Add IPC ParamTraits for c-style arrays

ipc/ipc_message_utils.h

Index: ipc/ipc_message_utils.h
diff --git a/ipc/ipc_message_utils.h b/ipc/ipc_message_utils.h
index 58d6aa26da34fb125d45bd9f2ddd3f767a2f7313..e60c4edfb23b746158d6720379eaa39a63cfd903 100644
--- a/ipc/ipc_message_utils.h
+++ b/ipc/ipc_message_utils.h
@@ -307,6 +307,44 @@ struct IPC_EXPORT ParamTraits<double> {
   static void Log(const param_type& p, std::string* l);
 };
 
+template <class P, size_t Size>
+struct ParamTraits<P[Size]> {
+  using param_type = P[Size];
+  static void GetSize(base::PickleSizer* sizer, const param_type& p) {
+    GetParamSize(sizer, static_cast<int>(Size));

[dcheng, 2017/02/08 00:13:37]

Let's use strict_cast instead of static_cast throughout. I think it's highly
unlikely we'll have an array that will overflow the size here... but it's a
compile-time check, it should be free and can't hurt.

[ericrk, 2017/02/08 18:57:49]

So, strict_cast seems to only consider the types being cast between, not the
value being cast (even when the value is a compile-time constant), so
strict_cast raises an error here, as you can't safely go from size_t > int for
arbitrary values.

I added a static_assert at the beginning of the class to ensure that Size <=
std::numeric_limits<int>::max(), which should guard against the same issues.

+    for (size_t i = 0; i < Size; i++)
+      GetParamSize(sizer, p[i]);
+  }
+  static void Write(base::Pickle* m, const param_type& p) {
+    WriteParam(m, static_cast<int>(Size));
+    for (size_t i = 0; i < Size; i++)
+      WriteParam(m, p[i]);
+  }
+  static bool Read(const base::Pickle* m,
+                   base::PickleIterator* iter,
+                   param_type* r) {
+    int size;
+    // ReadLength() checks for < 0 itself.
+    if (!iter->ReadLength(&size))
+      return false;
+    // For a fixed size array, a size mismatch should be impossible.

[dcheng, 2017/02/08 00:13:37]

It's not impossible if the renderer sends us bad data on purpose =)

[ericrk, 2017/02/08 18:57:49]

heh, true enough, updated the comment

+    if (size != Size)
+      return false;
+    for (size_t i = 0; i < Size; i++) {
+      if (!ReadParam(m, iter, &(*r)[i]))
+        return false;
+    }
+    return true;
+  }
+  static void Log(const param_type& p, std::string* l) {
+    for (size_t i = 0; i < Size; ++i) {
+      if (i != 0)
+        l->append(" ");
+      LogParam((p[i]), l);
+    }
+  }
+};
+
 // STL ParamTraits -------------------------------------------------------------
 
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