Share field trial allocator on zygote-using Linuxes

base/metrics/field_trial.cc

 // Copyright (c) 2012 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 "base/metrics/field_trial.h"
 
 #include <algorithm>
 #include <utility>
 
 #include "base/base_switches.h"
 #include "base/build_time.h"
 #include "base/command_line.h"
 #include "base/debug/alias.h"
 #include "base/feature_list.h"
 #include "base/logging.h"
 #include "base/pickle.h"
 #include "base/process/memory.h"
 #include "base/rand_util.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_util.h"
 #include "base/strings/stringprintf.h"
 #include "base/strings/utf_string_conversions.h"
 
+// On systems that use the zygote process to spawn child processes, we must
+// retrieve the correct fd using the mapping in GlobalDescriptors.
+#if defined(OS_POSIX) && !defined(OS_NACL) && !defined(OS_MACOSX) && \
+    !defined(OS_ANDROID)
+#define POSIX_WITH_ZYGOTE 1
+#endif
+
+#if defined(POSIX_WITH_ZYGOTE)
+#include "base/posix/global_descriptors.h"
+#endif
+
 namespace base {
 
 namespace {
 
 // Define a separator character to use when creating a persistent form of an
 // instance.  This is intended for use as a command line argument, passed to a
 // second process to mimic our state (i.e., provide the same group name).
 const char kPersistentStringSeparator = '/';  // Currently a slash.
 
 // Define a marker character to be used as a prefix to a trial name on the
@@ skipping 192 matching lines @@
   }
 }
 
 #if defined(OS_WIN)
 HANDLE CreateReadOnlyHandle(FieldTrialList::FieldTrialAllocator* allocator) {
   HANDLE src = allocator->shared_memory()->handle().GetHandle();
   ProcessHandle process = GetCurrentProcess();
   DWORD access = SECTION_MAP_READ | SECTION_QUERY;
   HANDLE dst;
   if (!::DuplicateHandle(process, src, process, &dst, access, true, 0))
-    return nullptr;
+    return kInvalidPlatformHandle;
   return dst;
 }
 #endif
 
+#if defined(POSIX_WITH_ZYGOTE)
+int CreateReadOnlyHandle(FieldTrialList::FieldTrialAllocator* allocator) {
+  SharedMemoryHandle new_handle;
+  allocator->shared_memory()->ShareReadOnlyToProcess(GetCurrentProcessHandle(),
+                                                     &new_handle);
+  return SharedMemory::GetFdFromSharedMemoryHandle(new_handle);
+}
+#endif
+
 }  // namespace
 
 // statics
 const int FieldTrial::kNotFinalized = -1;
 const int FieldTrial::kDefaultGroupNumber = 0;
 bool FieldTrial::enable_benchmarking_ = false;
 
 int FieldTrialList::kNoExpirationYear = 0;
 
 //------------------------------------------------------------------------------
@@ skipping 482 matching lines @@
   }
   return true;
 }
 
 // static
 void FieldTrialList::CreateTrialsFromCommandLine(
     const CommandLine& cmd_line,
     const char* field_trial_handle_switch) {
   global_->create_trials_from_command_line_called_ = true;
 
-#if defined(OS_WIN) && !defined(OS_NACL)
   if (cmd_line.HasSwitch(field_trial_handle_switch)) {
-    std::string arg = cmd_line.GetSwitchValueASCII(field_trial_handle_switch);
-    int field_trial_handle = std::stoi(arg);
-    HANDLE handle = reinterpret_cast<HANDLE>(field_trial_handle);
-    bool result = CreateTrialsFromWindowsHandle(handle);
+    std::string handle_switch =
+        cmd_line.GetSwitchValueASCII(field_trial_handle_switch);
+    bool result = CreateTrialsFromHandleSwitch(handle_switch);
     DCHECK(result);
   }
-#endif
 
   if (cmd_line.HasSwitch(switches::kForceFieldTrials)) {
     bool result = FieldTrialList::CreateTrialsFromString(
         cmd_line.GetSwitchValueASCII(switches::kForceFieldTrials),
         std::set<std::string>());
     DCHECK(result);
   }
 }
 
 #if defined(OS_WIN)
 // static
 void FieldTrialList::AppendFieldTrialHandleIfNeeded(
     HandlesToInheritVector* handles) {
   if (!global_)
     return;
   if (kUseSharedMemoryForFieldTrials) {
     InstantiateFieldTrialAllocatorIfNeeded();
     if (global_->readonly_allocator_handle_)
       handles->push_back(global_->readonly_allocator_handle_);
   }
 }
 #endif
 
+#if defined(OS_POSIX) && !defined(OS_NACL)
+// static
+int FieldTrialList::GetFieldTrialHandle() {
+  if (!global_)
+    return kInvalidPlatformFile;
+  if (kUseSharedMemoryForFieldTrials) {
+    InstantiateFieldTrialAllocatorIfNeeded();
+    if (global_->readonly_allocator_handle_ != kInvalidPlatformFile)
+      return global_->readonly_allocator_handle_;
+  }
+  return kInvalidPlatformFile;
+}
+#endif
+
 // static
 void FieldTrialList::CopyFieldTrialStateToFlags(
     const char* field_trial_handle_switch,
     CommandLine* cmd_line) {
   // TODO(lawrencewu): Ideally, having the global would be guaranteed. However,
   // content browser tests currently don't create a FieldTrialList because they
   // don't run ChromeBrowserMainParts code where it's done for Chrome.
   if (!global_)
     return;
 
@@ skipping 15 matching lines @@
     // retrieve the handle. See http://stackoverflow.com/a/153077
     auto uintptr_handle =
         reinterpret_cast<uintptr_t>(global_->readonly_allocator_handle_);
     std::string field_trial_handle = std::to_string(uintptr_handle);
     cmd_line->AppendSwitchASCII(field_trial_handle_switch, field_trial_handle);
     global_->field_trial_allocator_->UpdateTrackingHistograms();
     return;
   }
 #endif
 
+#if defined(POSIX_WITH_ZYGOTE)
+  // Use shared memory to pass the state if the feature is enabled, otherwise
+  // fallback to passing it via the command line as a string.
+  if (kUseSharedMemoryForFieldTrials) {

[Alexei Svitkine (slow), 2016/11/24 18:23:19]

This block seems to be almost identical to the one above.

Can we make it identical and not have two separate blocks? e.g. we can keep the
code same as for Windows with the cast - and then the codepath would be the
same?

[lawrencewu, 2016/11/24 18:48:59]

Made nearly identical, still have to do some windows-specific casting.

+    InstantiateFieldTrialAllocatorIfNeeded();
+    // If the readonly handle didn't get duplicated properly, then fallback to
+    // original behavior.
+    if (global_->readonly_allocator_handle_ == kInvalidPlatformFile) {
+      AddForceFieldTrialsFlag(cmd_line);
+      return;
+    }
+
+    std::string field_trial_handle =
+        std::to_string(global_->readonly_allocator_handle_);
+    cmd_line->AppendSwitchASCII(field_trial_handle_switch, field_trial_handle);
+    global_->field_trial_allocator_->UpdateTrackingHistograms();
+    return;
+  }
+#endif
+
   AddForceFieldTrialsFlag(cmd_line);
 }
 
 // static
 FieldTrial* FieldTrialList::CreateFieldTrial(
     const std::string& name,
     const std::string& group_name) {
   DCHECK(global_);
   DCHECK_GE(name.size(), 0u);
   DCHECK_GE(group_name.size(), 0u);
@@ skipping 66 matching lines @@
 }
 
 // static
 size_t FieldTrialList::GetFieldTrialCount() {
   if (!global_)
     return 0;
   AutoLock auto_lock(global_->lock_);
   return global_->registered_.size();
 }
 
+// static
+bool FieldTrialList::CreateTrialsFromHandleSwitch(
+    const std::string& handle_switch) {
 #if defined(OS_WIN)
+  int field_trial_handle = std::stoi(handle_switch);
+  HANDLE handle = reinterpret_cast<HANDLE>(field_trial_handle);
+  SharedMemoryHandle shm_handle(handle, GetCurrentProcId());
+  return FieldTrialList::CreateTrialsFromSharedMemoryHandle(shm_handle);
+#endif
+
+#if defined(POSIX_WITH_ZYGOTE)
+  int fd_key = std::stoi(handle_switch);
+  int fd = GlobalDescriptors::GetInstance()->Get(fd_key);
+  SharedMemoryHandle shm_handle(fd, true);
+  return FieldTrialList::CreateTrialsFromSharedMemoryHandle(shm_handle);
+#endif
+
+#if !defined(OS_WIN) && !defined(POSIX_WITH_ZYGOTE)
+  return false;
+#endif
+}
+
+#if !defined(OS_NACL)
 // static
-bool FieldTrialList::CreateTrialsFromWindowsHandle(HANDLE handle) {
-  SharedMemoryHandle shm_handle(handle, GetCurrentProcId());
-
+bool FieldTrialList::CreateTrialsFromSharedMemoryHandle(
+    SharedMemoryHandle shm_handle) {
   // shm gets deleted when it gets out of scope, but that's OK because we need
   // it only for the duration of this method.
   std::unique_ptr<SharedMemory> shm(new SharedMemory(shm_handle, true));
   if (!shm.get()->Map(kFieldTrialAllocationSize))
     TerminateBecauseOutOfMemory(kFieldTrialAllocationSize);
 
   return FieldTrialList::CreateTrialsFromSharedMemory(std::move(shm));
 }
 #endif
 
@@ skipping 69 matching lines @@
 #if !defined(OS_NACL)
 // static
 void FieldTrialList::InstantiateFieldTrialAllocatorIfNeeded() {
   if (!global_)
     return;
   AutoLock auto_lock(global_->lock_);
   // Create the allocator if not already created and add all existing trials.
   if (global_->field_trial_allocator_ != nullptr)
     return;
 
+  SharedMemoryCreateOptions options;
+  options.size = kFieldTrialAllocationSize;
+  options.share_read_only = true;
+
   std::unique_ptr<SharedMemory> shm(new SharedMemory());
-  if (!shm->CreateAndMapAnonymous(kFieldTrialAllocationSize))
+  if (!shm->Create(options))
+    TerminateBecauseOutOfMemory(kFieldTrialAllocationSize);
+
+  if (!shm->Map(kFieldTrialAllocationSize))
     TerminateBecauseOutOfMemory(kFieldTrialAllocationSize);
 
   global_->field_trial_allocator_.reset(
       new FieldTrialAllocator(std::move(shm), 0, kAllocatorName, false));
   global_->field_trial_allocator_->CreateTrackingHistograms(kAllocatorName);
 
   // Add all existing field trials.
   for (const auto& registered : global_->registered_) {
     AddToAllocatorWhileLocked(registered.second);
   }
 
-#if defined(OS_WIN)
+#if defined(OS_WIN) || defined(POSIX_WITH_ZYGOTE)
   // Set |readonly_allocator_handle_| so we can pass it to be inherited and
   // via the command line.
   global_->readonly_allocator_handle_ =
       CreateReadOnlyHandle(global_->field_trial_allocator_.get());
 #endif
 }
 #endif
 
 // static
 void FieldTrialList::AddToAllocatorWhileLocked(FieldTrial* field_trial) {
@@ skipping 94 matching lines @@
     return;
   }
   AutoLock auto_lock(global_->lock_);
   CHECK(!global_->PreLockedFind(trial->trial_name())) << trial->trial_name();
   trial->AddRef();
   trial->SetTrialRegistered();
   global_->registered_[trial->trial_name()] = trial;
 }
 
 }  // namespace base