feat(server): Log Nix output live during the builds

Instead of dumping all Nix output as one at the end of the build
process, stream it live as the lines come in.

This is a lot more useful for debugging stuff like where manifest
retrievals get stuck.

tools/nixery/server/builder/builder.go

@@ -18,6 +18,7 @@
 package builder
 
 import (
+	"bufio"
 	"bytes"
 	"context"
 	"encoding/json"
@@ -107,6 +108,15 @@ func convenienceNames(packages []string) []string {
 	return packages
 }
 
+// logNix logs each output line from Nix. It runs in a goroutine per
+// output channel that should be live-logged.
+func logNix(name string, r io.ReadCloser) {
+	scanner := bufio.NewScanner(r)
+	for scanner.Scan() {
+		log.Printf("\x1b[31m[nix - %s]\x1b[39m %s\n", name, scanner.Text())
+	}
+}
+
 // Call out to Nix and request that an image be built. Nix will, upon success,
 // return a manifest for the container image.
 func BuildImage(ctx *context.Context, cfg *config.Config, cache *LocalCache, image *Image, bucket *storage.BucketHandle) (*BuildResult, error) {
@@ -149,6 +159,7 @@ func BuildImage(ctx *context.Context, cfg *config.Config, cache *LocalCache, ima
 		if err != nil {
 			return nil, err
 		}
+		go logNix(image.Name, errpipe)
 
 		if err = cmd.Start(); err != nil {
 			log.Println("Error starting nix-build:", err)
@@ -157,11 +168,9 @@ func BuildImage(ctx *context.Context, cfg *config.Config, cache *LocalCache, ima
 		log.Printf("Started Nix image build for '%s'", image.Name)
 
 		stdout, _ := ioutil.ReadAll(outpipe)
-		stderr, _ := ioutil.ReadAll(errpipe)
 
 		if err = cmd.Wait(); err != nil {
-			// TODO(tazjin): Propagate errors upwards in a usable format.
-			log.Printf("nix-build execution error: %s\nstdout: %s\nstderr: %s\n", err, stdout, stderr)
+			log.Printf("nix-build execution error: %s\nstdout: %s\n", err, stdout)
 			return nil, err
 		}