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mojo/shell/application_manager/data_pipe_peek.cc

+// Copyright 2014 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 "mojo/shell/application_manager/data_pipe_peek.h"
+
+#include <stdint.h>
+
+#include "base/bind.h"
+#include "base/macros.h"
+
+namespace mojo {
+namespace shell {
+
+namespace {
+
+// Sleep for as long as max_sleep_micros if the deadline hasn't been reached
+// and the number of bytes read is still increasing. Returns true if sleep
+// was actually called.
+//
+// This class is a substitute for being able to wait until N bytes are available
+// from a data pipe. The MaybeSleep method is called when num_bytes_read are
+// available but more are needed by the Peek operation. If a second
+// Peek operation finds the same number of bytes after sleeping we assume
+// that there's no point in trying again.
+// TODO(hansmuller): this heuristic is weak. crbug.com/429377
+class PeekSleeper {
+ public:
+  explicit PeekSleeper(MojoTimeTicks deadline)
+      : deadline_(deadline),
+        last_number_bytes_read_(0) {}
+
+  bool MaybeSleep(uint32_t num_bytes_read) {
+    if (num_bytes_read > 0 && last_number_bytes_read_ >= num_bytes_read)
+      return false;
+    last_number_bytes_read_ = num_bytes_read;
+
+    MojoTimeTicks now(GetTimeTicksNow());
+    if (now > deadline_)
+      return false;
+
+    MojoTimeTicks sleep_time =
+        (deadline_ == 0) ? kMaxSleepMicros
+                         : std::min<int64>(deadline_ - now, kMaxSleepMicros);
+    base::PlatformThread::Sleep(base::TimeDelta::FromMicroseconds(sleep_time));
+    return true;
+  }
+
+ private:
+  static const MojoTimeTicks kMaxSleepMicros = 1000 * 10;  // 10 ms
+
+  const MojoTimeTicks deadline_;  // 0 => MOJO_DEADLINE_INDEFINITE
+  uint32_t last_number_bytes_read_;
+
+  DISALLOW_COPY_AND_ASSIGN(PeekSleeper);
+};
+
+const MojoTimeTicks PeekSleeper::kMaxSleepMicros;
+
+enum PeekStatus { kSuccess, kFail, kKeepReading };
+typedef const base::Callback<PeekStatus(const void*, uint32_t, std::string*)>&
+    PeekFunc;
+
+// When data is available on source, call peek_func and then either return true
+// and value, continue waiting for enough data to satisfy peek_func, or fail
+// and return false. Fail if the timeout is exceeded.
+// This function is not guaranteed to work correctly if applied to a data pipe
+// that's already been read from.
+bool BlockingPeekHelper(DataPipeConsumerHandle source,
+                        std::string* value,
+                        MojoDeadline timeout,
+                        PeekFunc peek_func) {
+  DCHECK(value);
+  value->clear();
+
+  MojoTimeTicks deadline =
+      (timeout == MOJO_DEADLINE_INDEFINITE)
+          ? 0
+          : 1 + GetTimeTicksNow() + static_cast<MojoTimeTicks>(timeout);
+  PeekSleeper sleeper(deadline);
+
+  MojoResult result;
+  do {
+    const void* buffer;
+    uint32_t num_bytes;
+    result =
+        BeginReadDataRaw(source, &buffer, &num_bytes, MOJO_READ_DATA_FLAG_NONE);
+
+    if (result == MOJO_RESULT_OK) {
+      PeekStatus status = peek_func.Run(buffer, num_bytes, value);
+      CHECK_EQ(EndReadDataRaw(source, 0), MOJO_RESULT_OK);
+      switch (status) {
+        case PeekStatus::kSuccess:
+          return true;
+        case PeekStatus::kFail:
+          return false;
+        case PeekStatus::kKeepReading:
+          break;
+      }
+      if (!sleeper.MaybeSleep(num_bytes))
+        return false;
+    } else if (result == MOJO_RESULT_SHOULD_WAIT) {
+      MojoTimeTicks now(GetTimeTicksNow());
+      if (timeout == MOJO_DEADLINE_INDEFINITE || now < deadline)
+        result =
+            Wait(source, MOJO_HANDLE_SIGNAL_READABLE, deadline - now, nullptr);
+    }
+  } while (result == MOJO_RESULT_OK);
+
+  return false;
+}
+
+PeekStatus PeekLine(size_t max_line_length,
+                    const void* buffer,
+                    uint32_t buffer_num_bytes,
+                    std::string* line) {
+  const char* p = static_cast<const char*>(buffer);
+  size_t max_p_index = std::min<size_t>(buffer_num_bytes, max_line_length);
+  for (size_t i = 0; i < max_p_index; i++) {
+    if (p[i] == '\n') {
+      *line = std::string(p, i + 1);  // Include the trailing newline.
+      return PeekStatus::kSuccess;
+    }
+  }
+  return (buffer_num_bytes >= max_line_length) ? PeekStatus::kFail
+                                               : PeekStatus::kKeepReading;
+}
+
+PeekStatus PeekNBytes(size_t bytes_length,
+                      const void* buffer,
+                      uint32_t buffer_num_bytes,
+                      std::string* bytes) {
+  if (buffer_num_bytes >= bytes_length) {
+    const char* p = static_cast<const char*>(buffer);
+    *bytes = std::string(p, bytes_length);
+    return PeekStatus::kSuccess;
+  }
+  return PeekStatus::kKeepReading;
+}
+
+}  // namespace
+
+bool BlockingPeekNBytes(DataPipeConsumerHandle source,
+                        std::string* bytes,
+                        size_t bytes_length,
+                        MojoDeadline timeout) {
+  PeekFunc peek_nbytes = base::Bind(PeekNBytes, bytes_length);
+  return BlockingPeekHelper(source, bytes, timeout, peek_nbytes);
+}
+
+bool BlockingPeekLine(DataPipeConsumerHandle source,
+                      std::string* line,
+                      size_t max_line_length,
+                      MojoDeadline timeout) {
+  PeekFunc peek_line = base::Bind(PeekLine, max_line_length);
+  return BlockingPeekHelper(source, line, timeout, peek_line);
+}
+
+}  // namespace shell
+}  // namespace mojo