fix(build.go): default to slug builds

this commit also splits the build selection code into a separate func,
so that it can be more easily tested

Author: Aaron Schlesinger

pkg/gitreceive/build.go

@@ -123,14 +123,18 @@ func build(conf *Config, builderKey, gitSha string) error {
 		return fmt.Errorf("running %s (%s)", strings.Join(tarCmd.Args, " "), err)
 	}
 
-	usingDockerfile := true
-	rawProcFile, err := ioutil.ReadFile(fmt.Sprintf("%s/Procfile", tmpDir))
-	if err == nil {
-		usingDockerfile = false
-	}
+	bType := getBuildTypeForDir(tmpDir)
+	usingDockerfile := bType == buildTypeDockerfile
+
 	var procType pkg.ProcessType
-	if err := yaml.Unmarshal(rawProcFile, &procType); err != nil {
-		return fmt.Errorf("procfile %s/ProcFile is malformed (%s)", tmpDir, err)
+	if bType == buildTypeProcfile {
+		rawProcFile, err := ioutil.ReadFile(fmt.Sprintf("%s/Procfile", tmpDir))
+		if err != nil {
+			return fmt.Errorf("reading %s/Procfile", tmpDir)
+		}
+		if err := yaml.Unmarshal(rawProcFile, &procType); err != nil {
+			return fmt.Errorf("procfile %s/ProcFile is malformed (%s)", tmpDir, err)
+		}
 	}
 
 	var srcManifest string

pkg/gitreceive/build_type.go

@@ -0,0 +1,25 @@
+package gitreceive
+
+import (
+	"fmt"
+	"os"
+)
+
+type buildType string
+
+func (b buildType) String() string {
+	return string(b)
+}
+
+const (
+	buildTypeProcfile   buildType = "procfile"
+	buildTypeDockerfile buildType = "dockerfile"
+)
+
+func getBuildTypeForDir(dirName string) buildType {
+	_, err := os.Stat(fmt.Sprintf("%s/Dockerfile", dirName))
+	if err == nil {
+		return buildTypeDockerfile
+	}
+	return buildTypeProcfile
+}

pkg/gitreceive/build_type_test.go

@@ -0,0 +1,27 @@
+package gitreceive
+
+import (
+	"os"
+	"testing"
+)
+
+func TestGetBuildTypeForDir(t *testing.T) {
+	tmpDir := os.TempDir()
+	bType := getBuildTypeForDir(tmpDir)
+	if bType != buildTypeProcfile {
+		t.Fatalf("expected procfile build, got %s", bType)
+	}
+	if _, err := os.Create(tmpDir + "/Dockerfile"); err != nil {
+		t.Fatalf("error creating %s/Dockerfile (%s)", tmpDir, err)
+	}
+	defer func() {
+		if err := os.Remove(tmpDir + "/Dockerfile"); err != nil {
+			t.Fatalf("failed to remove Dockerfile from %s (%s)", tmpDir, err)
+		}
+	}()
+
+	bType = getBuildTypeForDir(tmpDir)
+	if bType != buildTypeDockerfile {
+		t.Fatalf("expected dockerfile build, got %s", bType)
+	}
+}