Revert "Change the substream implementation in pb_decode."

This reverts commit dc2da0edc5.

Add pb_close_string_substream() for copying back the state.
This makes adding error messages easier in the future, as also
them need to be propagated back from the substream.

example_unions/decode.c

@@ -45,10 +45,13 @@ const pb_field_t* decode_unionmessage_type(pb_istream_t *stream)
 bool decode_unionmessage_contents(pb_istream_t *stream, const pb_field_t fields[], void *dest_struct)
 {
     pb_istream_t substream;
+    bool status;
     if (!pb_make_string_substream(stream, &substream))
         return false;
     
-    return pb_decode(&substream, fields, dest_struct);
+    status = pb_decode(&substream, fields, dest_struct);
+    pb_close_string_substream(stream, &substream);
+    return status;
 }
 
 int main()

pb_decode.c

@@ -186,29 +186,29 @@ static bool checkreturn read_raw_value(pb_istream_t *stream, pb_wire_type_t wire
     }
 }
 
-/* Decode string length from stream and return a substream with limited length. */
-static bool substream_callback(pb_istream_t *stream, uint8_t *buf, size_t count)
-{
-    pb_istream_t *parent = (pb_istream_t*)stream->state;
-    return pb_read(parent, buf, count);
-}
-
+/* Decode string length from stream and return a substream with limited length.
+ * Remember to close the substream using pb_close_string_substream().
+ */
 bool checkreturn pb_make_string_substream(pb_istream_t *stream, pb_istream_t *substream)
 {
     uint32_t size;
     if (!pb_decode_varint32(stream, &size))
         return false;
     
-    if (stream->bytes_left < size)
+    *substream = *stream;
+    if (substream->bytes_left < size)
         return false;
     
-    substream->callback = &substream_callback;
-    substream->state = stream;
     substream->bytes_left = size;
-    
+    stream->bytes_left -= size;
     return true;
 }
 
+void pb_close_string_substream(pb_istream_t *stream, pb_istream_t *substream)
+{
+    stream->state = substream->state;
+}
+
 /* Iterator for pb_field_t list */
 typedef struct {
     const pb_field_t *start; /* Start of the pb_field_t array */
@@ -293,6 +293,7 @@ static bool checkreturn decode_field(pb_istream_t *stream, pb_wire_type_t wire_t
                 && PB_LTYPE(iter->current->type) <= PB_LTYPE_LAST_PACKABLE)
             {
                 /* Packed array */
+                bool status;
                 size_t *size = (size_t*)iter->pSize;
                 pb_istream_t substream;
                 if (!pb_make_string_substream(stream, &substream))
@@ -305,7 +306,9 @@ static bool checkreturn decode_field(pb_istream_t *stream, pb_wire_type_t wire_t
                         return false;
                     (*size)++;
                 }
-                return (substream.bytes_left == 0);
+                status = (substream.bytes_left == 0);
+                pb_close_string_substream(stream, &substream);
+                return status;
             }
             else
             {
@@ -339,6 +342,7 @@ static bool checkreturn decode_field(pb_istream_t *stream, pb_wire_type_t wire_t
                         return false;
                 }
                 
+                pb_close_string_substream(stream, &substream);
                 return true;
             }
             else
@@ -602,6 +606,7 @@ bool checkreturn pb_dec_string(pb_istream_t *stream, const pb_field_t *field, vo
 
 bool checkreturn pb_dec_submessage(pb_istream_t *stream, const pb_field_t *field, void *dest)
 {
+    bool status;
     pb_istream_t substream;
     
     if (!pb_make_string_substream(stream, &substream))
@@ -610,5 +615,7 @@ bool checkreturn pb_dec_submessage(pb_istream_t *stream, const pb_field_t *field
     if (field->ptr == NULL)
         return false;
     
-    return pb_decode(&substream, (pb_field_t*)field->ptr, dest);
+    status = pb_decode(&substream, (pb_field_t*)field->ptr, dest);
+    pb_close_string_substream(stream, &substream);
+    return status;
 }

pb_decode.h

@@ -23,6 +23,10 @@
  * 
  * 3) You can use state to store your own data (e.g. buffer pointer),
  * and rely on pb_read to verify that no-body reads past bytes_left.
+ * 
+ * 4) Your callback may be used with substreams, in which case bytes_left
+ * is different than from the main stream. Don't use bytes_left to compute
+ * any pointers.
  */
 struct _pb_istream_t
 {
@@ -69,6 +73,7 @@ bool pb_decode_fixed64(pb_istream_t *stream, void *dest);
 
 /* Make a limited-length substream for reading a PB_WT_STRING field. */
 bool pb_make_string_substream(pb_istream_t *stream, pb_istream_t *substream);
+void pb_close_string_substream(pb_istream_t *stream, pb_istream_t *substream);
 
 /* --- Internal functions ---
  * These functions are not terribly useful for the average library user, but