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Earlier, there was only one valid instruction to restart the emulator
in an appup/relup script, 'restart_new_emulator'. A new instuction,
'restart_emulator', is now added, and the meaning is as follows:
'restart_new_emulator' is mainly for the core applications (erts,
kernel, stdlib and sasl), and it indicates that there is new core
functionality in the release and the emulator needs to be restarted
before executing the upgrade scripts. If this instruction exists, a
temporary release will be created which consists of the new version
erts, kernel, stdlib and sasl, and the old versions of all other
applications. After restarting the emulator with this temporary
release, the rest of the upgrade instructions are executed, including
loading of new versions.
'restart_emulator' can be used by any application if a restart of the
emulator is needed after the upgrade instructions have been
executed. In this case, the emulator will be restarted with the new
release (i.e. not a tempoarary one) after all other upgrade
instructions for all applications have been excecuted.
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If the version of erts differs between two releases, the release
handler automatically adds a 'restart_new_emulator' instruction to the
upgrade script (relup). Earlier, this instruction was always added at
the end of the upgrade script, causing the following sequence of
operations during an upgrade (a bit simplified):
1. suspend processes
2. load new code
3. execute code_change functions
4. resume processes
5. restart emulator with new erts version
Obviously, this caused the new code to be loaded into the old emulator
and this would fail if the beam format had been changed in the new
version of the emulator.
To overcome this problem, this commit changes the order of the
instructions, so for upgrade with changed erts version we now have:
1. restart emulator with new erts, kernel, stdlib and sasl versions,
but old versions of all other applications.
2. suspend processes
3. load new code
4. execute code_change functions
5. resume processes
This is implemented by creating a temporary release, including new
erts, kernel, stdlib and sasl from the new release and all other
applications from the old release. A new boot file for this temporary
release is created, which includes a new 'apply' instruction to run
release_handler:new_emulator_upgrade/2. Then the emulator is restarted
using this boot file - and release_handler:new_emulator_upgrade/2
executes the rest of the upgrade script.
For downgrade, the order will be as before:
1. suspend processes
2. execute code_change functions with 'down'-indication
3. load old code
4. resume processes
5. restart emulator with old erts version
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Previously, relup file always included an
application:start(Application, permanent) apply instruction for every
application that appear in the UpTo/DowFrom release file, whatever
their start type in the release file.
The new implementation fixes this bug by honoring the start type
according to the rel(5) format. If the start type is none, no apply
line is included in the relup. If the start type is load, the relup
includes instruction to only load the application. Otherwise, the
relup includes an instruction to start the application to the
according type.
The fix is implemented by adding a new parameter to the
add_application high level appup instruction. This new parameter is
documented in appup(5).
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