Attempt to fix the sporadic uvar test failures on Linux

We identify when the universal variable file has changed out from under us by
comparing a bunch of fields from its stat: inode, device, size, high-precision
timestamp, generation. Linux aggressively reuses inodes, and the size may be
the same by coincidence (which is the case in the tests). Also, Linux
officially has nanosecond precision, but in practice it seems to only uses
millisecond precision for storing mtimes. Thus if there are three or more
updates within a millisecond, every field we check may be the same, and we are
vulnerable to the ABA problem. I believe this explains the occasional test
failures.

The solution is to manually set the nanosecond field of the mtime timestamp to
something unlikely to be duplicated, like a random number, or better yet, the
current time (with nanosecond precision). This is more in the spirit of the
timestamp, and it means we're around a million times less likely to collide.
This seems to fix the tests.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/env_universal_common.h b/src/env_universal_common.h
index 203f952b..3428c693 100644
--- a/src/env_universal_common.h
+++ b/src/env_universal_common.h
@@ -168,7 +168,7 @@ public:
 };
 
 bool universal_log_enabled();
-#define UNIVERSAL_LOG(x) if (universal_log_enabled()) fprintf(stderr, "UNIVERSAL LOG: %s\n", x)
+#define UNIVERSAL_LOG(x) do { if (universal_log_enabled()) fprintf(stderr, "UNIVERSAL LOG: %s\n", x); } while (0)
 
 /* Environment variable for requesting a particular universal notifier. See fetch_default_strategy_from_environment for names. */
 #define UNIVERSAL_NOTIFIER_ENV_NAME "fish_universal_notifier"