Client + Log: Implement a log rotater

1 files changed, 109 insertions, 0 deletions

diff --git a/internal/log/rotate.go b/internal/log/rotate.go
new file mode 100644
index 0000000..fb93c3c
--- /dev/null
+++ b/internal/log/rotate.go
@@ -0,0 +1,109 @@
+package log
+
+import (
+	"io"
+	"os"
+	"sync"
+
+	"codeberg.org/eduVPN/eduvpn-common/internal/atomicfile"
+)
+
+var (
+	// MaxSize is the maximum size in bytes from when to start trimming
+	MaxSize int64 = 1024 * 1024
+
+	// TrimSize denotes how much to trim from the beginning
+	TrimSize = MaxSize / 2
+
+	// TrimMsg is the message to display when it was trimmed
+	TrimMsg = "--- previous part was trimmed by eduvpn-common as the file was too big ---\n"
+)
+
+// FileRotater is a file that is trimmed when a maximum size is reached
+// This is for logging useful
+type FileRotater struct {
+	filename string
+	file     *os.File
+	mu       sync.Mutex
+}
+
+// NewFileRotater creates a new log file rotater
+func NewFileRotater(filename string) (*FileRotater, error) {
+	fr := &FileRotater{
+		filename: filename,
+	}
+
+	err := fr.open()
+	if err != nil {
+		return nil, err
+	}
+	return fr, nil
+}
+
+func (fr *FileRotater) open() error {
+	f, err := os.OpenFile(fr.filename, os.O_RDWR|os.O_CREATE|os.O_APPEND, 0o666)
+	if err != nil {
+		return err
+	}
+	fr.file = f
+	return nil
+}
+
+func (fr *FileRotater) trim() error {
+	fr.mu.Lock()
+	defer fr.mu.Unlock()
+	// We need to seek to the trim size to skip over that part as we discard it
+	_, err := fr.file.Seek(TrimSize, io.SeekStart)
+	if err != nil {
+		return err
+	}
+
+	// get the part of the file that we want to keep
+	keep, err := io.ReadAll(fr.file)
+	if err != nil {
+		return err
+	}
+
+	all := []byte(TrimMsg)
+	all = append(all, keep...)
+	err = atomicfile.WriteFile(fr.file.Name(), all, 0o666)
+	if err != nil {
+		return err
+	}
+
+	// re-open the handle as the file was renamed
+	err = fr.file.Close()
+	if err != nil {
+		return err
+	}
+	err = fr.open()
+	if err != nil {
+		return err
+	}
+	return nil
+}
+
+// Write implements io.Writer for the log rotater
+func (fr *FileRotater) Write(p []byte) (n int, err error) {
+	fi, err := fr.file.Stat()
+	if err != nil {
+		return 0, err
+	}
+
+	if fi.Size() >= MaxSize {
+		err = fr.trim()
+		if err != nil {
+			return 0, err
+		}
+	}
+	// we don't write atomically here as we want it to be as fast as possible
+	// and if we lose a part of one log statement it's not a big deal
+	return fr.file.Write(p)
+}
+
+// Close closes the file in a safe way by locking and unlocking the mutex
+func (fr *FileRotater) Close() error {
+	fr.mu.Lock()
+	defer fr.mu.Unlock()
+	return fr.file.Close()
+}