database: don't call socket.getfqdn() on every write (#46554)

We've seen `getfqdn()` cause slowdowns on macOS in CI when added elsewhere. It's also
called by database.py every time we write the DB file.

- [x] replace the call with a memoized version so that it is only called once per process.

Signed-off-by: Todd Gamblin <tgamblin@llnl.gov>

lib/spack/spack/database.py

@@ -50,6 +50,7 @@
     pass
 
 import llnl.util.filesystem as fs
+import llnl.util.lang
 import llnl.util.tty as tty
 
 import spack.deptypes as dt
@@ -121,6 +122,17 @@
 )
 
 
+@llnl.util.lang.memoized
+def _getfqdn():
+    """Memoized version of `getfqdn()`.
+
+    If we call `getfqdn()` too many times, DNS can be very slow. We only need to call it
+    one time per process, so we cache it here.
+
+    """
+    return socket.getfqdn()
+
+
 def reader(version: vn.StandardVersion) -> Type["spack.spec.SpecfileReaderBase"]:
     reader_cls = {
         vn.Version("5"): spack.spec.SpecfileV1,
@@ -1084,7 +1096,7 @@ def _write(self, type, value, traceback):
             self._state_is_inconsistent = True
             return
 
-        temp_file = self._index_path + (".%s.%s.temp" % (socket.getfqdn(), os.getpid()))
+        temp_file = self._index_path + (".%s.%s.temp" % (_getfqdn(), os.getpid()))
 
         # Write a temporary database file them move it into place
         try: