test-tool genzeros: generate large amounts of data more efficiently

In this developer's tests, producing one gigabyte worth of NULs in a
busy loop that writes out individual bytes, unbuffered, took ~27sec.
Writing chunked 256kB buffers instead only took ~0.6sec

This matters because we are about to introduce a pair of test cases that
want to be able to produce 5GB of NULs, and we cannot use `/dev/zero`
because of the HP NonStop platform's lack of support for that device.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/t/helper/test-genzeros.c b/t/helper/test-genzeros.c
index b1197e91a89d31255f8690c576983c5fc639d761..8ca988d6216e7895d8afea772e110b7eeaa875d1 100644 (file)
--- a/t/helper/test-genzeros.c
+++ b/t/helper/test-genzeros.c
@@ -3,7 +3,10 @@
 
 int cmd__genzeros(int argc, const char **argv)
 {
+       /* static, so that it is NUL-initialized */
+       static const char zeros[256 * 1024];
        intmax_t count;
+       ssize_t n;
 
        if (argc > 2) {
                fprintf(stderr, "usage: %s [<count>]\n", argv[0]);
@@ -12,9 +15,19 @@ int cmd__genzeros(int argc, const char **argv)
 
        count = argc > 1 ? strtoimax(argv[1], NULL, 0) : -1;
 
-       while (count < 0 || count--) {
-               if (putchar(0) == EOF)
+       /* Writing out individual NUL bytes is slow... */
+       while (count < 0)
+               if (write(1, zeros, ARRAY_SIZE(zeros)) < 0)
                        return -1;
+
+       while (count > 0) {
+               n = write(1, zeros, count < ARRAY_SIZE(zeros) ?
+                         count : ARRAY_SIZE(zeros));
+
+               if (n < 0)
+                       return -1;
+
+               count -= n;
        }
 
        return 0;