From a8f5328bb7c784b044cc9649643d56d97ad2334c Mon Sep 17 00:00:00 2001
From: Nuno Diegues <nmld@ist.utl.pt>
Date: Wed, 20 Jan 2021 16:03:52 +0000
Subject: [PATCH] Make MultiLevelWriter resilient to individual log failure
 (#282)

Fixes #281

Currently MultiLevelWriter would give up if any individual backing logger
failed. This could result in no logging ever happening if the bad logger
was set up in the first position.

As a motivating factor, this can happen in "normal" circumstances, such
as running as a Windows Service where stderr is not available and therefore
the ConsoleWriter will fail. If you happen to set it as the first logger,
then no logging ever takes place. To make matters worse, connecting a
debugger creates an stderr, so it made it a pretty daunting situation
to overcome.

The proposed solution is to go through all underlying loggers and return
the last error obtained, if any. In practice there isn't much being done
with those errors anyway, as the best way to address logging errors is
to hook a ErrorHandler, which will still be triggered despite this change.

Co-authored-by: Nuno Diegues <nuno@cloudflare.com>
---
 writer.go      | 32 +++++++++----------
 writer_test.go | 84 ++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 100 insertions(+), 16 deletions(-)

diff --git a/writer.go b/writer.go
index 381b0f6..98c932e 100644
--- a/writer.go
+++ b/writer.go
@@ -56,30 +56,30 @@ type multiLevelWriter struct {
 
 func (t multiLevelWriter) Write(p []byte) (n int, err error) {
 	for _, w := range t.writers {
-		n, err = w.Write(p)
-		if err != nil {
-			return
-		}
-		if n != len(p) {
-			err = io.ErrShortWrite
-			return
+		if _n, _err := w.Write(p); err == nil {
+			n = _n
+			if _err != nil {
+				err = _err
+			} else if _n != len(p) {
+				err = io.ErrShortWrite
+			}
 		}
 	}
-	return len(p), nil
+	return n, err
 }
 
 func (t multiLevelWriter) WriteLevel(l Level, p []byte) (n int, err error) {
 	for _, w := range t.writers {
-		n, err = w.WriteLevel(l, p)
-		if err != nil {
-			return
-		}
-		if n != len(p) {
-			err = io.ErrShortWrite
-			return
+		if _n, _err := w.WriteLevel(l, p); err == nil {
+			n = _n
+			if _err != nil {
+				err = _err
+			} else if _n != len(p) {
+				err = io.ErrShortWrite
+			}
 		}
 	}
-	return len(p), nil
+	return n, err
 }
 
 // MultiLevelWriter creates a writer that duplicates its writes to all the
diff --git a/writer_test.go b/writer_test.go
index 7d06e4b..6d55ad1 100644
--- a/writer_test.go
+++ b/writer_test.go
@@ -4,6 +4,8 @@
 package zerolog
 
 import (
+	"errors"
+	"io"
 	"reflect"
 	"testing"
 )
@@ -27,3 +29,85 @@ func TestMultiSyslogWriter(t *testing.T) {
 		t.Errorf("Invalid syslog message routing: want %v, got %v", want, got)
 	}
 }
+
+var writeCalls int
+
+type mockedWriter struct {
+	wantErr bool
+}
+
+func (c mockedWriter) Write(p []byte) (int, error) {
+	writeCalls++
+
+	if c.wantErr {
+		return -1, errors.New("Expected error")
+	}
+
+	return len(p), nil
+}
+
+// Tests that a new writer is only used if it actually works.
+func TestResilientMultiWriter(t *testing.T) {
+	tests := []struct {
+		name    string
+		writers []io.Writer
+	}{
+		{
+			name:    "All valid writers",
+			writers: []io.Writer{
+				mockedWriter {
+					wantErr: false,
+				},
+				mockedWriter {
+					wantErr: false,
+				},
+			},
+		},
+		{
+			name:    "All invalid writers",
+			writers: []io.Writer{
+				mockedWriter {
+					wantErr: true,
+				},
+				mockedWriter {
+					wantErr: true,
+				},
+			},
+		},
+		{
+			name:    "First invalid writer",
+			writers: []io.Writer{
+				mockedWriter {
+					wantErr: true,
+				},
+				mockedWriter {
+					wantErr: false,
+				},
+			},
+		},
+		{
+			name:    "First valid writer",
+			writers: []io.Writer{
+				mockedWriter {
+					wantErr: false,
+				},
+				mockedWriter {
+					wantErr: true,
+				},
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		writers := tt.writers
+		multiWriter := MultiLevelWriter(writers...)
+
+		logger := New(multiWriter).With().Timestamp().Logger().Level(InfoLevel)
+		logger.Info().Msg("Test msg")
+
+		if len(writers) != writeCalls {
+			t.Errorf("Expected %d writers to have been called but only %d were.", len(writers), writeCalls)
+		}
+		writeCalls = 0
+	}
+}
\ No newline at end of file