monitor: increase buffer to deal with very big messages

This commit incrases the buffer os the osbuild message scanner
to deal with very big messages. The default size of 64kb is too
small for the `dracut` stage which will generate 5447 messages
that /dev/log is not available and they come so rapidly that
osbuild will pick them up in one big chunk.

This is a short term fix, the deeper fix should be to tweak
the osbuild monitor to have a smaller buffer size when sending
stdout/stderr output from stages to the monitor. This will
fix a crash in the bootc-image-builder anaconda builds that
is currently breaking the integration test in konflux.

See also osbuild/osbuild#1976

pkg/osbuild/monitor.go

@@ -62,8 +62,15 @@ type Progress struct {
 // NewStatusScanner returns a StatusScanner that can parse osbuild
 // jsonseq monitor status messages
 func NewStatusScanner(r io.Reader) *StatusScanner {
+	scanner := bufio.NewScanner(r)
+	// osbuild can currently generate very long messages, the default
+	// 64kb is too small for e.g. the dracut stage (see also
+	// https://github.com/osbuild/osbuild/issues/1976). Increase for
+	// but to unblock us.
+	buf := make([]byte, 0, 512_000)
+	scanner.Buffer(buf, 512_000)
 	return &StatusScanner{
-		scanner:         bufio.NewScanner(r),
+		scanner:         scanner,
 		contextMap:      make(map[string]*contextJSON),
 		stageContextMap: make(map[string]*stageContextJSON),
 	}

pkg/osbuild/monitor_test.go

@@ -2,7 +2,9 @@ package osbuild_test
 
 import (
 	"bytes"
+	"fmt"
 	"os"
+	"strings"
 	"testing"
 	"time"
 
@@ -116,3 +118,16 @@ func TestScannerSmoke(t *testing.T) {
 		assert.NotEqual(t, time.Time{}, st.Timestamp)
 	}
 }
+
+func TestScannerVeryLongLines(t *testing.T) {
+	buf := bytes.NewBuffer(nil)
+	fmt.Fprint(buf, `{"message": "`)
+	fmt.Fprint(buf, strings.Repeat("1", 128_000))
+	fmt.Fprint(buf, `"}`)
+
+	scanner := osbuild.NewStatusScanner(buf)
+	st, err := scanner.Status()
+	assert.NoError(t, err)
+	require.NotNil(t, st)
+	assert.Equal(t, 128_000, len(st.Trace))
+}