[RFC/PATCH] pipe_command(): mark stdin descriptor as non-blocking

Our pipe_command() helper lets you both write to and read from a child process on its stdin/stdout. It's supposed to work without deadlocks because we use poll() to check when descriptors are ready for reading or writing. But there's a bug: if both the data to be written and the data to be read back exceed the pipe buffer, we'll deadlock.

The issue is that the code assumes that if you have, say, a 2MB buffer to write and poll() tells you that the pipe descriptor is ready for writing, that calling:

  write(cmd->in, buf, 2*1024*1024);

will do a partial write, filling the pipe buffer and then returning what it did write. And that is what it would do on a socket, but not for a pipe. When writing to a pipe, at least on Linux, it will block waiting for the child process to read() more. And now we have a potential deadlock, because the child may be writing back to us, waiting for us to read() ourselves.

An easy way to trigger this is:

  git -c add.interactive.useBuiltin=true \
      -c interactive.diffFilter=cat \
      checkout -p HEAD~200

The diff against HEAD~200 will be big, and the filter wants to write all of it back to us (obviously this is a dummy filter, but in the real world something like diff-highlight would similarly stream back a big output).

If you set add.interactive.useBuiltin to false, the problem goes away, because now we're not using pipe_command() anymore (instead, that part happens in perl). But this isn't a bug in the interactive code at all. It's the underlying pipe_command() code which is broken, and has been all along.

We presumably didn't notice because most calls only do input _or_ output, not both. And the few that do both, like gpg calls, may have large inputs or outputs, but never both at the same time (e.g., consider signing, which has a large payload but a small signature comes back).

The obvious fix is to put the descriptor into non-blocking mode, and indeed, that makes the problem go away. Callers shouldn't need to care, because they never see the descriptor (they hand us a buffer to feed into it).

Signed-off-by: Jeff King <peff@peff.net>
---
+cc Dscho for two reasons:

  - this is a fallout of the builtin add-interactive, though I again
    emphasize that it's just triggering a lurking bug (which is mine, no
    less!). But I thought you'd want to know.

  - more importantly, I'm not sure of the portability implications of
    the fix. This is our first use of O_NONBLOCK outside of the
    compat/simple-ipc unix-socket code. Do we need to abstract this
    behind a compat/ layer for Windows?

 run-command.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)