setvbuf: close and reopen stream before change

Fix setvbuf code by closing and reopening stream before changing buffer.

But we need to review what this code is doing embedded inside a library
function rather than the simpler original form being run independently
at the top of main() by tools that need it.

WHATS_NEW

@@ -1,5 +1,6 @@
 Version 2.02.98 -
 =================================
+  Fix setvbuf code by closing and reopening stream before changing buffer.
   Disable private buffering when using liblvm.
   When private stdin/stdout buffering is not used always use silent mode.
   Add log/silent to lvm.conf equivalent to -qq.

lib/commands/toolcontext.c

@@ -1245,6 +1245,37 @@ static void _init_globals(struct cmd_context *cmd)
 	init_mirror_in_sync(0);
 }
 
+/*
+ * Close and reopen stream on file descriptor fd.
+ */
+static int _reopen_stream(FILE *stream, int fd, const char *mode, const char *name, FILE **new_stream)
+{
+	int fd_copy, new_fd;
+
+	if ((fd_copy = dup(fd)) < 0) {
+		log_sys_error("dup", name);
+		return 0;
+	}
+
+	if (fclose(stream))
+		log_sys_error("fclose", name);
+
+	if ((new_fd = dup2(fd_copy, fd)) < 0)
+		log_sys_error("dup2", name);
+	else if (new_fd != fd)
+		log_error("dup2(%d, %d) returned %d", fd_copy, fd, new_fd);
+
+	if (close(fd_copy) < 0)
+		log_sys_error("close", name);
+
+	if (!(*new_stream = fdopen(fd, mode))) {
+		log_sys_error("fdopen", name);
+		return 0;
+	}
+
+	return 1;
+}
+
 /* Entry point */
 struct cmd_context *create_toolcontext(unsigned is_long_lived,
 				       const char *system_dir,
@@ -1252,6 +1283,7 @@ struct cmd_context *create_toolcontext(unsigned is_long_lived,
 				       unsigned threaded)
 {
 	struct cmd_context *cmd;
+	FILE *new_stream;
 
 #ifdef M_MMAP_MAX
 	mallopt(M_MMAP_MAX, 0);
@@ -1293,9 +1325,20 @@ struct cmd_context *create_toolcontext(unsigned is_long_lived,
 			log_error("Failed to allocate line buffer.");
 			goto out;
 		}
-		if ((setvbuf(stdin, cmd->linebuffer, _IOLBF, linebuffer_size) ||
+
-		     setvbuf(stdout, cmd->linebuffer + linebuffer_size,
+		if (!_reopen_stream(stdin, STDIN_FILENO, "r", "stdin", &new_stream))
-			     _IOLBF, linebuffer_size))) {
+			goto_out;
+		stdin = new_stream;
+		if (setvbuf(stdin, cmd->linebuffer, _IOLBF, linebuffer_size)) {
+			log_sys_error("setvbuf", "");
+			goto out;
+		}
+
+		if (!_reopen_stream(stdout, STDOUT_FILENO, "w", "stdout", &new_stream))
+			goto_out;
+		stdout = new_stream;
+		if (setvbuf(stdout, cmd->linebuffer + linebuffer_size,
+			     _IOLBF, linebuffer_size)) {
 			log_sys_error("setvbuf", "");
 			goto out;
 		}
@@ -1546,6 +1589,7 @@ int refresh_toolcontext(struct cmd_context *cmd)
 void destroy_toolcontext(struct cmd_context *cmd)
 {
 	struct dm_config_tree *cft_cmdline;
+	FILE *new_stream;
 
 	if (cmd->dump_filter)
 		persistent_filter_dump(cmd->filter, 1);
@@ -1570,9 +1614,18 @@ void destroy_toolcontext(struct cmd_context *cmd)
 
 	if (cmd->linebuffer) {
 		/* Reset stream buffering to defaults */
-		setlinebuf(stdin);
+		if (_reopen_stream(stdin, STDIN_FILENO, "r", "stdin", &new_stream)) {
-		fflush(stdout);
+			stdin = new_stream;
-		setlinebuf(stdout);
+			setlinebuf(stdin);
+		} else
+			cmd->linebuffer = NULL;	/* Leave buffer in place (deliberate leak) */
+
+		if (_reopen_stream(stdout, STDOUT_FILENO, "w", "stdout", &new_stream)) {
+			stdout = new_stream;
+			setlinebuf(stdout);
+		} else
+			cmd->linebuffer = NULL;	/* Leave buffer in place (deliberate leak) */
+
 		dm_free(cmd->linebuffer);
 	}