Regression: panic when calling UpdateContext on a context not yet initialized

[hairyhenderson]

I'm seeing a new panic with v0.19.0 that I didn't experience with v0.18.0:

panic: runtime error: index out of range [-1]

goroutine 1 [running]:
github.com/rs/zerolog/internal/json.Encoder.AppendKey(...)
        /Users/hairyhenderson/go/pkg/mod/github.com/rs/zerolog@v1.19.0/internal/json/base.go:7
github.com/rs/zerolog.Context.Str(0x1144480, 0xc000098590, 0xff, 0x0, 0x0, 0xc0000cc400, 0x0, 0x1f4, 0x0, 0x0, ...)
        /Users/hairyhenderson/go/pkg/mod/github.com/rs/zerolog@v1.19.0/context.go:74 +0x202
main.main.func3(0x1144480, 0xc000098590, 0xff, 0x0, 0x0, 0xc0000cc400, 0x0, 0x1f4, 0x0, 0x0, ...)
        /Users/hairyhenderson/zlbug/main.go:21 +0xb8
github.com/rs/zerolog.(*Logger).UpdateContext(0xc0000d9e48, 0x1126520)
        /Users/hairyhenderson/go/pkg/mod/github.com/rs/zerolog@v1.19.0/log.go:255 +0xef
main.main()
        /Users/hairyhenderson/zlbug/main.go:20 +0x1b0
exit status 2

This code can reproduce it:

package main

import (
	"os"

	"github.com/rs/zerolog"
)

func main() {
	log := zerolog.New(zerolog.ConsoleWriter{
		Out:             os.Stdout,
		NoColor:         true,
		FormatTimestamp: func(i interface{}) string { return "" },
		FormatLevel:     func(i interface{}) string { return "" },
	})

	log.Info().Msg("Hello world")
	log.UpdateContext(func(c zerolog.Context) zerolog.Context {
		return c.Str("foo", "bar")
	})
	log.Info().Msg("kaboom")
}

If I add a line like log = log.With().Logger() before calling log.UpdateContext, it's OK.

This was introduced by #226, specifically here. The dst byte array is no longer being length-checked before attempting to access index len(dst)-1 (-1 when the array is empty - i.e. when it hasn't been initailized yet).

Note that I only ran into this in a unit test, and is easily worked around by calling log.With(), but it's reasonable to assume this will affect other code as well.

This workaround only works because enc.AppendBeginMarker is now called by With, and it tolerates a zero-length array and produces an array of length 1.

/cc @ffenix113 @rs

[hairyhenderson]

Thanks @ffenix113 and @rs!