Introduce callback for logging

Introduce an API for custom log handler functions.  This allows libfuse
applications to send messages to syslog(3) or other logging systems.
See include/fuse_log.h for details.

Convert libfuse from fprintf(stderr, ...) to log_fuse(level, ...).  Most
messages are error messages with FUSE_LOG_ERR log level.  There are also
some debug messages which now use the FUSE_LOG_DEBUG log level.

Note that lib/mount_util.c is used by both libfuse and fusermount3.
Since fusermount3 does not link against libfuse, we cannot call
fuse_log() from lib/mount_util.c.  This file will continue to use
fprintf(stderr, ...) until someone figures out how to split it up.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 41 insertions, 0 deletions

diff --git a/lib/fuse_log.c b/lib/fuse_log.c
new file mode 100644
index 0000000..09178e4
--- /dev/null
+++ b/lib/fuse_log.c
@@ -0,0 +1,41 @@
+/*
+  FUSE: Filesystem in Userspace
+  Copyright (C) 2019  Red Hat, Inc.
+
+  Logging API.
+
+  This program can be distributed under the terms of the GNU LGPLv2.
+  See the file COPYING.LIB
+*/
+
+#include "fuse_log.h"
+#include "fuse_i.h"
+
+#include <stdarg.h>
+#include <stdio.h>
+
+static void default_log_func(
+		__attribute__(( unused )) enum fuse_log_level level,
+		const char *fmt, va_list ap)
+{
+	vfprintf(stderr, fmt, ap);
+}
+
+static fuse_log_func_t log_func = default_log_func;
+
+void fuse_set_log_func(fuse_log_func_t func)
+{
+	if (!func)
+		func = default_log_func;
+
+	log_func = func;
+}
+
+void fuse_log(enum fuse_log_level level, const char *fmt, ...)
+{
+	va_list ap;
+
+	va_start(ap, fmt);
+	log_func(level, fmt, ap);
+	va_end(ap);
+}