Make the resolver(3) and many associated interfaces much more reentrant.

The getaddrinfo(3), getipnodebyname(3) and resolver(3) can coincide now
with what should be totally reentrant, and h_errno values will now
be preserved correctly, but this does not affect interfaces such as
gethostbyname(3) which are still mostly non-reentrant.

In all of these relevant functions, the thread-safety has been pushed
down as far as it seems possible right now.  This means that operations
that are selected via nsdispatch(3) (i.e. files, yp, dns) are protected
still under global locks that getaddrinfo(3) defines, but where possible
the locking is greatly reduced.  The most noticeable improvement is
that multiple DNS lookups can now be run at the same time, and this
shows major improvement in performance of DNS-lookup threaded programs,
and solves the "Mozilla tab serialization" problem.

No single-threaded applications need to be recompiled.  Multi-threaded
applications that reference "_res" to change resolver(3) options will
need to be recompiled, and ones which reference "h_errno" will also
if they desire the correct h_errno values.  If the applications already
understood that _res and h_errno were not thread-safe and had their own
locking, they will see no performance improvement but will not
actually break in any way.

Please note that when NSS modules are used, or when nsdispatch(3)
defaults to adding any lookups of its own to the individual libc
_nsdispatch() calls, those MUST be reentrant as well.

1 files changed, 6 insertions, 0 deletions

diff --git a/lib/libc/net/gethostbyname.3 b/lib/libc/net/gethostbyname.3
index e703e3221275..a4486bc6e9f8 100644
--- a/lib/libc/net/gethostbyname.3
+++ b/lib/libc/net/gethostbyname.3
@@ -369,6 +369,12 @@ version 4.9.4.
 These functions use static data storage;
 if the data is needed for future use, it should be
 copied before any subsequent calls overwrite it.
+Threaded applications should never use them, as they will also conflict
+with the
+.Xr getaddrinfo 3
+and
+.Xr getipnodebyname 3
+families of functions (which should be used instead).
 Only the Internet
 address format is currently understood.
 .Pp