Require a bit less free memory before checking for upgrades.
The general goal is to avoid displaying "Not enough memory available to check for upgrades" too often due to over-cautious memory requirements checked in the wrapper. The specific goal is to avoid having to bump the hardware requirements for Tails 1.1 (Wheezy) to >> 1 GiB of RAM, as this would 1. not be very nice for users of oldish hardware; and 2. force us to implement #5502 ("Notify user if hardware requirements are not met") in time for the 1.1 freeze. My experiments, documented on #5390, indicate that even lower limits would probably work. Let's not be too adventurous to start with, though: my plan is to lower the limit to something that is low enough for the wrapper to dare running the check for upgrades, but still quite cautious. Later on, we could try lowering the limits even more, or even drop it entirely: this would require a mechanism to detect when the check for upgrades fails due to memory exhaustion, which don't have yet. Note that, if an automatic upgrade is available, Tails Upgrader checks memory again, to ensure there's enough free memory to apply the upgrade. This commit assumes that Tails Upgrader's own check is cautious enough, and that we were not implicitly relying on the check done earlier, in the wrapper, to ensure upgrade safety. This assumption might be wrong. My plan is to use the incremental upgrade to 1.1~rc1 as a test bench to verify this, as I have no time to fully test this fully right now (still, I successfully applied a small — 50 MiB — IUK on top of a current build from this very branch, with 126 MiB of free memory, and 700 MiB total free memory). The symptom of a failed upgrade due to lack of memory (and then, of a too low $mem_factor value in Frontend.pm) would be to see the upgrader that simply die in the middle of the download, or (worse) in the middle of the upgrade. In the worst case, the resulting (partially upgraded) system may not boot anymore, but no user data will be affected, and the user can still fix their stick by doing a full upgrade.
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