[erlang-patches] The 'start_phases' entry in .app files is incorrectly set to 'undefined' when generating a release

[Ulf Wiger]

Hmm, there's an old catch here. Someone will have to hollar if it is no longer relevant - but if it isn't, some documentation patch is called for.

In http://www.erlang.org/doc/apps/kernel/application.html#Module:start-2

StartType defines the type of start:

normal if it's a normal startup.
normal also if the application is distributed and started at the current node due to a failover from another node, and the application specification key start_phases == undefined.
{takeover,Node} if the application is distributed and started at the current node due to a takeover from Node, either because application:takeover/2 has been called or because the current node has higher priority than Node.
{failover,Node} if the application is distributed and started at the current node due to a failover from Node, and the application specification key start_phases /= undefined.
Note that StartType = {failover, Node} is only used if start_phases /= undefined. I will guess that it is for this reason that start_phases is actually set to undefined as default. If it isn't, it was a happy coincidence, since it actually preserves backward compatibility.

This particular oddity was introduced many years ago, in the 90s - possibly even before Erlang was released as Open Source, so the legacy argument may not be that relevant.

OTOH, I don't think many people use the start_phases attribute, so even new code might break if the Mod:start/2 function is suddenly called with a StartType that was never seen before.

BR,
Ulf W


On 3 Jan 2012, at 05:21, Juan Jose Comellas wrote:

> The default value for 'start_phases' in .app files should be [], but
> it is incorrectly set to 'undefined' when generating a release. This
> happens when the original .app file does not set a value for
> 'start_phases' explicitly.
> 
> The reltool application defines its own copy of a record to handle
> .app files (#app_info{}, defined in lib/reltool/src/reltool.hrl) that
> has different default values than the one used by systools
> (#application{}, defined in lib/sasl/src/systools.hrl). In particular,
> the 'start_phases' entry is assumed by all of OTP to be [] when it is
> not explicitly set but reltool sets it to 'undefined' by default. This
> causes badmatch errors when trying to generate release upgrades.
> 
> Without this patch, all of the rebar examples that generate release
> upgrades that I've found on the web will fail. The bug was introduced
> in R14A.
> 
> git fetch git://github.com/jcomellas/otp.git jc-fix-default-start-phases
> 
> Juanjo
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