pkt-line: eliminate the need for static buffer in packet_write_gently()

Teach `packet_write_gently()` to write the pkt-line header and the actual
buffer in 2 separate calls to `write_in_full()` and avoid the need for a
static buffer, thread-safe scratch space, or an excessively large stack
buffer.

Change `write_packetized_from_fd()` to allocate a temporary buffer rather
than using a static buffer to avoid similar issues here.

These changes are intended to make it easier to use pkt-line routines in
a multi-threaded context with multiple concurrent writers writing to
different streams.

Signed-off-by: Jeff Hostetler <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: jeffhostetler

pkt-line.c

@@ -196,17 +196,26 @@ int packet_write_fmt_gently(int fd, const char *fmt, ...)
 
 static int packet_write_gently(const int fd_out, const char *buf, size_t size)
 {
-	static char packet_write_buffer[LARGE_PACKET_MAX];
+	char header[4];
 	size_t packet_size;
 
-	if (size > sizeof(packet_write_buffer) - 4)
+	if (size > LARGE_PACKET_DATA_MAX)
 		return error(_("packet write failed - data exceeds max packet size"));
 
 	packet_trace(buf, size, 1);
 	packet_size = size + 4;
-	set_packet_header(packet_write_buffer, packet_size);
-	memcpy(packet_write_buffer + 4, buf, size);
-	if (write_in_full(fd_out, packet_write_buffer, packet_size) < 0)
+
+	set_packet_header(header, packet_size);
+
+	/*
+	 * Write the header and the buffer in 2 parts so that we do
+	 * not need to allocate a buffer or rely on a static buffer.
+	 * This also avoids putting a large buffer on the stack which
+	 * might have multi-threading issues.
+	 */
+
+	if (write_in_full(fd_out, header, 4) < 0 ||
+	    write_in_full(fd_out, buf, size) < 0)
 		return error(_("packet write failed"));
 	return 0;
 }
@@ -244,20 +253,23 @@ void packet_buf_write_len(struct strbuf *buf, const char *data, size_t len)
 
 int write_packetized_from_fd(int fd_in, int fd_out)
 {
-	static char buf[LARGE_PACKET_DATA_MAX];
+	char *buf = xmalloc(LARGE_PACKET_DATA_MAX);
 	int err = 0;
 	ssize_t bytes_to_write;
 
 	while (!err) {
-		bytes_to_write = xread(fd_in, buf, sizeof(buf));
-		if (bytes_to_write < 0)
+		bytes_to_write = xread(fd_in, buf, LARGE_PACKET_DATA_MAX);
+		if (bytes_to_write < 0) {
+			free(buf);
 			return COPY_READ_ERROR;
+		}
 		if (bytes_to_write == 0)
 			break;
 		err = packet_write_gently(fd_out, buf, bytes_to_write);
 	}
 	if (!err)
 		err = packet_flush_gently(fd_out);
+	free(buf);
 	return err;
 }