bug#18505: 24.3.93; intermittent unexec failures when building on Mac OS X 10.10 beta, Xcode 6.0

[David Caldwell]

Hello,

I tried to build the latest pretest on Mac OS X Yosemite Beta with the
new Xcode 6.0 (GM) tools and ran into this error during the unexec step:

unexec: not enough room for load commands for new __DATA segments

(Full unexec log is attached).

Poking around I discovered a couple discrepancies. The "headerpad_extra"
setting in configure.ac says entries are 56 bytes, but they are actually
78 bytes (probably because the comment was originally written in the 32
bit days). Also the number of load commands needed is variable now--it
depends on how many malloc segments were found and how many it could
merge (see find_emacs_zone_regions() and unexec_regions_merge() in
unexmacosx.c).

I printed num_unexec_regions in unexmacosx.c and observed anywhere
from about 14 to 34 regions needed. But given that it seems to be
different on every run and it's super annoying to debug when you run out
of space, I suggest bumping headerpad_extra up to 0x1000. This gives
about 1.5KB of headroom (or wasted space, depending on how you look at
it) and allows for 52 load commands, which should be future proof for
quite a while. I have attached a patch for configure.ac that does this
and changes the comment to reflect my understanding.

I've also provided a patch that makes the unexec error more helpful: it
computes the size needed for headerpad_extra given the number of load
commands that it needed for that particular unexec run.

I assume this affects the trunk code, too, though I didn't look at it.

-David

increase_headerpad_extra.patch
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helpful_unexec_error.patch
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unexec-log.txt
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smime.p7s
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