[PATCH 4/5] strbuf_reset: don't write to slopbuf with ThreadSanitizer

["Martin Ågren" <martin.agren@gmail.com>]

If two threads have one freshly initialized string buffer each and call
strbuf_reset on them at roughly the same time, both threads will be
writing a '\0' to strbuf_slopbuf. That is not a problem in practice
since it doesn't matter in which order the writes happen. But
ThreadSanitizer will consider this a race.

When compiling with GIT_THREAD_SANITIZER, avoid writing to
strbuf_slopbuf. Let's instead assert on the first byte of strbuf_slopbuf
being '\0', since it ensures the promised invariant of "buf[len] ==
'\0'". (Writing to strbuf_slopbuf is normally bad, but could become even
more bad if we stop covering it up in strbuf_reset.)

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
---
 strbuf.h | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/strbuf.h b/strbuf.h
index e705b94db..295654d39 100644
--- a/strbuf.h
+++ b/strbuf.h
@@ -153,7 +153,19 @@ static inline void strbuf_setlen(struct strbuf *sb, size_t len)
 /**
  * Empty the buffer by setting the size of it to zero.
  */
+#ifdef GIT_THREAD_SANITIZER
+#define strbuf_reset(sb)						\
+	do {								\
+		struct strbuf *_sb = sb; 				\
+		_sb->len = 0;						\
+		if (_sb->buf == strbuf_slopbuf)				\
+			assert(!strbuf_slopbuf[0]);			\
+		else							\
+			_sb->buf[0] = '\0';				\
+	} while (0)
+#else
 #define strbuf_reset(sb)  strbuf_setlen(sb, 0)
+#endif
 
 
 /**
-- 
2.14.1.151.gdfeca7a7e