…B. (#1486)

(cherry picked from commit dc653b0)

This PR adds a new `db.MaxVersion()` API that returns the
maximum version across all keys in the DB.

(cherry picked from commit 36af85f)

(cherry picked from commit f07dcfc)

Add --num_versions flag to set NumVersionsToKeep option in badger.

(cherry picked from commit c68fbc1)

The untyped const causes conversion to int, for which the value is too large.
Use a typed const instead that is large enough to store value.

Similar to #1541, but for v2.2007 branch.

If a Go program imports both badger (v1) and badger/v2, a warning will
be produced at init time:

	WARNING: proto: file "pb.proto" is already registered
	A future release will panic on registration conflicts. See:
	https://developers.google.com/protocol-buffers/docs/reference/go/faq#namespace-conflict

The problem is the "pb.proto" filename; it's registered globally, which
makes it very likely to cause conflicts with other protobuf-generated
packages in a Go binary.

Coincidentally, this is a problem with badger's pb package in the v1 module,
since that too uses the name "pb.proto". Instead, call the file
"badgerpb2.proto", which should be unique enough, and it's also the name
we use for the Protobuf package.

Finally, update gen.sh to work out of the box via just "bash gen.sh"
without needing extra tweaks, thanks to the "paths=source_relative"
option. It forces output files to be produced next to the input files,
which is what we want.

(cherry picked from commit 3e6a4b7)

Co-authored-by: Daniel Martí <mvdan@mvdan.cc>

(cherry picked from commit 6c1e5bc)

Plan 9 doesn't have mmap, so we just return an error.

File locking is implemented using the exclusive mode bit.
From http://man.cat-v.org/plan_9/5/open:

  If the file is marked for exclusive use (see stat(5)), only
  one client can have the file open at any time.  That is,
  after such a file has been opened, further opens will fail
  until fid has been clunked.

Initial attempt at implementing directory locking using this method was
not successful because badger seems to be opening the directory multiple
times (at least while running the test suite). As a workaround, we lock
the pid lock file instead of the directory itself, same as the windows
implementation.

Co-authored-by: Ibrahim Jarif <ibrahim@dgraph.io>
(cherry picked from commit 07fa3eb)

Co-authored-by: Fazlul Shahriar <fshahriar@gmail.com>

Co-authored-by: Fazlul Shahriar <fshahriar@gmail.com>

…std (#1736)

Backports the changes from #1706 and #1709 to the v2 branch of badger.

For context, https://github.com/caddyserver/caddy uses https://github.com/smallstep/certificates which uses https://github.com/smallstep/nosql which uses badger.

See smallstep/nosql#12 for the discussion. It's not currently viable for smallstep to upgrade from v2 to v3 of badger, so it would be nice to backport this zstd change to the v2 branch to remove the implicit (yet optional) CGO dependency on Caddy.

I targeted the release/v2.2007 branch, I couldn't really see a more relevant branch to target, so I hope that's okay.

(cherry picked from commit ee6065b)