Am 22.12.2016 um 22:37 schrieb Johannes Schindelin:
Hi Hannes,
On Thu, 22 Dec 2016, Johannes Sixt wrote:
Am 22.12.2016 um 18:09 schrieb Johannes Schindelin:
+static HANDLE swap_osfhnd(int fd, HANDLE new_handle)
+{
+ /*
+ * Create a copy of the original handle associated with fd
+ * because the original will get closed when we dup2().
+ */
+ HANDLE handle = (HANDLE)_get_osfhandle(fd);
+ HANDLE duplicate = duplicate_handle(handle);
+ /* Create a temp fd associated with the already open "new_handle". */
+ int new_fd = _open_osfhandle((intptr_t)new_handle, O_BINARY);
+ assert((fd == 1) || (fd == 2));
+ /*
+ * Use stock dup2() to re-bind fd to the new handle. Note that
+ * this will implicitly close(1) and close both fd=1 and the
+ * originally associated handle. It will open a new fd=1 and
+ * call DuplicateHandle() on the handle associated with new_fd.
+ * It is because of this implicit close() that we created the
+ * copy of the original.
+ *
+ * Note that the OS can recycle HANDLE (numbers) just like it
+ * recycles fd (numbers), so we must update the cached value
+ * of "console". You can use GetFileType() to see that
+ * handle and _get_osfhandle(fd) may have the same number
+ * value, but they refer to different actual files now.
Certainly, the OS does not recycle handle values that are in use (open). Then
I do not quite get the point of this paragraph. See...
+ *
+ * Note that dup2() when given target := {0,1,2} will also
+ * call SetStdHandle(), so we don't need to worry about that.
+ */
+ dup2(new_fd, fd);
+ if (console == handle)
+ console = duplicate;
... This is where "the cached value of console is updated", right? If console
== handle, then this is not because a handle value was recycled, but because
fd *was* console. Since the old value of console has been closed by the
dup2(), we must refer to the back-up value in the future. Am I missing
something?
You are correct, we must update `console` because `handle` is no longer
the handle we want.
The comment above only meant to reinforce that we have to forget about the
previous handle, too, as we might access something completely different
than a console otherwise.
It is like accessing a pointer after free().
When I read the paragraph for the first time, I expected some
information to be saved, then some handles to be closed and re-opened,
which would possibly recycle a handle value, and then same state to be
resurrected depending on the information saved earlier.
But nothing of this kind happens, only:
dup2(new_fd, fd);
if (console == handle)
console = duplicate;
which is necessary and, once one has understood the context, obvious.
Would you have a suggestion how to rephrase the comment to make it less
confusing?
Perhaps
* This might close the cached console handle.
* We must cache the live duplicate instead.
Then update the cache before the dup2, because at this time all 3 of
console, handle, and duplicate are live and cannot be recycled:
if (console == handle)
console = duplicate;
dup2(new_fd, fd);
-- Hannes
