Check snapshots for feature compatibility.

runtime/vm/snapshot.cc

Index: runtime/vm/snapshot.cc
diff --git a/runtime/vm/snapshot.cc b/runtime/vm/snapshot.cc
index 33570ae8d4c29cf603116073a5913cf89435c7bc..8c1029c16e161a251663ea670b7d2b02e90c23db 100644
--- a/runtime/vm/snapshot.cc
+++ b/runtime/vm/snapshot.cc
@@ -603,6 +603,10 @@ RawApiError* SnapshotReader::ReadFullSnapshot() {
   if (error != ApiError::null()) {
     return error;
   }
+  error = VerifyFeatures();
+  if (error != ApiError::null()) {
+    return error;
+  }
 
   // The version string matches. Read the rest of the snapshot.
 
@@ -674,6 +678,10 @@ RawObject* SnapshotReader::ReadScriptSnapshot() {
   if (error != ApiError::null()) {
     return error;
   }
+  error = VerifyFeatures();
+  if (error != ApiError::null()) {
+    return error;
+  }
 
   // The version string matches. Read the rest of the snapshot.
   obj_ = ReadObject();
@@ -706,7 +714,7 @@ RawApiError* SnapshotReader::VerifyVersion() {
     OS::SNPrint(message_buffer,
                 kMessageBufferSize,
                 "No full snapshot version found, expected '%s'",
-                Version::SnapshotString());
+                expected_version);
     // This can also fail while bringing up the VM isolate, so make sure to
     // allocate the error message in old space.
     const String& msg = String::Handle(String::New(message_buffer, Heap::kOld));
@@ -723,7 +731,7 @@ RawApiError* SnapshotReader::VerifyVersion() {
                 kMessageBufferSize,
                 "Wrong %s snapshot version, expected '%s' found '%s'",
                 (Snapshot::IsFull(kind_)) ? "full" : "script",
-                Version::SnapshotString(),
+                expected_version,
                 actual_version);
     free(actual_version);
     // This can also fail while bringing up the VM isolate, so make sure to
@@ -736,6 +744,41 @@ RawApiError* SnapshotReader::VerifyVersion() {
 }
 
 
+RawApiError* SnapshotReader::VerifyFeatures() {
+  // If the features string doesn't match, return an error.
+  // Note: New things are allocated only if we're going to return an error.
+
+  const char* expected_features = Dart::FeaturesString(kind_);
+  ASSERT(expected_features != NULL);
+  const intptr_t expected_len = strlen(expected_features);
+
+  const char* features = reinterpret_cast<const char*>(CurrentBufferAddress());
+  ASSERT(features != NULL);
+  intptr_t buffer_len = strnlen(features, PendingBytes());
+  if (strncmp(features, expected_features,
+              expected_len < buffer_len ? expected_len : buffer_len)) {

[siva, 2016/05/16 16:48:00]

why not write this as 
if ((expected_len > buffer_len) ||
    strncmp(features, expected_features, expected_len)) {
  .....
  .....
}

[rmacnak, 2016/05/16 20:59:56]

Done.

+    const intptr_t kMessageBufferSize = 256;
+    char message_buffer[kMessageBufferSize];
+    char* actual_features = OS::StrNDup(features, buffer_len < 128 ? buffer_len
+                                                                   : 128);
+    OS::SNPrint(message_buffer,
+                kMessageBufferSize,
+                "Wrong features in snapshot, expected '%s' found '%s'",
+                expected_features,
+                actual_features);
+    free(const_cast<char*>(expected_features));
+    free(actual_features);
+    // This can also fail while bringing up the VM isolate, so make sure to
+    // allocate the error message in old space.
+    const String& msg = String::Handle(String::New(message_buffer, Heap::kOld));
+    return ApiError::New(msg, Heap::kOld);
+  }
+  free(const_cast<char*>(expected_features));
+  Advance(expected_len + 1);
+  return ApiError::null();
+}
+
+
 #define ALLOC_NEW_OBJECT_WITH_LEN(type, length)                                \
   ASSERT(Snapshot::IsFull(kind_));                                            \
   ASSERT_NO_SAFEPOINT_SCOPE();                                                 \
@@ -1719,6 +1762,10 @@ RawApiError* VmIsolateSnapshotReader::ReadVmIsolateSnapshot() {
   if (error != ApiError::null()) {
     return error;
   }
+  error = VerifyFeatures();
+  if (error != ApiError::null()) {
+    return error;
+  }
 
   // The version string matches. Read the rest of the snapshot.
 
@@ -2114,6 +2161,7 @@ void FullSnapshotWriter::WriteVmIsolateSnapshot() {
 
     // Write out the version string.
     writer.WriteVersion();
+    writer.WriteFeatures();
 
     /*
      * Now Write out the following
@@ -2165,6 +2213,7 @@ void FullSnapshotWriter::WriteIsolateFullSnapshot() {
 
     // Write out the version string.
     writer.WriteVersion();
+    writer.WriteFeatures();
 
     // Write out the full snapshot.
 
@@ -2696,6 +2745,16 @@ void SnapshotWriter::WriteVersion() {
 }
 
 
+void SnapshotWriter::WriteFeatures() {
+  const char* expected_features = Dart::FeaturesString(kind_);
+  ASSERT(expected_features != NULL);
+  const intptr_t features_len = strlen(expected_features);
+  WriteBytes(reinterpret_cast<const uint8_t*>(expected_features),
+             features_len + 1);
+  free(const_cast<char*>(expected_features));
+}
+
+
 ScriptSnapshotWriter::ScriptSnapshotWriter(uint8_t** buffer,
                                            ReAlloc alloc)
     : SnapshotWriter(Thread::Current(),
@@ -2727,6 +2786,7 @@ void ScriptSnapshotWriter::WriteScriptSnapshot(const Library& lib) {
 
     // Write out the version string.
     WriteVersion();
+    WriteFeatures();
 
     // Write out the library object.
     {