src/: define ceph_release_t and use it

we have following pains when it comes to ceph release related
programming:

* we use int, uint8_t, uint32_t, unsigned int for representing the ceph
  release, i.e., jewel, luminous, nautilus, in different places in our
  source tree.
* we always need to add a comment aside of `uint8_t release` to help
  the folks to understand that it is CEPH_RELEASE_*.
* also we keep forgetting that "os << release" actually prints the
  release as an ASCII.
* and it's painful to remember that we have to translate the release
  number using `ceph_release_name()` before print it out in the human
  readable format.
* we replicate the n+2 upgrade policy in multiple places

in this change, `ceph_release_t` and some helper functions are
intruduced to alleviate the pains above.

* add a scoped enum for representing ceph releases, so the release
  is typed . which means that we can attach different function to
  it. and in future, we can even replace `ceph_release_t` with
  a class if we need to support more fancy features which cannot be
  implemented using free functions.
* add `ostream<<()` operator for `ceph_release_t`, so we can simply
  send it to `ostream`
* add `can_upgrade_from()` so we don't need to repeat ourselves.
* move ceph_release_from_name() to ceph_release.{h,cc}, as currently,
  ceph_release.cc uses `ceph_release_name()` for implementing
  `ostream<<()`, and after this change, `ceph_release_from_name()`
  will return `ceph_release_t`, so if we keep `ceph_release_from_name()`
  where it was, these two headers will be included by each other,
  which is a no-go.
* reimplement `ceph_release_from_name()` using a loop. before this
  change, `ceph_release_from_name()` was implemented using a manually
  unrolled if-else structure, which is more performant, but the
  downside is that, it replicates mapping between release number
  and its name. so after this change, a loop is used instead.
  as this function is not used in the critical path, so this change
  should not have visible impact on the performance.
* always use ceph_release_t::unknown as the default value of the
  "release" member variables. before this change, sometimes, we use
  "0" and sometimes we use "1", after inspecting the code, i found that
  "0" is good enough to cover all the use cases. and since "0" is a
  magic number in this context, it is replaced using
  `ceph_release_t::unknown`. to facilidate the checking against
  `ceph_release_t::unknown`, `operator!()` is added.
* ceph::to_string() and ceph::to_integer<>() are added to help
  to remove the asssumption of the underlying type of `ceph_release_t`,
  ideally, users of `ceph_release_t` should not use `static_cast<>` to
  cast it into integer types, instead, they should use
  `ceph::to_integer<>()` to do this job. if, in future, we want to
  use a `class` to represent `ceph_release_t`, we can get this done
  with minimum change, if `ceph::to_string()` and `ceph::to_string()`
  are used. we can not specialize them in `std` naming space. as
  it's claimed that it's undefined behavior to do so. see
  https://en.cppreference.com/w/cpp/language/extending_std .

Signed-off-by: Kefu Chai <kchai@redhat.com>

1 files changed, 4 insertions, 11 deletions

diff --git a/src/ceph_mon.cc b/src/ceph_mon.cc
index f51dce57e62..49b6750f206 100644
--- a/src/ceph_mon.cc
+++ b/src/ceph_mon.cc
@@ -571,17 +571,10 @@ int main(int argc, const char **argv)
     string val;
     int r = store->read_meta("min_mon_release", &val);
     if (r >= 0 && val.size()) {
-      int min = atoi(val.c_str());
-      if (min &&
-	  min + 2 < (int)ceph_release()) {
-	derr << "recorded min_mon_release is " << min
-	     << " (" << ceph_release_name(min)
-	     << ") which is >2 releases older than installed "
-	     << ceph_release() << " (" << ceph_release_name(ceph_release())
-	     << "); you can only upgrade 2 releases at a time" << dendl;
-	derr << "you should first upgrade to "
-	     << (min + 1) << " (" << ceph_release_name(min + 1) << ") or "
-	     << (min + 2) << " (" << ceph_release_name(min + 2) << ")" << dendl;
+      ceph_release_t from_release = ceph_release_from_name(val);
+      ostringstream err;
+      if (!can_upgrade_from(from_release, "min_mon_release", err)) {
+	derr << err.str() << dendl;
 	prefork.exit(1);
       }
     }