logtail: require Buffer.Write to not retain the provided slice (#11617)

Buffer.Write has the exact same signature of io.Writer.Write.
The latter requires that implementations to never retain
the provided input buffer, which is an expectation that most
users will have when they see a Write signature.

The current behavior of Buffer.Write where it does retain
the input buffer is a risky precedent to set.
Switch the behavior to match io.Writer.Write.

There are only two implementations of Buffer in existence:
* logtail.memBuffer
* filch.Filch

The former can be fixed by cloning the input to Write.
This will cause an extra allocation in every Write,
but we can fix that will pooling on the caller side
in a follow-up PR.

The latter only passes the input to os.File.Write,
which does respect the io.Writer.Write requirements.

Updates #cleanup
Updates tailscale/corp#18514

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

logtail/buffer.go

@@ -4,6 +4,7 @@
 package logtail
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
 	"sync"
@@ -19,8 +20,7 @@ type Buffer interface {
 	TryReadLine() ([]byte, error)
 
 	// Write writes a log line into the ring buffer.
-	//
+	// Implementations must not retain the provided buffer.
-	// Write takes ownership of the provided slice.
 	Write([]byte) (int, error)
 }
 
@@ -62,7 +62,7 @@ func (m *memBuffer) Write(b []byte) (int, error) {
 	defer m.dropMu.Unlock()
 
 	ent := qentry{
-		msg:       b,
+		msg:       bytes.Clone(b),
 		dropCount: m.dropCount,
 	}
 	select {