Buried in this large patch is a logical fix:
```
- if (!map)
- return glnx_throw_errno_prefix (error, "mmap");
+ if (map == (void*)-1)
+ return glnx_null_throw_errno_prefix (error, "mmap");
```
Which would have helped me debug another patch I was working
on. But it turns out that actually correctly checking for
errors from `mmap()` triggers lots of other bugs - basically
because we sometimes handle zero-length variants (in detached
metadata). When we start actually returning errors due to
this, things break. (It wasn't a problem in practice before
because most things looked at the zero size, not the data).
Anyways there's a bigger picture issue here - a while ago
we made a fix to only use `mmap()` for reading metadata from disk
only if it was large enough (i.e. `>16k`). But that didn't
help various other paths in the pull code and others that were
directly doing the `mmap()`.
Fix this by having a proper low level fs helper that does "read all data from
fd+offset into GBytes", which handles the size check. Then the `GVariant` bits
are just a clean layer on top of this. (At the small cost of an additional
allocation)
Side note: I had to remind myself, but the reason we can't just use
`GMappedFile` here is it doesn't support passing an offset into `mmap()`.
Closes: #1251
Approved by: jlebon
Colin Walters
5c7d2dd8be