Re: Improve LWLock tranche name visibility across backends

I haven't followed the latest discussion, but I took a look at the patch.

+      It is possible to use a <literal>tranche_id</literal> that was not retrieved
+      using <function>LWLockNewTrancheId</function>, but this is not recommended.
+      The ID may clash with an already registered tranche name, or the specified
+      name may not be found. In such cases, looking up the name will return a generic
+      "extension" tranche name.

Is there any reason to continue allowing this?  For example, maybe we could
ERROR if LWLockInitialize()/GetLWTrancheName() are given a tranche_id
greater than the number allocated.  I guess I'm not following why we should
gracefully handle these kinds of coding errors, especially when they result
in unhelpful behavior like an "extension" tranche.

+#else
+elog(ERROR, "injection points not supported");
+#endif

This causes compilation to fail when injection points are not enabled.

I haven't combed through the patch character-by-character, but upon a
read-through, the general shape looks reasonable to me.  As a general note,
I'd suggest adding more commentary throughout and finding opportunities to
simplify and/or clean up the code as much as possible.

-- 
nathan