message: more iterators, less problems

This seems like a pretty nice way to do this, since it lets us produce
the sliced version of each value rather than the somewhat janky many
pointer.

examples/headers.zig

@@ -15,14 +15,20 @@ pub fn main() !void {
 
     try message.deleteHeader("My-Key3");
 
-    var iter = try message.headerIterator();
-    defer iter.destroy();
+    {
+        var iter = try message.headerIterator();
+        defer iter.destroy();
 
-    while (try iter.next()) |pair| {
-        std.debug.print(
-            "Key: '{s}', Value: '{s}'\n",
-            .{ pair.key, pair.value orelse "null" },
-        );
+        while (iter.next()) |resolv| {
+            var val_iter = try resolv.getValueIterator();
+            defer val_iter.destroy();
+
+            std.debug.print("key '{s}' got: ", .{resolv.key});
+            while (val_iter.next()) |value| {
+                std.debug.print("'{s}', ", .{value});
+            }
+            std.debug.print("\n", .{});
+        }
     }
 
     const subscription = try connection.subscribeSync("subject");
@@ -33,13 +39,12 @@ pub fn main() !void {
     defer received.destroy();
 
     {
-        const vals1 = try received.getAllHeaderValues("My-Key1");
-        defer std.heap.raw_c_allocator.free(vals1);
-        std.debug.print("For key 'My-Key1' got: ", .{});
-        for (vals1) |value| {
-            const val = std.mem.sliceTo(value, 0);
+        var iter = try received.getHeaderValueIterator("My-Key1");
+        defer iter.destroy();
 
-            std.debug.print("'{s}', ", .{val});
+        std.debug.print("For key 'My-Key1' got: ", .{});
+        while (iter.next()) |value| {
+            std.debug.print("'{s}', ", .{value});
         }
         std.debug.print("\n", .{});
     }

src/message.zig

@@ -76,6 +76,10 @@ pub const Message = opaque {
         };
     }
 
+    pub fn getHeaderValueIterator(self: *Message, key: [:0]const u8) Error!HeaderValueIterator {
+        return .{ .values = try self.getAllHeaderValues(key) };
+    }
+
     pub fn getAllHeaderKeys(self: *Message) Error![][*:0]const u8 {
         var keys: [*c][*c]const u8 = undefined;
         var count: c_int = 0;
@@ -93,23 +97,56 @@ pub const Message = opaque {
         };
     }
 
+    pub const HeaderValueIterator = struct {
+        values: [][*:0]const u8,
+        index: usize = 0,
+
+        pub fn destroy(self: HeaderValueIterator) void {
+            std.heap.raw_c_allocator.free(self.values);
+        }
+
+        pub fn next(self: *HeaderValueIterator) ?[:0]const u8 {
+            if (self.index >= self.values.len) return null;
+            defer self.index += 1;
+
+            return std.mem.sliceTo(self.values[self.index], 0);
+        }
+    };
+
     pub const HeaderIterator = struct {
         message: *Message,
         keys: [][*:0]const u8,
         index: usize = 0,
 
+        pub const ValueResolver = struct {
+            message: *Message,
+            key: [:0]const u8,
+
+            pub fn getValue(self: ValueResolver) Error![:0]const u8 {
+                // TODO: if we didn't care about the lifecycle of self.message, we
+                // could do catch unreachable here and make this error-free
+                return try self.message.getHeaderValue(self.key);
+            }
+
+            pub fn getValueIterator(self: ValueResolver) Error!HeaderValueIterator {
+                return .{
+                    .values = try self.message.getAllHeaderValues(self.key),
+                };
+            }
+        };
+
         pub fn destroy(self: *HeaderIterator) void {
             std.heap.raw_c_allocator.free(self.keys);
         }
 
-        pub fn next(self: *HeaderIterator) Error!?struct { key: [:0]const u8, value: ?[:0]const u8 } {
+        pub fn next(self: *HeaderIterator) ?ValueResolver {
             if (self.index >= self.keys.len) return null;
             defer self.index += 1;
 
             const sliced = std.mem.sliceTo(self.keys[self.index], 0);
             return .{
+                .message = self.message,
                 .key = sliced,
-                .value = try self.message.getHeaderValue(sliced),
             };
         }