Initial work towards various IO cleanups with an eye to fixing https://github.com/fish-shell/fish-shell/issues/110

share/functions/eval.fish

@@ -19,6 +19,19 @@ function eval -S -d "Evaluate parameters as a command"
 		status --job-control full
 	end
 
+        # rfish: To eval 'foo', we construct a block "begin ; foo; end <&3 3<&-"
+        # The 'eval2_inner' is a param to 'begin' itself; I believe it does nothing.
+        # Note the redirections are also within the quotes.
+        #
+        # We then pipe this to 'source 3<&0' which dup2's 3 to stdin.
+        #
+        # You might expect that the dup2(3, stdin) should overwrite stdin,
+        # and therefore prevent 'source' from reading the piped-in block. This doesn't happen
+        # because when you pipe to a builtin, we don't overwrite stdin with the read end
+        # of the block; instead we set a separate fd in a variable 'builtin_stdin', which is
+        # what it reads from. So builtins are magic in that, in pipes, their stdin
+        # is not fd 0.
+
 	echo "begin; $argv "\n" ;end eval2_inner <&3 3<&-" | source 3<&0
 	set -l res $status