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mcp: fix a race condition in logging #761

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Merged
maciej-kisiel merged 1 commit into from
Jan 25, 2026
Merged

mcp: fix a race condition in logging #761

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9 changes: 6 additions & 3 deletions mcp/logging.go
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Original file line number Diff line number Diff line change
Expand Up @@ -10,6 +10,7 @@ import (
"context"
"encoding/json"
"log/slog"
"slices"
"sync"
"time"
)
Expand Down Expand Up @@ -165,8 +166,7 @@ func (h *LoggingHandler) Handle(ctx context.Context, r slog.Record) error {

func (h *LoggingHandler) handle(ctx context.Context, r slog.Record) error {
// Observe the rate limit.
// TODO(jba): use golang.org/x/time/rate. (We can't here because it would require adding
// golang.org/x/time to the go.mod file.)
// TODO(jba): use golang.org/x/time/rate.
h.mu.Lock()
skip := time.Since(h.lastMessageSent) < h.opts.MinInterval
h.mu.Unlock()
Expand All @@ -175,6 +175,7 @@ func (h *LoggingHandler) handle(ctx context.Context, r slog.Record) error {
}

var err error
var data json.RawMessage
// Make the buffer reset atomic with the record write.
// We are careful here in the unlikely event that the handler panics.
// We don't want to hold the lock for the entire function, because Notify is
Expand All @@ -185,6 +186,8 @@ func (h *LoggingHandler) handle(ctx context.Context, r slog.Record) error {
defer h.mu.Unlock()
h.buf.Reset()
err = h.handler.Handle(ctx, r)
// Clone the buffer as Bytes() references the internal buffer.
data = json.RawMessage(slices.Clone(h.buf.Bytes()))
}()
if err != nil {
return err
Expand All @@ -197,7 +200,7 @@ func (h *LoggingHandler) handle(ctx context.Context, r slog.Record) error {
params := &LoggingMessageParams{
Logger: h.opts.LoggerName,
Level: slogLevelToMCP(r.Level),
Data: json.RawMessage(h.buf.Bytes()),
Data: data,
}
// We pass the argument context to Notify, even though slog.Handler.Handle's
// documentation says not to.
Expand Down