Support single-stage builds for older Docker versions

Summary:
Refactored the Docker compiler to omit the stage name (`AS <stage>`)
from the `FROM` instruction when no dependent artifacts are declared.
This allows for use with older versions of Docker that don't support
multi-stage builds.

Test Plan: Run `go test` under the `docker` directory.

Reviewers: thcipriani, mobrovac, hashar, Jrbranaa, mmodell, #release-engineering-team

Reviewed By: mobrovac

Tags: #release-engineering-team

Differential Revision: https://phabricator.wikimedia.org/D684

2 files changed, 56 insertions, 2 deletions

diff --git a/docker/compiler.go b/docker/compiler.go
index 9f1e5d1..a129ec4 100644
--- a/docker/compiler.go
+++ b/docker/compiler.go
@@ -13,9 +13,14 @@ func Compile(cfg *config.Config, variant string) *bytes.Buffer {
 	vcfg, err := config.ExpandVariant(cfg, variant)
 
 	if err == nil {
+		// omit the main stage name unless multi-stage is required below
+		mainStage := ""
+
 		// write multi-stage sections for each artifact dependency
 		for _, artifact := range vcfg.Artifacts {
 			if artifact.From != "" {
+				mainStage = variant
+
 				dependency, err := config.ExpandVariant(cfg, artifact.From)
 
 				if err == nil {
@@ -24,14 +29,20 @@ func Compile(cfg *config.Config, variant string) *bytes.Buffer {
 			}
 		}
 
-		CompileStage(buffer, variant, vcfg)
+		CompileStage(buffer, mainStage, vcfg)
 	}
 
 	return buffer
 }
 
 func CompileStage(buffer *bytes.Buffer, stage string, vcfg *config.VariantConfig) {
-	Writeln(buffer, "FROM ", vcfg.Base, " AS ", stage)
+	baseAndStage := vcfg.Base
+
+	if stage != "" {
+		baseAndStage += " AS " + stage
+	}
+
+	Writeln(buffer, "FROM ", baseAndStage)
 
 	Writeln(buffer, "USER root")
 
diff --git a/docker/compiler_test.go b/docker/compiler_test.go
new file mode 100644
index 0000000..f8b467f
--- /dev/null
+++ b/docker/compiler_test.go
@@ -0,0 +1,43 @@
+package docker_test
+
+import (
+	"testing"
+	"gopkg.in/stretchr/testify.v1/assert"
+
+	"phabricator.wikimedia.org/source/blubber.git/config"
+	"phabricator.wikimedia.org/source/blubber.git/docker"
+)
+
+func TestSingleStageHasNoName(t *testing.T) {
+	cfg, err := config.ReadConfig([]byte(`---
+base: foo/bar
+variants:
+  development: {}
+`))
+
+	assert.Nil(t, err)
+
+	dockerfile := docker.Compile(cfg, "development").String()
+
+	assert.Contains(t, dockerfile, "FROM foo/bar\n")
+}
+
+func TestMultiStageIncludesStageNames(t *testing.T) {
+	cfg, err := config.ReadConfig([]byte(`---
+base: foo/bar
+variants:
+  build: {}
+  production:
+    artifacts:
+      - from: build
+        source: .
+        destination: .
+`))
+
+	assert.Nil(t, err)
+
+	dockerfile := docker.Compile(cfg, "production").String()
+
+	assert.Contains(t, dockerfile, "FROM foo/bar AS build\n")
+	assert.Contains(t, dockerfile, "FROM foo/bar AS production\n")
+}