Downgrade ErrorCtx to Warn on canceled context

A client disconnecting or aborting a request cancels the request
context; code further down the call chain often surfaces that as an
error (failed query, failed decode) and logs it via ErrorCtx. That's
not a server-side failure, so log it at Warn instead. Deadline
exceeded is left at Error since it more often points at a real
problem (slow query, downstream latency).

Signed-off-by: Bryan Frimin <bryan@probo.com>

Author: gearnode

log/log.go

@@ -18,6 +18,7 @@ package log
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"io"
 	"log/slog"
@@ -256,8 +257,15 @@ func (l *Logger) Named(name string, options ...Option) *Logger {
 }
 
 // Log logs a message at the specified level with optional attributes,
-// adding trace and span IDs if the context has a span.
+// adding trace and span IDs if the context has a span. An Error
+// logged against a context canceled by its caller (e.g. a client
+// disconnect) is downgraded to Warn, since it reflects the caller
+// going away rather than a failure on our end.
 func (l *Logger) Log(ctx context.Context, level Level, msg string, args ...Attr) {
+	if level == LevelError && errors.Is(context.Cause(ctx), context.Canceled) {
+		level = LevelWarn
+	}
+
 	if l.match != nil && l.match(level, msg, args) {
 		return
 	}

log/log_test.go

@@ -299,6 +299,44 @@ func TestCtxConvenienceMethods(t *testing.T) {
 	}
 }
 
+func TestErrorCtxDowngradesOnCanceledContext(t *testing.T) {
+	var buf bytes.Buffer
+	l := NewLogger(WithOutput(&buf))
+
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+
+	l.ErrorCtx(ctx, "client went away")
+
+	m := parseJSONLog(t, &buf)
+	assert.Equal(t, "WARN", m["level"])
+	assert.Equal(t, "client went away", m["msg"])
+}
+
+func TestErrorCtxKeepsErrorOnDeadlineExceeded(t *testing.T) {
+	var buf bytes.Buffer
+	l := NewLogger(WithOutput(&buf))
+
+	ctx, cancel := context.WithTimeout(context.Background(), 0)
+	defer cancel()
+	<-ctx.Done()
+
+	l.ErrorCtx(ctx, "slow downstream call")
+
+	m := parseJSONLog(t, &buf)
+	assert.Equal(t, "ERROR", m["level"])
+}
+
+func TestErrorCtxKeepsErrorOnLiveContext(t *testing.T) {
+	var buf bytes.Buffer
+	l := NewLogger(WithOutput(&buf))
+
+	l.ErrorCtx(context.Background(), "real failure")
+
+	m := parseJSONLog(t, &buf)
+	assert.Equal(t, "ERROR", m["level"])
+}
+
 func TestConvenienceMethodsWithAttrs(t *testing.T) {
 	var buf bytes.Buffer
 	l := NewLogger(WithOutput(&buf))