[RaphaelFakhri]

TsnetSetLogFD wraps the caller's fd in os.NewFile. An *os.File closes its fd when it is garbage collected, so after the logger is replaced (a second tailscale_set_logfd call, for example with -1) or the server is closed, the next GC cycle closes an fd that belongs to the C caller. With tailscale_set_logfd(sd, 2) this closes the process's stderr; if the fd number has been reused in the meantime, it closes an unrelated file or socket.

Change

• TsnetSetLogFD writes to the fd with write(2) through a small fdWriter type, which never closes it. Log output is unchanged.
• TestSetLogFD (in tsnetctest) passes one end of a pipe to tailscale_set_logfd, resets the logger with -1, closes the server, runs GC, and checks that the fd is still open.

Testing

go test -run TestSetLogFD -count=3 .   # without the fix: 3 of 3 fail, "log fd 7 was closed by libtailscale: bad file descriptor"
go test -run TestSetLogFD -count=3 .   # with the fix: 3 of 3 pass
go test -count=1 -v .                  # TestConn, TestGetRemoteAddr, TestSetLogFD, TestExtractIP pass
gofmt -l . && go vet . ./tsnetctest/   # clean