader) - 1);

[Alexei Svitkine (slow), 2017/05/26 18:01:52]

This is very cryptic.

You get the size as size_t.

Then you take sizeof(RecordHeader)-1, which should be 31 (i.e. the 0b11111) and
essentially clearing those bottom 5 bits from the size... I don't really follow
why this is needed...

[bcwhite, 2017/05/29 18:32:26]

The tilde (~) before it flips the bits so it's clearing the bottom bits.  This
is the typical way of stripping off any extra bytes so that the value is rounded
down to a multiple of sizeof(RecordHeader).  It basically guarantees that there
will be space for another header if there is any space at all.

It could be removed since the allocator will always meet that requirement. 
Perhaps it would be better just to document it instead.  Done.

+    size_t amount = std::min(size, kMaxRecordSize);
+    amount = std::min(amount, alloc_size - end_offset_ - sizeof(header));

[Alexei Svitkine (slow), 2017/05/26 18:01:52]

Nit: size_to_write instead of amount

[bcwhite, 2017/05/29 18:32:26]

Done.

+
+    // Write the data and the record header.
+    header.as_parts.amount = amount;
+    header.as_parts.continued = (amount < size);
+    size_t offset = end_offset_;
+    end_offset_ += CalcRecordSize(amount);
+    DCHECK_GE(alloc_size, end_offset_);
+    if (end_offset_ < alloc_size) {
+      // An empty record header has to be next before this one gets written.
+      base::subtle::NoBarrier_Store(
+          reinterpret_cast<base::subtle::Atomic32*>(block + end_offset_), 0);
+    }
+    memcpy(block + offset + sizeof(header), data, amount);

[Alexei Svitkine (slow), 2017/05/26 18:01:52]

I feel like this code would be clearer with another helper function.

At a high-level, what this is doing is:

while (remaining_size > 0 && MaybeAdvanceSegment()) { 
  data = WriteData(data, &remaining_size);
}

Can you refactor the code so it's a bit more like that? i.e. split a helper to
do the actual copying of data from the logic of making sure we have a segment to
write to.

[bcwhite, 2017/05/29 18:32:26]

Done.

+    base::subtle::Release_Store(
+        reinterpret_cast<base::subtle::Atomic32*>(block), header.as_atomic);
+
+    // Account for what was stored and prepare for follow-on records with any
+    // remaining data.
+    data += amount;
+    size -= amount;
+
+    // If the block got filled, create a new one after the current one. Failure
+    // will cause a nullptr |block| when next accessed.
+    if (end_offset_ == alloc_size)
+      AddSegment(size);
+  } while (size > 0);
+
+  return true;
+}
+
+bool PersistentSystemProfile::RecordAllocator::Read(RecordType* type,
+                                                    std::string* record) const {
+  RecordType found_type = kUnusedSpace;
+  record->clear();
+
+  // Find something to read. Failure will cause a nullptr |block| when accessed
+  // below and will then return false.
+  if (tail_reference_ == 0)
+    NextSegment();
+
+  bool continued;
+  do {
+    char* block =
+        allocator_->GetAsArray<char>(tail_reference_, kTypeIdSystemProfile,
+                                     base::PersistentMemoryAllocator::kSizeAny);
+    if (!block)
+      return false;  // Can't read anything more.
+
+    // Get the actual alloc size to verify that record doesn't extend past it.
+    const size_t alloc_size =
+        allocator_->GetAllocSize(tail_reference_) & ~(sizeof(RecordHeader) - 1);
+    DCHECK_GT(alloc_size, end_offset_);
+
+    // Get and validate the record header.
+    RecordHeader header;
+    header.as_atomic = base::subtle::Acquire_Load(
+        reinterpret_cast<base::subtle::Atomic32*>(block + end_offset_));
+    continued = !!header.as_parts.continued;
+    if (header.as_parts.type == kUnusedSpace) {

[Alexei Svitkine (slow), 2017/05/26 18:01:52]

I don't see where kUnusedSpace is written? Is it meant to be implicit - if so
that should be documented in the enum.

[bcwhite, 2017/05/29 18:32:26]

Done.

+      return false;  // End of records.
+    } else if (found_type == kUnusedSpace) {
+      found_type = static_cast<RecordType>(header.as_parts.type);
+    } else if (found_type != header.as_parts.type) {
+      NOTREACHED();  // Continuation didn't match start of record.
+      return false;
+    }
+    size_t amount = header.as_parts.amount;
+    if (amount < sizeof(header) ||
+        end_offset_ + sizeof(header) + amount > alloc_size) {
+      NOTREACHED();  // Invalid header amount.
+      return false;
+    }
+
+    // Append the record data to the output string.
+    record->append(block + sizeof(header), amount);
+    end_offset_ += CalcRecordSize(amount);
+    DCHECK_GE(alloc_size, end_offset_);
+
+    // If the end of the block has been reached, advance to the next one.
+    // Failure will cause a nullptr |block| when next accessed.
+    if (end_offset_ == alloc_size)
+      NextSegment();
+  } while (continued);
+
+  *type = found_type;
+  return true;
+}
+
+bool PersistentSystemProfile::RecordAllocator::NextSegment() const {
+  base::PersistentMemoryAllocator::Iterator iter(allocator_, tail_reference_);
+  tail_reference_ = iter.GetNextOfType(kTypeIdSystemProfile);
+  end_offset_ = 0;
+  return tail_reference_ != 0;
+}
+
+bool PersistentSystemProfile::RecordAllocator::AddSegment(size_t min_size) {
+  if (NextSegment()) {
+    // The first record-header should have been zeroed as part of the allocation
+    // or by the "reset" procedure.
+    DCHECK_EQ(0, base::subtle::NoBarrier_Load(
+                     allocator_->GetAsArray<base::subtle::Atomic32>(
+                         tail_reference_, kTypeIdSystemProfile, 1)));
+    return true;
+  }
+
+  DCHECK_EQ(0U, tail_reference_);
+  DCHECK_EQ(0U, end_offset_);
+
+  size_t size = std::max(CalcRecordSize(min_size), kSystemProfileAllocSize);
+
+  uint32_t ref = allocator_->Allocate(size, kTypeIdSystemProfile);
+  if (!ref)
+    return false;  // Allocator must be full.
+  allocator_->MakeIterable(ref);
+
+  tail_reference_ = ref;
+  return true;
+}
+
+PersistentSystemProfile::PersistentSystemProfile() {}
+
+PersistentSystemProfile::~PersistentSystemProfile() {}
+
+void PersistentSystemProfile::RegisterPersistentAllocator(
+    base::PersistentMemoryAllocator* memory_allocator) {
+  DCHECK_CALLED_ON_VALID_THREAD(thread_checker_);
+
+  // Create and store the allocator. A |min_size| of "1" ensures that a memory
+  // block is reserved now.
+  RecordAllocator allocator(memory_allocator, 1);
+  allocators_.push_back(std::move(allocator));
+}
+
+void PersistentSystemProfile::DeregisterPersistentAllocator(
+    base::PersistentMemoryAllocator* memory_allocator) {
+  DCHECK_CALLED_ON_VALID_THREAD(thread_checker_);
+
+  // This would be more efficient with a std::map but it's not expected that
+  // allocators will get deregistered with any frequency, if at all.
+  for (auto iter = allocators_.begin(); iter != allocators_.end(); ++iter) {
+    if (iter->Matches(memory_allocator)) {
+      allocators_.erase(iter);
+      return;
+    }
+  }

[Alexei Svitkine (slow), 2017/05/26 18:01:52]

Nit: Use EraseIf() from base/stl_util.h?

[bcwhite, 2017/05/29 18:32:26]

Done.

+  NOTREACHED();
+}
+
+void PersistentSystemProfile::SetSystemProfile(
+    const SystemProfileProto& system_profile) {
+  DCHECK_CALLED_ON_VALID_THREAD(thread_checker_);
+
+  std::string serialized_profile;
+  if (!system_profile.SerializeToString(&serialized_profile)) {
+    NOTREACHED();
+    return;
+  }
+
+  for (auto& allocator : allocators_) {
+    // A full system profile always starts fresh.
+    allocator.Reset();
+    // Write out the serialized profile.
+    allocator.Write(kSystemProfileProto, serialized_profile);
+  }
+}
+
+// static
+bool PersistentSystemProfile::GetSystemProfile(
+    SystemProfileProto* system_profile,
+    const base::PersistentMemoryAllocator* memory_allocator) {
+  const RecordAllocator records(memory_allocator);
+
+  RecordType type;
+  std::string record;
+  if (!records.Read(&type, &record))
+    return false;
+  if (type != kSystemProfileProto)
+    return false;
+
+  system_profile->ParseFromString(record);
+  return true;

[Alexei Svitkine (slow), 2017/05/26 18:01:52]

Return the result of system_profile->ParseFromString()? In case there's data
corruption for example.

[bcwhite, 2017/05/29 18:32:26]

Done.

+}
+
+}  // namespace metrics