[tig PATCH] continue updates when pipe read has errno "Success"

[tig PATCH] continue updates when pipe read has errno "Success"

[Jeff King]

When we are reading from a pipe and receive a signal, our
read call fails and ferror() returns true. The current
behavior is to call end_update and report failure. However,
we can detect this situation by checking that errno is set
to success and continue the reading process.

You can provoke this behavior by running a "tig blame" that
takes a few seconds and then resizing the terminal that tig
is running in (you should get an incomplete blame output and
the error "Failed to read: Success").

Signed-off-by: Jeff King <peff@peff.net>
---
I am not convinced this is the right solution. Specifically:

  - there are a few other calls to ferror. Maybe they should be
    converted, too, which implies that perhaps there is a better idiom
    for checking this.

  - I have no idea how portable this is. Do all stdio implementations
    fail to restart on signal? Do they all set ferror and have errno ==
    0 (I would have expected EINTR, or at the very least a 0-read
    without ferror set)?

But it works for me (Linux, glibc 2.7).

 tig.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/tig.c b/tig.c
index 6b111e4..8362ecf 100644
--- a/tig.c
+++ b/tig.c
@@ -2392,7 +2392,7 @@ update_view(struct view *view)
 	update_view_title(view);
 
 check_pipe:
-	if (ferror(view->pipe)) {
+	if (ferror(view->pipe) && errno != 0) {
 		report("Failed to read: %s", strerror(errno));
 		end_update(view, TRUE);
 
-- 
1.6.0.98.g0d3cc

Re: [tig PATCH] continue updates when pipe read has errno "Success"

[Jonas Fonseca]

On Thu, Aug 21, 2008 at 03:40, Jeff King <peff@peff.net> wrote:
> When we are reading from a pipe and receive a signal, our
> read call fails and ferror() returns true. The current
> behavior is to call end_update and report failure. However,
> we can detect this situation by checking that errno is set
> to success and continue the reading process.

Thanks for the fix/workaround!

> I am not convinced this is the right solution. Specifically:
>
>  - there are a few other calls to ferror. Maybe they should be
>    converted, too, which implies that perhaps there is a better idiom
>    for checking this.

Well, perhaps something like this can work around the issue for tig-0.12:

bool check_ferror(FILE *file) { return ferror(file) && errno != 0; }

For a possible "better" fix, I have been working on moving tig to use
the run-command.[ch] code from git, which means that ferror() will no
longer be needed. It is still not ready but looks promising.

>  - I have no idea how portable this is. Do all stdio implementations
>    fail to restart on signal? Do they all set ferror and have errno ==
>    0 (I would have expected EINTR, or at the very least a 0-read
>    without ferror set)?
>
> But it works for me (Linux, glibc 2.7).

I have tested it on FreeBSD which doesn't seem to have this problem
with ferror(). Anyway, the workaround doesn't break anything so
applied.

-- 
Jonas Fonseca

Re: [tig PATCH] continue updates when pipe read has errno "Success"

[Jeff King]

On Fri, Aug 22, 2008 at 12:10:35PM +0200, Jonas Fonseca wrote:

> Thanks for the fix/workaround!

Thank you for making tig. ;)

> For a possible "better" fix, I have been working on moving tig to use
> the run-command.[ch] code from git, which means that ferror() will no
> longer be needed. It is still not ready but looks promising.

I think that would be much cleaner, as the semantics of system calls and
signals tend to be better defined.

-Peff