Allow customizing companion commands

cmd/companion/log.go

@@ -0,0 +1,65 @@
+package main
+
+import (
+	"io"
+	"log"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// setupFileLogging tees the standard logger to a file alongside the config.
+// Returns the log file path. Stderr is kept as a secondary sink so dev runs
+// (linux, console) still print, while windowsgui builds rely on the file.
+func setupFileLogging(cfgDir string) (string, error) {
+	if err := os.MkdirAll(cfgDir, 0755); err != nil {
+		return "", err
+	}
+	p := filepath.Join(cfgDir, "companion.log")
+	f, err := os.OpenFile(p, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644)
+	if err != nil {
+		return "", err
+	}
+	log.SetOutput(io.MultiWriter(f, os.Stderr))
+	return p, nil
+}
+
+// tailLog returns the last n lines of the log file. Reads only the trailing
+// chunk of the file so the cost is bounded regardless of log size.
+func tailLog(p string, n int) ([]string, error) {
+	const tailBytes = 32 * 1024
+	f, err := os.Open(p)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+	info, err := f.Stat()
+	if err != nil {
+		return nil, err
+	}
+	size := info.Size()
+	var off int64
+	if size > tailBytes {
+		off = size - tailBytes
+	}
+	buf := make([]byte, size-off)
+	if _, err := f.ReadAt(buf, off); err != nil && err != io.EOF {
+		return nil, err
+	}
+	s := string(buf)
+	// Drop the (likely partial) first line when we didn't start at 0.
+	if off > 0 {
+		if i := strings.IndexByte(s, '\n'); i >= 0 {
+			s = s[i+1:]
+		}
+	}
+	s = strings.TrimRight(s, "\n")
+	if s == "" {
+		return nil, nil
+	}
+	lines := strings.Split(s, "\n")
+	if len(lines) > n {
+		lines = lines[len(lines)-n:]
+	}
+	return lines, nil
+}