The following piece of code handle environment variable reloads in Spring

What it does is:

• iterate over environment variables from when spring was started and remove those that rails did not change
• iterate over client environment variables and if those are not present in the environment already, replace them

My problem is that I have a Rails application where there is an environment variable that triggers if the app is in master mode or slave mode. I use ENV! to manage environment variable, and if a variable is not set, ENV! sets a default value. The following sequence of events cause an issue:

• spring server starts without the MASTER environment variable
• app sets MASTER=false which is its default value
• some time later...
• I start the app again with MASTER=true on the command-line
• Spring server receives a client request with MASTER=true
• the process above is followed:
  • it considers MASTER to be unset when the spring server started but it has now MASTER=false. It received a client request with MASTER=true
  • it does not removes MASTER from the environment because it sees the variable changed (fron no value to false)
  • it does not replace MASTER=false with MASTER=true because it already has a value.
• the app is started in slave mode despite having specified MASTER=true on the command-line.

I don't know the rationale behind this code, but in some cases it is completely wrong.

I believe this might be a reason behing the #420 issue