feat: add a buffer writer to the logger, for internal use (#6551)

Identical to console, file or conn but writes to a buffer instead.

It is useful in two contexts:

- tests that need to assert the logs in a way that is simpler than
  LogChecker.
- capturing the logs of a given task to display in the web UI,
  return from the API, etc.

Since all logged events at a given level are written to the buffer by default, it is best used with WriterMode.Expression to only keep the log lines of interest.

## Checklist

The [contributor guide](https://forgejo.org/docs/next/contributor/) contains information that will be helpful to first time contributors. There also are a few [conditions for merging Pull Requests in Forgejo repositories](https://codeberg.org/forgejo/governance/src/branch/main/PullRequestsAgreement.md). You are also welcome to join the [Forgejo development chatroom](https://matrix.to/#/#forgejo-development:matrix.org).

### Tests

- I added test coverage for Go changes...
  - [x] in their respective `*_test.go` for unit tests.
  - [ ] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I added test coverage for JavaScript changes...
  - [ ] in `web_src/js/*.test.js` if it can be unit tested.
  - [ ] in `tests/e2e/*.test.e2e.js` if it requires interactions with a live Forgejo server (see also the [developer guide for JavaScript testing](https://codeberg.org/forgejo/forgejo/src/branch/forgejo/tests/e2e/README.md#end-to-end-tests)).

### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [x] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [x] I do not want this change to show in the release notes.
- [ ] I want the title to show in the release notes with a link to this pull request.
- [ ] I want the content of the `release-notes/<pull request number>.md` to be be used for the release notes instead of the title.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/6551
Reviewed-by: Antonin Delpeuch <wetneb@noreply.codeberg.org>
Co-authored-by: Earl Warren <contact@earl-warren.org>
Co-committed-by: Earl Warren <contact@earl-warren.org>

3 files changed, 59 insertions, 0 deletions

diff --git a/.deadcode-out b/.deadcode-out
index ae0d358b51..64741ec7ac 100644
--- a/.deadcode-out
+++ b/.deadcode-out
@@ -169,6 +169,9 @@ code.gitea.io/gitea/modules/json
 	StdJSON.NewDecoder
 	StdJSON.Indent
 
+code.gitea.io/gitea/modules/log
+	NewEventWriterBuffer
+
 code.gitea.io/gitea/modules/markup
 	GetRendererByType
 	RenderString
diff --git a/modules/log/event_writer_buffer.go b/modules/log/event_writer_buffer.go
new file mode 100644
index 0000000000..28857c2189
--- /dev/null
+++ b/modules/log/event_writer_buffer.go
@@ -0,0 +1,22 @@
+// Copyright 2025 The Forgejo Authors.
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package log
+
+import (
+	"bytes"
+)
+
+type EventWriterBuffer struct {
+	*EventWriterBaseImpl
+	Buffer *bytes.Buffer
+}
+
+var _ EventWriter = (*EventWriterBuffer)(nil)
+
+func NewEventWriterBuffer(name string, mode WriterMode) *EventWriterBuffer {
+	w := &EventWriterBuffer{EventWriterBaseImpl: NewEventWriterBase(name, "buffer", mode)}
+	w.Buffer = new(bytes.Buffer)
+	w.OutputWriteCloser = nopCloser{w.Buffer}
+	return w
+}
diff --git a/modules/log/event_writer_buffer_test.go b/modules/log/event_writer_buffer_test.go
new file mode 100644
index 0000000000..5c0343b8a8
--- /dev/null
+++ b/modules/log/event_writer_buffer_test.go
@@ -0,0 +1,34 @@
+// Copyright 2025 The Forgejo Authors.
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package log_test
+
+import (
+	"context"
+	"testing"
+
+	"code.gitea.io/gitea/modules/log"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestBufferLogger(t *testing.T) {
+	prefix := "TestPrefix "
+	level := log.INFO
+	expected := "something"
+
+	bufferWriter := log.NewEventWriterBuffer("test-buffer", log.WriterMode{
+		Level:      level,
+		Prefix:     prefix,
+		Expression: expected,
+	})
+
+	logger := log.NewLoggerWithWriters(context.Background(), "test", bufferWriter)
+
+	logger.SendLogEvent(&log.Event{
+		Level:         log.INFO,
+		MsgSimpleText: expected,
+	})
+	logger.Close()
+	assert.Contains(t, bufferWriter.Buffer.String(), expected)
+}