Implement a BinarySink writing to a fixed-size buffer.

This is one of marshal.c's small collection of handy BinarySink
adapters to existing kinds of thing, alongside stdio_sink and
bufchain_sink. It writes into a fixed-size buffer, discarding all
writes after the buffer fills up, and sets a flag to let you know if
it overflowed.

There was one of these in Windows Pageant a while back, under the name
'struct PageantReply' (introduced in commit b6cbad89fc, removed
again in 98538caa39 when the named-pipe revamp made it
unnecessary). This is the same idea but centralised for reusability.

defs.h

@@ -91,6 +91,7 @@ typedef struct BinarySink BinarySink;
 typedef struct BinarySource BinarySource;
 typedef struct stdio_sink stdio_sink;
 typedef struct bufchain_sink bufchain_sink;
+typedef struct buffer_sink buffer_sink;
 typedef struct handle_sink handle_sink;
 
 typedef struct IdempotentCallback IdempotentCallback;

marshal.h

@@ -353,7 +353,14 @@ struct bufchain_sink {
     bufchain *ch;
     BinarySink_IMPLEMENTATION;
 };
+struct buffer_sink {
+    char *out;
+    size_t space;
+    bool overflowed;
+    BinarySink_IMPLEMENTATION;
+};
 void stdio_sink_init(stdio_sink *sink, FILE *fp);
 void bufchain_sink_init(bufchain_sink *sink, bufchain *ch);
+void buffer_sink_init(buffer_sink *sink, void *buffer, size_t len);
 
 #endif /* PUTTY_MARSHAL_H */

utils/marshal.c

@@ -336,3 +336,23 @@ void bufchain_sink_init(bufchain_sink *sink, bufchain *ch)
     sink->ch = ch;
     BinarySink_INIT(sink, bufchain_sink_write);
 }
+
+static void buffer_sink_write(BinarySink *bs, const void *data, size_t len)
+{
+    buffer_sink *sink = BinarySink_DOWNCAST(bs, buffer_sink);
+    if (len > sink->space) {
+        len = sink->space;
+        sink->overflowed = true;
+    }
+    memcpy(sink->out, data, len);
+    sink->space -= len;
+    sink->out += len;
+}
+
+void buffer_sink_init(buffer_sink *sink, void *buffer, size_t len)
+{
+    sink->out = buffer;
+    sink->space = len;
+    sink->overflowed = false;
+    BinarySink_INIT(sink, buffer_sink_write);
+}