[PATCH 1/3] libtracefs: Add more methods for writing to files

["Yordan Karadzhov (VMware)" <y.karadz@gmail.com>]

For the moment the library provides only one method for writing to
tracefs files, that is:

int tracefs_instance_file_write()

This method first truncates the size of the file to 0 (clearing it),
then writes new content to this file. Essentially this method overwrites
(clear + write). In this patch we add two additional methids:

int tracefs_instance_file_append()

which writes without clearing and

int tracefs_instance_file_clear()

which clears without writing anything. Those two new APIs have various
use-cases. For example one can use the two methods when adding/clearing
kprobes.

Signed-off-by: Yordan Karadzhov (VMware) <y.karadz@gmail.com>
---
 include/tracefs.h      |  4 +++
 src/tracefs-instance.c | 74 +++++++++++++++++++++++++++++++++---------
 2 files changed, 63 insertions(+), 15 deletions(-)

diff --git a/include/tracefs.h b/include/tracefs.h
index 55ee867..551c37c 100644
--- a/include/tracefs.h
+++ b/include/tracefs.h
@@ -33,6 +33,10 @@ 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);
+int tracefs_instance_file_clear(struct tracefs_instance *instance,
+				const char *file);
 char *tracefs_instance_file_read(struct tracefs_instance *instance,
 				 const char *file, int *psize);
 int tracefs_instance_file_read_number(struct tracefs_instance *instance,
diff --git a/src/tracefs-instance.c b/src/tracefs-instance.c
index b8ce36f..2aeb529 100644
--- a/src/tracefs-instance.c
+++ b/src/tracefs-instance.c
@@ -337,21 +337,41 @@ const char *tracefs_instance_get_trace_dir(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, int flags)
 {
-	int ret;
+	int ret = 0;
 	int fd;
 
-	fd = open(file, O_WRONLY | O_TRUNC);
+	fd = open(file, flags);
 	if (fd < 0) {
 		tracefs_warning("Failed to open '%s'", file);
 		return -1;
 	}
-	ret = write(fd, str, strlen(str));
+
+	if (str)
+		ret = write(fd, str, strlen(str));
+
 	close(fd);
 	return ret;
 }
 
+static int instance_file_write(struct tracefs_instance *instance,
+			       const char *file, const char *str, int flags)
+{
+	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, flags);
+	tracefs_put_tracing_file(path);
+
+	return ret;
+}
 
 /**
  * tracefs_instance_file_write - Write in trace file of specific instance.
@@ -364,19 +384,43 @@ static int write_file(const char *file, const char *str)
 int tracefs_instance_file_write(struct tracefs_instance *instance,
 				 const char *file, const char *str)
 {
-	struct stat st;
-	char *path;
-	int ret;
+	return instance_file_write(instance, file, str, O_WRONLY | O_TRUNC);
+}
 
-	path = tracefs_instance_get_file(instance, file);
-	if (!path)
-		return -1;
-	ret = stat(path, &st);
-	if (ret == 0)
-		ret = write_file(path, str);
-	tracefs_put_tracing_file(path);
+/**
+ * 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 appended to the file.
+ *
+ * Returns the number of appended bytes, or -1 in case of an error.
+ */
+int tracefs_instance_file_append(struct tracefs_instance *instance,
+				 const char *file, const char *str)
+{
+	return instance_file_write(instance, file, str, O_WRONLY);
+}
 
-	return ret;
+/**
+ * tracefs_instance_file_clear - Clear a trace file of specific instance.
+ * Note, it only opens with O_TRUNC and closes the file. If the file has
+ * content that does not get cleared in this way, this will not have any
+ * effect. For example, set_ftrace_filter can have probes that are not
+ * cleared by O_TRUNC:
+ *
+ * echo "schedule:stacktrace" > set_ftrace_filter
+ *
+ * This function will not clear the above "set_ftrace_filter" after that
+ * command.
+ * @instance: ftrace instance, can be NULL for the top instance.
+ * @file: name of the file to clear.
+ *
+ * Returns 0 on success, or -1 in case of an error.
+ */
+int tracefs_instance_file_clear(struct tracefs_instance *instance,
+				const char *file)
+{
+	return instance_file_write(instance, file, NULL, O_WRONLY | O_TRUNC);
 }
 
 /**
-- 
2.25.1