Set default log to src-cli.debug.log if build tag debug is used

DEVELOPMENT.md

@@ -78,3 +78,17 @@ To remove it and then force the upstream image on Docker Hub to be used again:
 ```sh
 docker rmi sourcegraph/src-batch-change-volume-workspace
 ```
+
+## Use `debug` build tag to debug batch changes functionality
+
+Since `src batch apply` and `src batch preview` start up a TUI that gets updated repeatedly it's nearly impossible to do printf-debugging by printing debug information - the TUI would hide those or overwrite them.
+
+To help with that you can compile your src binary (or run the tests) with the `debug` build flag:
+
+```
+go build -tags debug -o ~/src ./cmd/src
+```
+
+This will cause the `./internal/batches/debug.go` file to be included in the build. In that file the `log` default package logger is setup to log to `~/.sourcegraph/src-cli.debug.log`.
+
+That allows you to `tail -f ~/.sourcegraph/src-cli.debug.log` and use `log.Println()` in your code to debug.

cmd/src/batch_common.go

@@ -201,7 +201,6 @@ type executeBatchSpecOpts struct {
 // Sourcegraph, including execution as needed and applying the resulting batch
 // spec if specified.
 func executeBatchSpec(ctx context.Context, opts executeBatchSpecOpts) error {
-	batches.DebugOut = opts.out
 	svc := service.New(&service.Opts{
 		AllowUnsupported: opts.flags.allowUnsupported,
 		AllowIgnored:     opts.flags.allowIgnored,

internal/batches/debug.go

@@ -1,9 +1,27 @@
+// +build debug
+
 package batches
 
 import (
-	"github.com/sourcegraph/sourcegraph/lib/output"
+	"log"
+	"os"
+	"path/filepath"
 )
 
-// DebugOut can be used to print debug messages in development to the TUI.
-// For that it needs to be set to an actual *output.Output.
-var DebugOut output.Writer = output.NoopWriter{}
+// In builds with the debug flag (i.e. `go build -tags debug -o src ./cmd/src`)
+// init() sets up the default logger to log to a file in ~/.sourcegraph.
+func init() {
+	homedir, err := os.UserHomeDir()
+	if err != nil {
+		log.Fatalf("getting user home directory: %s", err)
+	}
+
+	fullPath := filepath.Join(homedir, ".sourcegraph", "src-cli.debug.log")
+
+	f, err := os.OpenFile(fullPath, os.O_RDWR|os.O_CREATE|os.O_APPEND, 0666)
+	if err != nil {
+		log.Fatalf("setting debug log file failed: %s", err)
+	}
+
+	log.SetOutput(f)
+}