streams: close-stream only caches non-nil result.

This is motivated by trying to implement a struct delegate
stream which performs reference counting in close, in
order to close the real stream when the count hits zero.

The caching behavior of close-stream is a problem.

* stream.c (strm_base_init): Initialize close_result to nil,
rather than nao.
(strm_base_mark): Don't check close_result for nao.
(close_stream): Suppress the call to op->close if close_result
has a non-nil value, rather than a value other than nao.

* tests/018/close-delegate.tl,
* tests/018/close-delegate.expected: New files.

* txr.1: Document that only a non-nil return is cached
by close-stream.

2 files changed, 23 insertions, 0 deletions

diff --git a/tests/018/close-delegate.expected b/tests/018/close-delegate.expected
new file mode 100644
index 00000000..227f9f67
--- /dev/null
+++ b/tests/018/close-delegate.expected
@@ -0,0 +1,3 @@
+close called, count 2
+close called, count 1
+20
diff --git a/tests/018/close-delegate.tl b/tests/018/close-delegate.tl
new file mode 100644
index 00000000..4cf1d650
--- /dev/null
+++ b/tests/018/close-delegate.tl
@@ -0,0 +1,20 @@
+(load "../common")
+
+(defstruct refcount-close stream-wrap
+  stream
+  (count 1)
+
+  (:method close (me throw-on-error-p)
+    (put-line `close called, count @{me.count}`)
+    (when (plusp me.count)
+      (if (zerop (dec me.count))
+        (close-stream me.stream throw-on-error-p)))))
+
+(flow
+  (with-stream (s (make-struct-delegate-stream
+                    (new refcount-close
+                         count 2
+                         stream (open-file *load-path*))))
+    (get-lines s))
+  len
+  prinl)