Add buffered writer

The buffered writer adopts a signature identical to `write_cb`,
so that it can be plugged into anywhere `write_cb` appears.

test/unit/buf_writer.c

@@ -0,0 +1,64 @@
+#include "test/jemalloc_test.h"
+
+#define TEST_BUF_SIZE 16
+#define UNIT_MAX (TEST_BUF_SIZE * 3)
+
+static size_t test_write_len;
+static char test_buf[TEST_BUF_SIZE];
+static uint64_t arg_store;
+
+static void test_write_cb(void *cbopaque, const char *s) {
+	size_t prev_test_write_len = test_write_len;
+	test_write_len += strlen(s); /* only increase the length */
+	arg_store = *(uint64_t *)cbopaque; /* only pass along the argument */
+	assert_zu_le(prev_test_write_len, test_write_len,
+	    "Test write overflowed");
+}
+
+TEST_BEGIN(test_buf_write) {
+	char s[UNIT_MAX + 1];
+	size_t n_unit, remain, i;
+	ssize_t unit;
+	uint64_t arg = 4; /* Starting value of random argument. */
+	buf_writer_arg_t test_buf_arg =
+	    {test_write_cb, &arg, test_buf, TEST_BUF_SIZE - 1, 0};
+
+	memset(s, 'a', UNIT_MAX);
+	arg_store = arg;
+	for (unit = UNIT_MAX; unit >= 0; --unit) {
+		/* unit keeps decreasing, so strlen(s) is always unit. */
+		s[unit] = '\0';
+		for (n_unit = 1; n_unit <= 3; ++n_unit) {
+			test_write_len = 0;
+			remain = 0;
+			for (i = 1; i <= n_unit; ++i) {
+				arg = prng_lg_range_u64(&arg, 64);
+				buffered_write_cb(&test_buf_arg, s);
+				remain += unit;
+				if (remain > test_buf_arg.buf_size) {
+					/* Flushes should have happened. */
+					assert_u64_eq(arg_store, arg, "Call "
+					    "back argument didn't get through");
+					remain %= test_buf_arg.buf_size;
+					if (remain == 0) {
+						/* Last flush should be lazy. */
+						remain += test_buf_arg.buf_size;
+					}
+				}
+				assert_zu_eq(test_write_len + remain, i * unit,
+				    "Incorrect length after writing %zu strings"
+				    " of length %zu", i, unit);
+			}
+			buf_writer_flush(&test_buf_arg);
+			assert_zu_eq(test_write_len, n_unit * unit,
+			    "Incorrect length after flushing at the end of"
+			    " writing %zu strings of length %zu", n_unit, unit);
+		}
+	}
+}
+TEST_END
+
+int
+main(void) {
+	return test(test_buf_write);
+}