io: don't leave errno as a random value when we hit EOF.

It's used inside io_finish; setting to 0 allows that to know we hit
EOF on a read.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

ccan/io/io.c

@@ -202,8 +202,12 @@ struct io_plan *io_write_(struct io_conn *conn, const void *data, size_t len,
 static int do_read(int fd, struct io_plan_arg *arg)
 {
 	ssize_t ret = read(fd, arg->u1.cp, arg->u2.s);
-	if (ret <= 0)
+	if (ret <= 0) {
+		/* Errno isn't set if we hit EOF, so set it to distinct value */
+		if (ret == 0)
+			errno = 0;
 		return -1;
+	}
 
 	arg->u1.cp += ret;
 	arg->u2.s -= ret;
@@ -230,8 +234,12 @@ struct io_plan *io_read_(struct io_conn *conn,
 static int do_read_partial(int fd, struct io_plan_arg *arg)
 {
 	ssize_t ret = read(fd, arg->u1.cp, *(size_t *)arg->u2.vp);
-	if (ret <= 0)
+	if (ret <= 0) {
+		/* Errno isn't set if we hit EOF, so set it to distinct value */
+		if (ret == 0)
+			errno = 0;
 		return -1;
+	}
 
 	*(size_t *)arg->u2.vp = ret;
 	return 1;

ccan/io/io.h

@@ -223,7 +223,8 @@ struct io_plan *io_write_(struct io_conn *conn,
  *
  * This creates a plan to read data into a buffer.  Once it's all
  * read, the @next function will be called: on an error, the finish
- * function is called instead.
+ * function is called instead.  If read() returns 0 (EOF) errno is set
+ * to 0.
  *
  * Note that the I/O may actually be done immediately.
  *
@@ -256,7 +257,8 @@ struct io_plan *io_read_(struct io_conn *conn,
  *
  * This creates a plan to read data into a buffer.  Once any data is
  * read, @len is updated and the @next function will be called: on an
- * error, the finish function is called instead.
+ * error, the finish function is called instead.  If read() returns 0 (EOF)
+ * errno is set to 0.
  *
  * Note that the I/O may actually be done immediately.
  *

ccan/io/test/run-45-eof.c

+#include <ccan/io/io.h>
+/* Include the C files directly. */
+#include <ccan/io/poll.c>
+#include <ccan/io/io.c>
+#include <ccan/tap/tap.h>
+#include <sys/wait.h>
+#include <stdio.h>
+
+static size_t len;
+
+static void finished_read(struct io_conn *conn, int *expect)
+{
+	ok1(errno == *expect);
+}
+
+static struct io_plan *init_conn_read(struct io_conn *conn, int *expect)
+{
+	io_set_finish(conn, finished_read, expect);
+	return io_read(conn, &expect, sizeof(expect), io_never, expect);
+}
+
+static struct io_plan *init_conn_read_partial(struct io_conn *conn, int *expect)
+{
+	io_set_finish(conn, finished_read, expect);
+	return io_read_partial(conn, &expect, sizeof(expect), &len,
+			       io_never, expect);
+}
+
+int main(void)
+{
+	int fd, expect_errno = 0;
+
+	/* This is how many tests you plan to run */
+	plan_tests(2);
+	fd = open("/dev/null", O_RDONLY);
+	io_new_conn(NULL, fd, init_conn_read, &expect_errno);
+
+	fd = open("/dev/null", O_RDONLY);
+	io_new_conn(NULL, fd, init_conn_read_partial, &expect_errno);
+
+	io_loop(NULL, NULL);
+
+	/* This exits depending on whether all tests passed */
+	return exit_status();
+}