bash blocking on exec assigning an output file descriptor to a fifo

[Øyvind Hvidsten]

Please correct any mistakes in my wording, as I would very much like to 
be able to use the correct terms when describing this. Also, please ask 
if anything is unclear :)

My problem occurs when I do the following:
mkfifo foo; exec 3<"foo"; echo done

This blocks on the exec statement, and never reaches the echo statement, 
even though I don't think I've asked bash to read from that file 
descriptor (yet). The plan was to use "read -t 0 <&3" at a later stage 
to check if something is available there.

The following series of statements will however not block and echo 
"done" immediately:
mkfifo foo; exec 3<>"foo"; echo done

This also blocks:
mkfifo foo; exec 3>"foo"; exec 3<"foo"; echo done

Why is this?

My current version of bash:
GNU bash, version 4.1.5(1)-release (x86_64-pc-linux-gnu)