Add helpers for getting version and vcs information (#186)

After abcxyz/abc#149, I feel like we've reached
the point where there's significant enough complexity to warrant
centralizing this logic.

buildinfo/buildinfo.go

@@ -0,0 +1,79 @@
+// Copyright 2023 The Authors (see AUTHORS file)
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package buildinfo provides functions for setting build information for CLIs
+// and programs. Since CLIs and programs can be compiled, downloaded as a
+// binary, or installed via `go install`, there's some nuanced logic in getting
+// these values correct across all instances.
+//
+// Consumers are encouraged to create an internal package in their module at
+// "internal/version" with the following contents:
+//
+//	var (
+//	  Name = "my-program"
+//
+//	  Version = buildinfo.Version()
+//
+//	  Commit = buildinfo.Commit()
+//
+//	  OSArch = buildinfo.OSArch()
+//
+//	  HumanVersion = Name + " " + Version + " (" + Commit + ", " + OSArch + ")"
+//	)
+//
+// These variables can then be referenced throughout the program. The values can
+// still be overridden with LDFLAGS (which will take precedent over any values
+// defined here).
+package buildinfo
+
+import (
+	"log"
+	"runtime"
+	"runtime/debug"
+)
+
+// Version attempts to read the module version injected by the compiler. If no
+// information is present, it returns "source".
+func Version() string {
+	if info, ok := debug.ReadBuildInfo(); ok {
+		if v := info.Main.Version; v != "" {
+			return v // e.g. "v0.0.1-alpha6.0.20230815191505-8628f8201363"
+		}
+	}
+
+	return "source"
+}
+
+// Commit returns the VCS information, specifically the revision. Since most of
+// our modules use Git, this is the Git SHA. If no SHA exists (e.g. outside of a
+// repo), it returns "HEAD".
+func Commit() string {
+	if info, ok := debug.ReadBuildInfo(); ok {
+		for _, setting := range info.Settings {
+			log.Printf("setting: %#v\n", setting)
+
+			if setting.Key == "vcs.revision" {
+				return setting.Value
+			}
+		}
+	}
+
+	return "HEAD"
+}
+
+// OSArch returns the denormalized operating system and architecture, separated
+// by a slash (e.g. "linux/amd64").
+func OSArch() string {
+	return runtime.GOOS + "/" + runtime.GOARCH
+}