Deduplicate and fix up our use of mmap()

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

1 files changed, 2 insertions, 3 deletions

diff --git a/src/libotutil/ot-fs-utils.h b/src/libotutil/ot-fs-utils.h
index 98fcd6a2..7eb811cd 100644
--- a/src/libotutil/ot-fs-utils.h
+++ b/src/libotutil/ot-fs-utils.h
@@ -85,8 +85,7 @@ ot_map_anonymous_tmpfile_from_content (GInputStream *instream,
                                        GCancellable *cancellable,
                                        GError      **error);
 
-GBytes *ot_file_mapat_bytes (int dfd,
-                             const char *path,
-                             GError **error);
+GBytes *ot_fd_readall_or_mmap (int fd, goffset offset,
+                               GError **error);
 
 G_END_DECLS