A recent change to tmpdir.rb caused some problems for extensions installing:

diff --git a/lib/tmpdir.rb b/lib/tmpdir.rb
index 03c02fd..18d4fb6 100644
--- a/lib/tmpdir.rb
+++ b/lib/tmpdir.rb
@@ -138,7 +138,7 @@ def create(basename, *rest)
       end
       n = nil
       begin
-        path = File.expand_path(make_tmpname(basename, n), tmpdir)
+        path = File.join(tmpdir, make_tmpname(basename, n))

When installing an extension, RubyGems puts some requires and env setup into a temporary Ruby file it then loads:

    Tempfile.open %w"siteconf .rb", "." do |siteconf|
      t = siteconf
      siteconf.puts "require 'rbconfig'"
      siteconf.puts "dest_path = #{tmp_dest.dump}"
...

This Tempfile.open call attempts to make a temporary file in the current directory and open it as a normal file (since there's a block provided it can just delete the file when finished).

Tempfile.open circles back to Tempfile.initialize (in Java), which calls Dir::Tmpname.create (patched above). That proceeds to call a CallBlock in Tempfile that attempts to open the file:

    private class TempfileCallback implements BlockCallback {
        @Override
        public IRubyObject call(ThreadContext context, IRubyObject[] args, Block block) {
            Ruby runtime = context.runtime;
            IRubyObject tmpname = args[0], opts = args.length > 2 ? args[2] : context.nil;

...

            // let RubyFile do its init logic to open the channel
            Tempfile.super.initialize(context, new IRubyObject[]{tmpname, runtime.newFixnum(mode), opts}, Block.NULL_BLOCK);
            Tempfile.this.tmpname = tmpname;

            Tempfile.this.mode = runtime.newFixnum(mode & ~(OpenFlags.O_CREAT.intValue() | OpenFlags.O_EXCL.intValue()));
            Tempfile.this.opts = opts;

            return opts;

It is this Tempfile.super.initialize call that eventually blows up because the file has a "." but does not exist in the current directory. With the old tmpdir logic, the . got expanded based on cwd and worked properly.

The symptom I ran into was that any gems with extensions now fail to install, because that temporary environment-setting file can't be found.

I'll be reverting the tmpdir.rb line above for now.