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, 4 insertions, 1 deletions

diff --git a/src/ostree/ot-builtin-summary.c b/src/ostree/ot-builtin-summary.c
index abd1f86c..ed9e0b3d 100644
--- a/src/ostree/ot-builtin-summary.c
+++ b/src/ostree/ot-builtin-summary.c
@@ -217,7 +217,10 @@ ostree_builtin_summary (int argc, char **argv, GCancellable *cancellable, GError
       if (opt_raw)
         flags |= OSTREE_DUMP_RAW;
 
-      summary_data = ot_file_mapat_bytes (repo->repo_dir_fd, "summary", error);
+      glnx_fd_close int fd = -1;
+      if (!glnx_openat_rdonly (repo->repo_dir_fd, "summary", TRUE, &fd, error))
+        return FALSE;
+      summary_data = ot_fd_readall_or_mmap (fd, 0, error);
       if (!summary_data)
         return FALSE;