Fix hang in `named-compilezone | head`

I was truncating zone files for experimental purposes when I found
that `named-compilezone | head` got stuck. The full command line that
exhibited the problem was:

	dig axfr dotat.at |
	named-compilezone -o /dev/stdout dotat.at /dev/stdin |
	head

This requires a large enough zone to exhibit the problem, more than
about 70000 bytes of plain text output from named-compilezone.
I was running the command on Debian Stretch amd64.

This was puzzling since it looked like something was suppressing the
SIGPIPE. I used `strace` to examine what was happening at the hang.
The program was just calling write() a lot to print the zone file, and
the last write() hanged until I sent it a SIGINT.

During some discussion with friends, Ian Jackson guessed that opening
/dev/stdout O_RDRW might be the problem, and after some tests we found
that this does in fact suppress SIGPIPE.

Since `named-compilezone` only needs to write to its output file, the
fix is to omit the stdio "+" update flag.

diff --git a/bin/check/check-tool.c b/bin/check/check-tool.c
index e9ab4eb63303b57b77c98a61b6fd9477119f60b1..9585054456d71f961722ed455de1f3fb7dc1ff80 100644 (file)
--- a/bin/check/check-tool.c
+++ b/bin/check/check-tool.c
@@ -716,7 +716,7 @@ dump_zone(const char *zonename, dns_zone_t *zone, const char *filename,
        FILE *output = stdout;
        const char *flags;
 
-       flags = (fileformat == dns_masterformat_text) ? "w+" : "wb+";
+       flags = (fileformat == dns_masterformat_text) ? "w" : "wb";
 
        if (debug) {
                if (filename != NULL && strcmp(filename, "-") != 0)