From 7b9a4bcff781eaa865d1cd8a982620c2f64a0808 Mon Sep 17 00:00:00 2001
From: Dragos Carp <dragoscarp@gmail.com>
Date: Thu, 20 Dec 2018 12:22:49 +0100
Subject: [PATCH] Allow empty messages

---
 src/google/protobuf/common.d        |  8 +++++++-
 src/google/protobuf/decoding.d      | 11 +++++++++++
 src/google/protobuf/encoding.d      | 17 +++++++++++++++++
 src/google/protobuf/json_decoding.d | 12 ++++++++++++
 src/google/protobuf/json_encoding.d | 13 +++++++++++++
 5 files changed, 60 insertions(+), 1 deletion(-)

diff --git a/src/google/protobuf/common.d b/src/google/protobuf/common.d
index 21b75b7..5a5c791 100644
--- a/src/google/protobuf/common.d
+++ b/src/google/protobuf/common.d
@@ -144,7 +144,6 @@ template Message(T)
 
     alias fieldNames = staticMap!(fieldName, fields);
 
-    static assert(fields.length > 0, "Definition of '" ~ T.stringof ~ "' has no Proto field");
     static assert(allSatisfy!(validateField, fields), "'" ~ T.stringof ~ "' has invalid fields");
 
     private alias unsortedFields = getSymbolsByUDA!(T, Proto);
@@ -165,6 +164,13 @@ unittest
 
     static assert(Message!Test.fieldNames == AliasSeq!("bar", "foo"));
     static assert(Message!Test.protos == AliasSeq!(Proto(2, Wire.fixed, No.packed), Proto(3, Wire.none, No.packed)));
+
+    static class EmptyMessage
+    {
+    }
+
+    static assert(is(Message!EmptyMessage.fieldNames == AliasSeq!()));
+    static assert(is(Message!EmptyMessage.protos == AliasSeq!()));
 }
 
 template validateField(alias field)
diff --git a/src/google/protobuf/decoding.d b/src/google/protobuf/decoding.d
index 912f6b0..6c3fd4f 100644
--- a/src/google/protobuf/decoding.d
+++ b/src/google/protobuf/decoding.d
@@ -218,6 +218,17 @@ unittest
     assert(foo.qux);
 }
 
+unittest
+{
+    static class EmptyMessage
+    {
+    }
+
+    ubyte[] buff = [];
+    auto emptyMessage = buff.fromProtobuf!EmptyMessage;
+    assert(buff.empty);
+}
+
 private static void fromProtobufByField(alias field, T, R)(ref R inputRange, ref T message)
 if (isInputRange!R)
 {
diff --git a/src/google/protobuf/encoding.d b/src/google/protobuf/encoding.d
index 1aeb2f3..a2822c5 100644
--- a/src/google/protobuf/encoding.d
+++ b/src/google/protobuf/encoding.d
@@ -124,12 +124,17 @@ unittest
 auto toProtobuf(T)(T value)
 if (is(T == class) || is(T == struct))
 {
+    import std.meta : AliasSeq;
     import std.traits : hasMember;
 
     static if (hasMember!(T, "toProtobuf"))
     {
         return value.toProtobuf;
     }
+    else static if (is(Message!T.fields == AliasSeq!()))
+    {
+        return cast(ubyte[]) null;
+    }
     else
     {
         import std.algorithm : joiner;
@@ -188,6 +193,18 @@ unittest
     assert(foo.toProtobuf.array == [0x08, 0x05, 0x11, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x18, 0x01]);
 }
 
+unittest
+{
+    import std.array : array;
+
+    struct EmptyMessage
+    {
+    }
+
+    EmptyMessage emptyMessage;
+    assert(emptyMessage.toProtobuf.empty);
+}
+
 unittest
 {
     struct Foo
diff --git a/src/google/protobuf/json_decoding.d b/src/google/protobuf/json_decoding.d
index 430f679..4a334e2 100644
--- a/src/google/protobuf/json_decoding.d
+++ b/src/google/protobuf/json_decoding.d
@@ -294,6 +294,18 @@ unittest
     assertThrown!ProtobufException(fromJSONValue!Foo(parseJSON(`{"d":10, "e":"abc"}`)));
 }
 
+unittest
+{
+    import std.json : parseJSON;
+
+    struct EmptyMessage
+    {
+    }
+
+    assert(fromJSONValue!EmptyMessage(parseJSON(`{}`)) == EmptyMessage());
+    assert(fromJSONValue!EmptyMessage(parseJSON(`{"a":10, "b":"abc"}`)) == EmptyMessage());
+}
+
 private template oneofs(T)
 {
     import std.meta : NoDuplicates, staticMap;
diff --git a/src/google/protobuf/json_encoding.d b/src/google/protobuf/json_encoding.d
index 95c7269..87967c1 100644
--- a/src/google/protobuf/json_encoding.d
+++ b/src/google/protobuf/json_encoding.d
@@ -137,6 +137,19 @@ unittest
     assert(foo.toJSONValue == parseJSON(`{"a":10, "b":"abc"}`));
 }
 
+unittest
+{
+    import std.json : parseJSON;
+
+    struct EmptyMessage
+    {
+    }
+
+    EmptyMessage emptyMessage;
+
+    assert(emptyMessage.toJSONValue == parseJSON(`{}`));
+}
+
 unittest
 {
     import std.json : parseJSON;