fix: concurrent mutations to prefixWriter

[GrahamDennis]

When using the prefixed output format, I occasionally get a fatal error: concurrent map writes error due to concurrent modifications to the seen map used by Prefixed.

This change adds a mutex protecting the Prefixed struct.

[pd93]

@GrahamDennis Thanks for opening this. Could you test the following change for me?

diff --git a/internal/output/prefixed.go b/internal/output/prefixed.go
index 68d94156..fd2a2305 100644
--- a/internal/output/prefixed.go
+++ b/internal/output/prefixed.go
@@ -15,22 +15,21 @@ type Prefixed struct {
        logger  *logger.Logger
        seen    map[string]uint
        counter *uint
-       mutex   *sync.Mutex
+       mutex   sync.Mutex
 }
 
-func NewPrefixed(logger *logger.Logger) Prefixed {
+func NewPrefixed(logger *logger.Logger) *Prefixed {
        var counter uint
 
-       return Prefixed{
+       return &Prefixed{
                seen:    make(map[string]uint),
                counter: &counter,
                logger:  logger,
-               mutex:   &sync.Mutex{},
        }
 }
 
-func (p Prefixed) WrapWriter(stdOut, _ io.Writer, prefix string, _ *templater.Cache) (io.Writer, io.Writer, CloseFunc) {
-       pw := &prefixWriter{writer: stdOut, prefix: prefix, prefixed: &p}
+func (p *Prefixed) WrapWriter(stdOut, _ io.Writer, prefix string, _ *templater.Cache) (io.Writer, io.Writer, CloseFunc) {
+       pw := &prefixWriter{writer: stdOut, prefix: prefix, prefixed: p}
        return pw, pw, func(error) error { return pw.close() }
 }

No reason to use a pointer to a mutex here in my opinion.

[pd93]

Added the suggested changes in 8ce9bdc. Thanks @GrahamDennis!