Add BinarySink wrappers on existing forms of output.

There's now a stdio_sink, whose write function calls fwrite on the
given FILE *; a bufchain_sink, whose write function appends to the
given bufchain; and on Windows there's a handle_sink whose write
function writes to the given 'struct handle'. (That is, not the raw
Windows HANDLE, but our event-loop-friendly wrapper on it.)

Not yet used for anything, but they're about to be.

defs.h

@@ -40,6 +40,9 @@ typedef struct RSAKey RSAKey;
 
 typedef struct BinarySink BinarySink;
 typedef struct BinarySource BinarySource;
+typedef struct stdio_sink stdio_sink;
+typedef struct bufchain_sink bufchain_sink;
+typedef struct handle_sink handle_sink;
 
 typedef struct IdempotentCallback IdempotentCallback;
 

marshal.c

@@ -234,3 +234,27 @@ ptrlen BinarySource_get_pstring(BinarySource *src)
 
     return make_ptrlen(consume(len), len);
 }
+
+static void stdio_sink_write(BinarySink *bs, const void *data, size_t len)
+{
+    stdio_sink *sink = BinarySink_DOWNCAST(bs, stdio_sink);
+    fwrite(data, 1, len, sink->fp);
+}
+
+void stdio_sink_init(stdio_sink *sink, FILE *fp)
+{
+    sink->fp = fp;
+    BinarySink_INIT(sink, stdio_sink_write);
+}
+
+static void bufchain_sink_write(BinarySink *bs, const void *data, size_t len)
+{
+    bufchain_sink *sink = BinarySink_DOWNCAST(bs, bufchain_sink);
+    bufchain_add(sink->ch, data, len);
+}
+
+void bufchain_sink_init(bufchain_sink *sink, bufchain *ch)
+{
+    sink->ch = ch;
+    BinarySink_INIT(sink, bufchain_sink_write);
+}

marshal.h

@@ -3,6 +3,8 @@
 
 #include "defs.h"
 
+#include <stdio.h>
+
 /*
  * A sort of 'abstract base class' or 'interface' or 'trait' which is
  * the common feature of all types that want to accept data formatted
@@ -301,4 +303,21 @@ ptrlen BinarySource_get_pstring(BinarySource *);
 mp_int *BinarySource_get_mp_ssh1(BinarySource *src);
 mp_int *BinarySource_get_mp_ssh2(BinarySource *src);
 
+/*
+ * A couple of useful standard BinarySink implementations, which live
+ * as sensibly here as anywhere else: one that makes a BinarySink
+ * whose effect is to write to a stdio stream, and one whose effect is
+ * to append to a bufchain.
+ */
+struct stdio_sink {
+    FILE *fp;
+    BinarySink_IMPLEMENTATION;
+};
+struct bufchain_sink {
+    bufchain *ch;
+    BinarySink_IMPLEMENTATION;
+};
+void stdio_sink_init(stdio_sink *sink, FILE *fp);
+void bufchain_sink_init(bufchain_sink *sink, bufchain *ch);
+
 #endif /* PUTTY_MARSHAL_H */

windows/winhandl.c

@@ -715,3 +715,15 @@ void *handle_get_privdata(struct handle *h)
 {
     return h->u.g.privdata;
 }
+
+static void handle_sink_write(BinarySink *bs, const void *data, size_t len)
+{
+    handle_sink *sink = BinarySink_DOWNCAST(bs, handle_sink);
+    handle_write(sink->h, data, len);
+}
+
+void handle_sink_init(handle_sink *sink, struct handle *h)
+{
+    sink->h = h;
+    BinarySink_INIT(sink, handle_sink_write);
+}

windows/winstuff.h

@@ -24,6 +24,7 @@
 #endif
 
 #include "defs.h"
+#include "marshal.h"
 
 #include "tree234.h"
 
@@ -621,6 +622,12 @@ size_t handle_backlog(struct handle *h);
 void *handle_get_privdata(struct handle *h);
 struct handle *handle_add_foreign_event(HANDLE event,
                                         void (*callback)(void *), void *ctx);
+/* Analogue of stdio_sink in marshal.h, for a Windows handle */
+struct handle_sink {
+    struct handle *h;
+    BinarySink_IMPLEMENTATION;
+};
+void handle_sink_init(handle_sink *sink, struct handle *h);
 
 /*
  * winpgntc.c needs to schedule callbacks for asynchronous agent