[PATCH v3 7/7] kunit: Don't waste first attempt to format string in kunit_log_append()

[Richard Fitzgerald <rf@opensource.cirrus.com>]

It's wasteful to call vsnprintf() only to figure out the length of the
string. The string might fit in the available buffer space but we have to
vsnprintf() again to do that.

Instead, attempt to format the string to the available buffer at the same
time as finding the string length. Only if the string didn't fit the
available space is it necessary to extend the log and format the string
again to a new fragment buffer.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
---
 lib/kunit/test.c | 33 +++++++++++++++++----------------
 1 file changed, 17 insertions(+), 16 deletions(-)

diff --git a/lib/kunit/test.c b/lib/kunit/test.c
index 28d0048d6171..230ec5e9824f 100644
--- a/lib/kunit/test.c
+++ b/lib/kunit/test.c
@@ -196,11 +196,21 @@ void kunit_log_append(struct list_head *log, const char *fmt, ...)
 	if (!log)
 		return;
 
-	/* Evaluate length of line to add to log */
+	frag = list_last_entry(log, struct kunit_log_frag, list);
+	log_len = strlen(frag->buf);
+	len_left = sizeof(frag->buf) - log_len - 1;
+
+	/* Attempt to print formatted line to current fragment */
 	va_start(args, fmt);
-	len = vsnprintf(NULL, 0, fmt, args) + 1;
+	len = vsnprintf(frag->buf + log_len, len_left, fmt, args) + 1;
 	va_end(args);
 
+	if (len <= len_left)
+		goto out_newline;
+
+	/* Abandon the truncated result of vsnprintf */
+	frag->buf[log_len] = '\0';
+
 	if (len > sizeof(frag->buf) - 1) {
 		va_start(args, fmt);
 		tmp = kvasprintf(GFP_KERNEL, fmt, args);
@@ -212,24 +222,15 @@ void kunit_log_append(struct list_head *log, const char *fmt, ...)
 		return;
 	}
 
-	frag = list_last_entry(log, struct kunit_log_frag, list);
-	log_len = strlen(frag->buf);
-	len_left = sizeof(frag->buf) - log_len - 1;
-
-	if (len > len_left) {
-		frag = kunit_log_extend(log);
-		if (!frag)
-			return;
-
-		len_left = sizeof(frag->buf) - 1;
-		log_len = 0;
-	}
+	frag = kunit_log_extend(log);
+	if (!frag)
+		return;
 
 	/* Print formatted line to the log */
 	va_start(args, fmt);
-	vsnprintf(frag->buf + log_len, min(len, len_left), fmt, args);
+	vsnprintf(frag->buf, sizeof(frag->buf) - 1, fmt, args);
 	va_end(args);
-
+out_newline:
 	/* Add newline to end of log if not already present. */
 	kunit_log_newline(frag);
 }
-- 
2.30.2