dma(8): Fix race condition in multi-recipient delivery

[Simon 'corecode' Schubert]

Matthew Dillon wrote:
    dup() doesn't solve the problem.  A dup()'d descriptor still
    points to a common file pointer and thus a common seek position.
    Only open() will generate a new file pointer backing the file
    descriptor.
That's not true.

cheers
  simon
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#include <stdio.h>

int
main(void)
{
	FILE *f = fopen("duptmp", "w");
	FILE *f1, *f2;
	char line[100];

	if (f == NULL)
		return (1);

	fputs("1\n2\n3\n", f);
	fclose(f);

	f1 = fopen("duptmp", "r");
	if (f1 == NULL)
		return (1);
	f2 = fdopen(dup(fileno(f1)), "r");
	if (f2 == NULL)
		return (2);
	fgets(line, sizeof(line), f1);
	printf("read1: %s", line);
	fgets(line, sizeof(line), f2);
	printf("read2: %s", line);
	return (0);
}