Capture and expose build-time meta data

Summary:
The `go build` tool can accept linker options that dynamically set
variable values at build time. Let's make use of that in our `Makefile`
and `debian/rules` to know and expose meta data such as version and Git
commit at runtime.

Test Plan:
Run `make bin/blubber && bin/blubber --version` and verify that it outputs
"0.0.1-[git head commit]". Build and install the debian package and verify
the same using the installed binary.

Reviewers: thcipriani, hashar, Joe, #release-engineering-team

Reviewed By: thcipriani, #release-engineering-team

Tags: #release-engineering-team

Differential Revision: https://phabricator.wikimedia.org/D816

5 files changed, 33 insertions, 1 deletions

diff --git a/Makefile b/Makefile
index bfd7ab8..116e2b9 100644
--- a/Makefile
+++ b/Makefile
@@ -3,6 +3,10 @@ PACKAGE := phabricator.wikimedia.org/source/blubber
 export BUILD_DIR=$(GOPATH)/src/$(PACKAGE)
 BINARY :=$(CURDIR)/bin/blubber
 
+GO_LDFLAGS := \
+  -X $(PACKAGE)/meta.Version=$(shell cat VERSION) \
+  -X $(PACKAGE)/meta.GitCommit=$(shell git rev-parse --short HEAD)
+
 all: clean bin/blubber
 
 clean:
@@ -13,7 +17,7 @@ bin/blubber:
 	mkdir -p $(dir $(BUILD_DIR))
 	ln -s $(CURDIR) $(BUILD_DIR)
 	cd $(BUILD_DIR) && go get ./...
-	cd $(BUILD_DIR) && go build -v -i
+	cd $(BUILD_DIR) && go build -v -i -ldflags "$(GO_LDFLAGS)"
 	mkdir -p $(CURDIR)/bin && mv $(BUILD_DIR)/blubber $(BINARY)
 
 
diff --git a/VERSION b/VERSION
new file mode 100644
index 0000000..8acdd82
--- /dev/null
+++ b/VERSION
@@ -0,0 +1 @@
+0.0.1
diff --git a/debian/rules b/debian/rules
index 8cce5e0..1abf6fb 100755
--- a/debian/rules
+++ b/debian/rules
@@ -1,4 +1,12 @@
 #!/usr/bin/make -f
 
+PACKAGE := phabricator.wikimedia.org/source/blubber
+GO_LDFLAGS := \
+  -X $(PACKAGE)/meta.Version=$(shell cat VERSION) \
+  -X $(PACKAGE)/meta.GitCommit=$(shell git rev-parse --short HEAD)
+
 %:
 	dh $@ --buildsystem=golang --with=golang
+
+override_dh_auto_build:
+	dh_auto_build -O--buildsystem=golang -- -ldflags "$(GO_LDFLAGS)"
diff --git a/main.go b/main.go
index 5273668..a684d11 100644
--- a/main.go
+++ b/main.go
@@ -7,9 +7,15 @@ import (
 
 	"phabricator.wikimedia.org/source/blubber/config"
 	"phabricator.wikimedia.org/source/blubber/docker"
+	"phabricator.wikimedia.org/source/blubber/meta"
 )
 
 func main() {
+	if len(os.Args) > 1 && os.Args[1] == "--version" {
+		fmt.Println(meta.FullVersion())
+		os.Exit(0)
+	}
+
 	if len(os.Args) < 3 {
 		fmt.Println("Usage: blubber config.yaml variant")
 		os.Exit(1)
diff --git a/meta/meta.go b/meta/meta.go
new file mode 100644
index 0000000..a5ee581
--- /dev/null
+++ b/meta/meta.go
@@ -0,0 +1,13 @@
+package meta
+
+// Meta variables set dynamically by the linker at build time
+var (
+	Version   string
+	GitCommit string
+)
+
+// FullVersion returns the version string in the form of
+// ([major].[minor].[patch]+[commit])
+func FullVersion() string {
+	return Version + "+" + GitCommit
+}