windows socketpair tmpfile: use random prefix

fixes #1058

GetTempFileNameA() takes an optional prefix, and a "unique" long value
which can optionally be zero, which causes it to automatically
increment until a not-yet-existing filename is found.

When libevent creates many AF_UNIX socketpairs on windows,
it slows down dramatically, due to always using the same blank prefix,
and GetTempFileNameA() needing to iterate through all the existing
socketpair filenames. With a present and varying prefix, it will have
much less need to iterate.

It was also possible for a race with other processes also using
blank-prefix tmpfile names to result in both trying to start using
the same name at the same time (because libevent deletes the file
and then re-creates it as a unix socket), which should now be
much less likely.

Unfortuantely, "much" is just a factor of 32k, because the prefix
is only 3 characters, and windows filesystems are case-insensitive,
so doing better would require more sophisticated windows API usage
and charset trickyness.

1 files changed, 12 insertions, 2 deletions

diff --git a/evutil.c b/evutil.c
index 93093b0e..a9207188 100644
--- a/evutil.c
+++ b/evutil.c
@@ -212,11 +212,21 @@ create_tmpfile(char tmpfile[MAX_PATH])
 {
 	char short_path[MAX_PATH] = {0};
 	char long_path[MAX_PATH] = {0};
-	char *tmp_file = tmpfile;
+	char prefix[4] = {0};
+	// GetTempFileNameA() uses up to the first three characters of the prefix
+	// and windows filesystems are case-insensitive
+	const char *base32set = "abcdefghijklmnopqrstuvwxyz012345";
+	ev_uint16_t rnd;
+
+	evutil_secure_rng_get_bytes(&rnd, sizeof(rnd));
+	prefix[0] = base32set[(rnd      ) & 31];
+	prefix[1] = base32set[(rnd >>  5) & 31];
+	prefix[2] = base32set[(rnd >> 10) & 31];
+	prefix[3] = '\0';
 
 	GetTempPathA(MAX_PATH, short_path);
 	GetLongPathNameA(short_path, long_path, MAX_PATH);
-	if (!GetTempFileNameA(long_path, NULL, 0, tmp_file)) {
+	if (!GetTempFileNameA(long_path, prefix, 0, tmpfile)) {
 		event_warnx("GetTempFileName failed: %d", EVUTIL_SOCKET_ERROR());
 		return -1;
 	}