tests/dd: Do not use the OS provided dd utility on FIFOs #5453
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On *BSD and macOS, the system provided dd utility opens up the output file for both reading and writing. This means that the open/write to the FIFO does not block, and almost instantly completes. The system dd then exits, leaving nothing left to be read by the time the coreutils-rs dd tries to open/read the FIFO. Avoid this problem by just writing to the FIFO from the test case itself, rather than relying on the system provide dd.
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Reference code on FreeBSD's dd.c where it opens the output file for both reading and writing here. I could not test this fix on macOS (I tested on FreeBSD), but strongly suspect this will also work on macOS since Apple's dd implementation is doing the same thing (seen here). One potential issue with my proposed solution is that if the uutils/coreutils dd does not open the pipe at all (due to some catastrophic failure), the test case will hang (since the new write is on the main thread). I saw that this test was implemented using a separate thread to perform the write in the background. If you think that approach would be better here (or had some other approach in mind) please let me know and I can refactor. |
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Makes sense to me! It even cleans up the code!
Re: the hanging issue. I don't see much of a problem there at the moment. I think we'll notice the issue soon enough once the tests start hanging.
Just as word of caution: please be careful reading other implementations. In particular, we cannot read the GNU implementation because of license reasons. The BSD implementations are ok, I think, because they have a more compatible license.
On *BSD and macOS, the system provided dd utility opens up the output file for both reading and writing. This means that the open/write to the FIFO does not block, and almost instantly completes. The system dd then exits, leaving nothing left to be read by the time the coreutils-rs dd tries to open/read the FIFO.
Avoid this problem by just writing to the FIFO from the test case itself, rather than relying on the system provide dd.