tracing v2: building blocks for reading-back trace protobufs

components/tracing/core/proto_utils.cc

+// Copyright 2016 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/tracing/core/proto_utils.h"
+
+#include <limits>
+
+#include "base/sys_byteorder.h"
+
+#define CHECK_PTR_LE(a, b) \
+  CHECK_LE(reinterpret_cast<const void*>(a), reinterpret_cast<const void*>(b))
+namespace tracing {
+namespace v2 {
+namespace proto {
+
+const uint8_t* ParseVarInt(const uint8_t* start,
+                           const uint8_t* end,
+                           uint64_t* value) {
+  const uint8_t* pos = start;
+  uint64_t shift = 0;
+  *value = 0;
+  do {
+    CHECK_PTR_LE(pos, end - 1);
+    DCHECK_LT(shift, 64ull);
+    *value |= static_cast<uint64_t>(*pos & 0x7f) << shift;
+    shift += 7;
+  } while (*pos++ & 0x80);
+  return pos;
+}
+
+const uint8_t* ParseField(const uint8_t* start,
+                          const uint8_t* end,
+                          uint32_t* field_id,
+                          FieldType* field_type,
+                          uint64_t* field_intvalue) {
+  // The first byte of a proto field is structured as follows:
+  // The least 3 significant bits determine the field type.
+  // The most 5 significant bits determine the field id. If MSB == 1, the
+  // field id continues on the next bytes following the VarInt encoding.
+  const uint8_t kFieldTypeNumBits = 3;
+  const uint8_t kFieldTypeMask = (1 << kFieldTypeNumBits) - 1;  // 0000 0111;
+
+  const uint8_t* pos = start;
+  CHECK_PTR_LE(pos, end - 1);
+  *field_type = static_cast<FieldType>(*pos & kFieldTypeMask);
+
+  uint64_t raw_field_id;
+  pos = ParseVarInt(pos, end, &raw_field_id);
+  raw_field_id >>= kFieldTypeNumBits;
+
+  DCHECK_LE(raw_field_id, std::numeric_limits<uint32_t>::max());
+  *field_id = static_cast<uint32_t>(raw_field_id);
+
+  switch (*field_type) {
+    case kFieldTypeFixed64: {
+      CHECK_PTR_LE(pos + sizeof(uint64_t), end);
+      memcpy(field_intvalue, pos, sizeof(uint64_t));
+      *field_intvalue = base::ByteSwapToLE64(*field_intvalue);
+      pos += sizeof(uint64_t);
+      break;
+    }
+    case kFieldTypeFixed32: {
+      CHECK_PTR_LE(pos + sizeof(uint32_t), end);
+      uint32_t tmp;
+      memcpy(&tmp, pos, sizeof(uint32_t));
+      *field_intvalue = base::ByteSwapToLE32(tmp);
+      pos += sizeof(uint32_t);
+      break;
+    }
+    case kFieldTypeVarInt: {
+      pos = ParseVarInt(pos, end, field_intvalue);
+      break;
+    }
+    case kFieldTypeLengthDelimited: {
+      pos = ParseVarInt(pos, end, field_intvalue);
+      pos += *field_intvalue;
+      CHECK_PTR_LE(pos, end);
+      break;
+    }
+    default:
+      NOTREACHED() << "Unsupported proto field type " << *field_type;
+  }
+  return pos;
+}
+
+}  // namespace proto
+}  // namespace v2
+}  // namespace tracing