[RFC v4 03/17] kunit: test: add string_stream a std::stream like string builder

[Brendan Higgins <brendanhiggins@google.com>]

A number of test features need to do pretty complicated string printing
where it may not be possible to rely on a single preallocated string
with parameters.

So provide a library for constructing the string as you go similar to
C++'s std::string.

Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
---
Changes Since Last Version
 - None. There was some discussion about maybe trying to generalize this
   or replace it with something existing, but it didn't seem feasible to
   generalize this, and there wasn't really anything that is a great
   replacement.
---
 include/kunit/string-stream.h |  44 ++++++++++
 kunit/Makefile                |   3 +-
 kunit/string-stream.c         | 149 ++++++++++++++++++++++++++++++++++
 3 files changed, 195 insertions(+), 1 deletion(-)
 create mode 100644 include/kunit/string-stream.h
 create mode 100644 kunit/string-stream.c

diff --git a/include/kunit/string-stream.h b/include/kunit/string-stream.h
new file mode 100644
index 0000000000000..280ee67559588
--- /dev/null
+++ b/include/kunit/string-stream.h
@@ -0,0 +1,44 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * C++ stream style string builder used in KUnit for building messages.
+ *
+ * Copyright (C) 2019, Google LLC.
+ * Author: Brendan Higgins <brendanhiggins@google.com>
+ */
+
+#ifndef _KUNIT_STRING_STREAM_H
+#define _KUNIT_STRING_STREAM_H
+
+#include <linux/types.h>
+#include <linux/spinlock.h>
+#include <linux/kref.h>
+#include <stdarg.h>
+
+struct string_stream_fragment {
+	struct list_head node;
+	char *fragment;
+};
+
+struct string_stream {
+	size_t length;
+	struct list_head fragments;
+
+	/* length and fragments are protected by this lock */
+	spinlock_t lock;
+	struct kref refcount;
+	int (*add)(struct string_stream *this, const char *fmt, ...);
+	int (*vadd)(struct string_stream *this, const char *fmt, va_list args);
+	char *(*get_string)(struct string_stream *this);
+	void (*clear)(struct string_stream *this);
+	bool (*is_empty)(struct string_stream *this);
+};
+
+struct string_stream *new_string_stream(void);
+
+void destroy_string_stream(struct string_stream *stream);
+
+void string_stream_get(struct string_stream *stream);
+
+int string_stream_put(struct string_stream *stream);
+
+#endif /* _KUNIT_STRING_STREAM_H */
diff --git a/kunit/Makefile b/kunit/Makefile
index 5efdc4dea2c08..275b565a0e81f 100644
--- a/kunit/Makefile
+++ b/kunit/Makefile
@@ -1 +1,2 @@
-obj-$(CONFIG_KUNIT) +=			test.o
+obj-$(CONFIG_KUNIT) +=			test.o \
+					string-stream.o
diff --git a/kunit/string-stream.c b/kunit/string-stream.c
new file mode 100644
index 0000000000000..e90fb595a5607
--- /dev/null
+++ b/kunit/string-stream.c
@@ -0,0 +1,149 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * C++ stream style string builder used in KUnit for building messages.
+ *
+ * Copyright (C) 2019, Google LLC.
+ * Author: Brendan Higgins <brendanhiggins@google.com>
+ */
+
+#include <linux/list.h>
+#include <linux/slab.h>
+#include <kunit/string-stream.h>
+
+static int string_stream_vadd(struct string_stream *this,
+			       const char *fmt,
+			       va_list args)
+{
+	struct string_stream_fragment *fragment;
+	int len;
+	va_list args_for_counting;
+	unsigned long flags;
+
+	/* Make a copy because `vsnprintf` could change it */
+	va_copy(args_for_counting, args);
+
+	/* Need space for null byte. */
+	len = vsnprintf(NULL, 0, fmt, args_for_counting) + 1;
+
+	va_end(args_for_counting);
+
+	fragment = kmalloc(sizeof(*fragment), GFP_KERNEL);
+	if (!fragment)
+		return -ENOMEM;
+
+	fragment->fragment = kmalloc(len, GFP_KERNEL);
+	if (!fragment->fragment) {
+		kfree(fragment);
+		return -ENOMEM;
+	}
+
+	len = vsnprintf(fragment->fragment, len, fmt, args);
+	spin_lock_irqsave(&this->lock, flags);
+	this->length += len;
+	list_add_tail(&fragment->node, &this->fragments);
+	spin_unlock_irqrestore(&this->lock, flags);
+	return 0;
+}
+
+static int string_stream_add(struct string_stream *this, const char *fmt, ...)
+{
+	va_list args;
+	int result;
+
+	va_start(args, fmt);
+	result = string_stream_vadd(this, fmt, args);
+	va_end(args);
+	return result;
+}
+
+static void string_stream_clear(struct string_stream *this)
+{
+	struct string_stream_fragment *fragment, *fragment_safe;
+	unsigned long flags;
+
+	spin_lock_irqsave(&this->lock, flags);
+	list_for_each_entry_safe(fragment,
+				 fragment_safe,
+				 &this->fragments,
+				 node) {
+		list_del(&fragment->node);
+		kfree(fragment->fragment);
+		kfree(fragment);
+	}
+	this->length = 0;
+	spin_unlock_irqrestore(&this->lock, flags);
+}
+
+static char *string_stream_get_string(struct string_stream *this)
+{
+	struct string_stream_fragment *fragment;
+	size_t buf_len = this->length + 1; /* +1 for null byte. */
+	char *buf;
+	unsigned long flags;
+
+	buf = kzalloc(buf_len, GFP_KERNEL);
+	if (!buf)
+		return NULL;
+
+	spin_lock_irqsave(&this->lock, flags);
+	list_for_each_entry(fragment, &this->fragments, node)
+		strlcat(buf, fragment->fragment, buf_len);
+	spin_unlock_irqrestore(&this->lock, flags);
+
+	return buf;
+}
+
+static bool string_stream_is_empty(struct string_stream *this)
+{
+	bool is_empty;
+	unsigned long flags;
+
+	spin_lock_irqsave(&this->lock, flags);
+	is_empty = list_empty(&this->fragments);
+	spin_unlock_irqrestore(&this->lock, flags);
+
+	return is_empty;
+}
+
+void destroy_string_stream(struct string_stream *stream)
+{
+	stream->clear(stream);
+	kfree(stream);
+}
+
+static void string_stream_destroy(struct kref *kref)
+{
+	struct string_stream *stream = container_of(kref,
+						    struct string_stream,
+						    refcount);
+	destroy_string_stream(stream);
+}
+
+struct string_stream *new_string_stream(void)
+{
+	struct string_stream *stream = kzalloc(sizeof(*stream), GFP_KERNEL);
+
+	if (!stream)
+		return NULL;
+
+	INIT_LIST_HEAD(&stream->fragments);
+	spin_lock_init(&stream->lock);
+	kref_init(&stream->refcount);
+	stream->add = string_stream_add;
+	stream->vadd = string_stream_vadd;
+	stream->get_string = string_stream_get_string;
+	stream->clear = string_stream_clear;
+	stream->is_empty = string_stream_is_empty;
+	return stream;
+}
+
+void string_stream_get(struct string_stream *stream)
+{
+	kref_get(&stream->refcount);
+}
+
+int string_stream_put(struct string_stream *stream)
+{
+	return kref_put(&stream->refcount, &string_stream_destroy);
+}
+
-- 
2.21.0.rc0.258.g878e2cd30e-goog