From 5aa4f51b80bba3de6862c1914854b000b2195496 Mon Sep 17 00:00:00 2001
From: Thorsten Ball <mrnugget@gmail.com>
Date: Tue, 11 May 2021 17:21:35 +0200
Subject: [PATCH 1/2] Set default log to src-cli.debug.log if build tag debug
 is used

I don't care about the specifics - filename, build tag, etc. - but I
*need* this. And the reason why I need it is that it's hard to log
something when our TUI is active. Not because of how the TUI is
implemented, but because the whole concept of an updating TUI doesn't
gel with printf-debugging.

And this is perfect for me: I can `tail -f` the log file and then use
`log.Printf` whereever I want.
---
 cmd/src/batch_common.go   |  1 -
 internal/batches/debug.go | 26 ++++++++++++++++++++++----
 2 files changed, 22 insertions(+), 5 deletions(-)

diff --git a/cmd/src/batch_common.go b/cmd/src/batch_common.go
index 79eb3ee500..e9dae4e498 100644
--- a/cmd/src/batch_common.go
+++ b/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,
diff --git a/internal/batches/debug.go b/internal/batches/debug.go
index 7fd55f51e5..efface6287 100644
--- a/internal/batches/debug.go
+++ b/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)
+}

From 2d9b51736a0fc8d68e278313f8908924a2cc326a Mon Sep 17 00:00:00 2001
From: Thorsten Ball <mrnugget@gmail.com>
Date: Wed, 12 May 2021 11:04:33 +0200
Subject: [PATCH 2/2] Add info to DEVELOPMENT.md

---
 DEVELOPMENT.md | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/DEVELOPMENT.md b/DEVELOPMENT.md
index f291b4d719..be6a807670 100644
--- a/DEVELOPMENT.md
+++ b/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.