[PATCH 1/4] trace-cmd: Add new libtracefs API tracefs_instance_file_append()

["Tzvetomir Stoyanov (VMware)" <tz.stoyanov@gmail.com>]

The existing tracefs_instance_file_write() API truncates the file before writing to it.
The are use cases where the file must not be truncated. The new
  tracefs_instance_file_append()
API appends to the end of the file.

Signed-off-by: Tzvetomir Stoyanov (VMware) <tz.stoyanov@gmail.com>
---
 include/tracefs/tracefs.h      |  2 ++
 lib/tracefs/tracefs-instance.c | 40 ++++++++++++++++++++++++++++++----
 2 files changed, 38 insertions(+), 4 deletions(-)

diff --git a/include/tracefs/tracefs.h b/include/tracefs/tracefs.h
index bd3f732..16fcb66 100644
--- a/include/tracefs/tracefs.h
+++ b/include/tracefs/tracefs.h
@@ -30,6 +30,8 @@ tracefs_instance_get_file(struct tracefs_instance *instance, const char *file);
 char *tracefs_instance_get_dir(struct tracefs_instance *instance);
 int tracefs_instance_file_write(struct tracefs_instance *instance,
 				const char *file, const char *str);
+int tracefs_instance_file_append(struct tracefs_instance *instance,
+				 const char *file, const char *str);
 char *tracefs_instance_file_read(struct tracefs_instance *instance,
 				 char *file, int *psize);
 
diff --git a/lib/tracefs/tracefs-instance.c b/lib/tracefs/tracefs-instance.c
index b96ab61..e6b0a95 100644
--- a/lib/tracefs/tracefs-instance.c
+++ b/lib/tracefs/tracefs-instance.c
@@ -176,12 +176,16 @@ char *tracefs_instance_get_name(struct tracefs_instance *instance)
 	return NULL;
 }
 
-static int write_file(const char *file, const char *str)
+static int write_file(const char *file, const char *str, bool appned)
 {
 	int ret;
 	int fd;
 
-	fd = open(file, O_WRONLY | O_TRUNC);
+	if (appned)
+		fd = open(file, O_WRONLY | O_APPEND);
+	else
+		fd = open(file, O_WRONLY | O_TRUNC);
+
 	if (fd < 0) {
 		warning("Failed to open '%s'", file);
 		return -1;
@@ -198,7 +202,9 @@ static int write_file(const char *file, const char *str)
  * @file: name of the file
  * @str: nul terminated string, that will be written in the file.
  *
- * Returns the number of written bytes, or -1 in case of an error
+ * Returns the number of written bytes, or -1 in case of an error.
+ * The content of the file is replaced with the new one.
+ *
  */
 int tracefs_instance_file_write(struct tracefs_instance *instance,
 				 const char *file, const char *str)
@@ -212,7 +218,33 @@ int tracefs_instance_file_write(struct tracefs_instance *instance,
 		return -1;
 	ret = stat(path, &st);
 	if (ret == 0)
-		ret = write_file(path, str);
+		ret = write_file(path, str, false);
+	tracefs_put_tracing_file(path);
+
+	return ret;
+}
+
+/**
+ * tracefs_instance_file_append - Append to a trace file of specific instance.
+ * @instance: ftrace instance, can be NULL for the top instance
+ * @file: name of the file
+ * @str: nul terminated string, that will be written in the file.
+ *
+ * Returns the number of written bytes, or -1 in case of an error.
+ */
+int tracefs_instance_file_append(struct tracefs_instance *instance,
+				 const char *file, const char *str)
+{
+	struct stat st;
+	char *path;
+	int ret;
+
+	path = tracefs_instance_get_file(instance, file);
+	if (!path)
+		return -1;
+	ret = stat(path, &st);
+	if (ret == 0)
+		ret = write_file(path, str, true);
 	tracefs_put_tracing_file(path);
 
 	return ret;
-- 
2.24.1