stream filter: add "no more input" to the filters

Some filters may need to buffer the input and look-ahead inside it
to decide what to output, and they may consume more than zero bytes
of input and still not produce any output. After feeding all the
input, pass NULL as input as keep calling stream_filter() to let
such filters know there is no more input coming, and it is time for
them to produce the remaining output based on the buffered input.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

convert.c

@@ -838,7 +838,11 @@ static int null_filter_fn(struct stream_filter *filter,
 			  const char *input, size_t *isize_p,
 			  char *output, size_t *osize_p)
 {
-	size_t count = *isize_p;
+	size_t count;
+
+	if (!input)
+		return 0; /* we do not keep any states */
+	count = *isize_p;
 	if (*osize_p < count)
 		count = *osize_p;
 	if (count) {

convert.h

@@ -57,6 +57,13 @@ extern int is_null_stream_filter(struct stream_filter *);
  * Use as much input up to *isize_p and fill output up to *osize_p;
  * update isize_p and osize_p to indicate how much buffer space was
  * consumed and filled. Return 0 on success, non-zero on error.
+ *
+ * Some filters may need to buffer the input and look-ahead inside it
+ * to decide what to output, and they may consume more than zero bytes
+ * of input and still not produce any output. After feeding all the
+ * input, pass NULL as input and keep calling this function, to let
+ * such filters know there is no more input coming and it is time for
+ * them to produce the remaining output based on the buffered input.
  */
 extern int stream_filter(struct stream_filter *,
 			 const char *input, size_t *isize_p,

streaming.c

@@ -60,6 +60,7 @@ struct filtered_istream {
 	char obuf[FILTER_BUFFER];
 	int i_end, i_ptr;
 	int o_end, o_ptr;
+	int input_finished;
 };
 
 struct git_istream {
@@ -215,12 +216,30 @@ static read_method_decl(filtered)
 			fs->o_end = FILTER_BUFFER - to_receive;
 			continue;
 		}
+
+		/* tell the filter to drain upon no more input */
+		if (fs->input_finished) {
+			size_t to_receive = FILTER_BUFFER;
+			if (stream_filter(fs->filter,
+					  NULL, NULL,
+					  fs->obuf, &to_receive))
+				return -1;
+			fs->o_end = FILTER_BUFFER - to_receive;
+			if (!fs->o_end)
+				break;
+			continue;
+		}
 		fs->i_end = fs->i_ptr = 0;
 
 		/* refill the input from the upstream */
-		fs->i_end = read_istream(fs->upstream, fs->ibuf, FILTER_BUFFER);
-		if (fs->i_end <= 0)
-			break;
+		if (!fs->input_finished) {
+			fs->i_end = read_istream(fs->upstream, fs->ibuf, FILTER_BUFFER);
+			if (fs->i_end < 0)
+				break;
+			if (fs->i_end)
+				continue;
+		}
+		fs->input_finished = 1;
 	}
 	return filled;
 }
@@ -241,6 +260,7 @@ static struct git_istream *attach_stream_filter(struct git_istream *st,
 	fs->filter = filter;
 	fs->i_end = fs->i_ptr = 0;
 	fs->o_end = fs->o_ptr = 0;
+	fs->input_finished = 0;
 	ifs->size = -1; /* unknown */
 	return ifs;
 }