[tahoe-lafs-trac-stream] [Tahoe-LAFS] #2833: Enable using values from environment variables from tahoe.cfg

[Tahoe-LAFS]

#2833: Enable using values from environment variables from tahoe.cfg
-------------------------+---------------------------
 Reporter:  amontero     |          Owner:
     Type:  enhancement  |         Status:  new
 Priority:  normal       |      Milestone:  undecided
Component:  unknown      |        Version:  1.11.0
 Keywords:               |  Launchpad Bug:
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 As I progress through dockerizing my setup, I've noticed that being able
 to use values from environment vars would make Tahoe-LAFS more easily
 configurable.
 It would be beneficial to use arbitrary env var values to replace any
 configuration option value from inside tahoe.cfg (perhaps also other
 files, to deal with secrets, too).

 Also, Docker itself makes heavy use of env vars for host IPs and ports
 when linking containers.

 Some kind of placeholder or token syntax instead of literal values, such
 as %VARNAME or {VARNAME} could be added to declare a value which would be
 env var replaced. The icing on the cake would be Bash syntax, which allows
 defaults ${VARNAME:-default_value}.

 This would make Tahoe-LAFS a lot more dynamically configurable and more
 Docker-friendly, since env vars are commonplace.

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Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2833>
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