maintner: add RPC endpoint for listing Go releases

This change adds an RPC endpoint to maintnerd API server to list
the supported Go releases. This is needed and will be used by
cmd/coordinator to run subrepo trybots on previous Go release
(in addition to current).

It's implemented on top of the Gerrit project ref data that the
maintner corpus already tracks. A release is considered to be
exist for each git tag named "goX", "goX.Y", or "goX.Y.Z".

This functionality is also implemented in some other places using
data that is further away from the source of truth. Such places can
start to use this endpoint instead.

Add a subcommand to maintq to invoke the new list Go releases endpoint.
Its current output (against a local development maintnerd server):

	$ go run .../maintner/maintq -server=localhost:6344 list-releases
	major:1 minor:11 patch:1 tag_name:"go1.11.1" tag_commit:"26957168c4c0cdcc7ca4f0b19d0eb19474d224ac" branch_name:"release-branch.go1.11" branch_commit:"97781d2ed116d2cd9cb870d0b84fc0ec598c9abc"
	major:1 minor:10 patch:4 tag_name:"go1.10.4" tag_commit:"2191fce26a7fd1cd5b4975e7bd44ab44b1d9dd78" branch_name:"release-branch.go1.10" branch_commit:"e97b7d68f107ff60152f5bd5701e0286f221ee93"

Updates golang/go#17626

Change-Id: Ia9ea7f49d421ce0c7a9e85c423aba31572cea52b
Reviewed-on: https://go-review.googlesource.com/c/146137
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: dmitshur

maintner/maintnerd/apipb/api.pb.go

@@ -62,6 +62,28 @@ message GerritTryWorkItem {
   repeated string go_branch = 6;  // "master", "release-branch.go1.8", etc
 }
 
+// By default, ListGoReleases returns only the latest patches
+// of releases that are considered supported per policy.
+message ListGoReleasesRequest {}
+
+message ListGoReleasesResponse {
+  // Releases are Go releases, sorted with latest release first.
+  repeated GoRelease releases = 1;
+}
+
+message GoRelease {
+  int32 major = 1;
+  int32 minor = 2;
+  int32 patch = 3;
+  string tag_name = 4;       // "go1.11.1", etc.
+  string tag_commit = 5;     // "26957168c4c0cdcc7ca4f0b19d0eb19474d224ac"
+
+  // Release branch information for this major-minor version pair.
+  // Empty if the corresponding release branch doesn't exist.
+  string branch_name = 6;    // "release-branch.go1.11", etc.
+  string branch_commit = 7;  // most recent commit on the release branch, e.g., "edb6c16b9b62ed8586d2e3e422911d646095b7e5"
+}
+
 service MaintnerService {
   // HasAncestor reports whether one commit contains another commit
   // in its git history.
@@ -74,4 +96,8 @@ service MaintnerService {
 
   // GoFindTryWork finds trybot work for the coordinator to build & test.
   rpc GoFindTryWork(GoFindTryWorkRequest) returns (GoFindTryWorkResponse);
+
+  // ListGoReleases lists Go releases. A release is considered to exist
+  // if a tag for it exists.
+  rpc ListGoReleases(ListGoReleasesRequest) returns (ListGoReleasesResponse);
 }