Author izbyshev
Recipients gregory.p.smith, izbyshev, kevans, koobs, nanjekyejoannah, pablogsal, serhiy.storchaka, vstinner
Date 2019-01-17.00:38:47
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> One of the issue that I have with using posix_spawn() is that the *exact* behavior of subprocess is not properly defined by test_subprocess. Should we more more tests, or document that the exact behavior is "an implementation detail"?

Regarding using PATH from "env" instead of parent's environment, it may be considered documented because subprocess docs refer to os.execvp(), where it's explicitly documented:

"""
The variants which include a ā€œpā€ near the end (execlp(), execlpe(), execvp(), and execvpe()) will use the PATH environment variable to locate the program file. When the environment is being replaced (using one of the exec*e variants, discussed in the next paragraph), the new environment is used as the source of the PATH variable.
"""

The problem is that it differs from execvp() in libc (and POSIX), so I would consider such behavior a bug if it wasn't so old to become a feature. Thanks to Victor for noticing that, I missed it.

So, if we can't change os.execvp() and/or current subprocess behavior, posix_spawnp seems to be ruled out. (I don't consider temporarily changing the parent environment as a solution). A naive emulation of posix_spawnp would be repeatedly calling posix_spawn for each PATH entry, but that's prohibitively expensive. Could we use a following hybrid approach instead?

Iterate over PATH entries and perform a quick check for common exec errors (ENOENT, ENOTDIR, EACCES) manually (e.g. by stat()). If the check fails, exec would also fail so just skip the entry. (It's important not to get false negatives here, but the child process would have the same privileges as the parent since we don't use POSIX_SPAWN_RESETIDS, so I can't think of one). Otherwise, call posix_spawn with the absolute path. If it fails, skip the entry.

Looks ugly, but are there other problems? This would seem to work just as well as posix_spawnp in the common case, but in the worst (contrived) case it would be equivalent to calling posix_spawn for each PATH entry.
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Date User Action Args
2019-01-17 00:38:49izbyshevsetrecipients: + izbyshev, gregory.p.smith, vstinner, serhiy.storchaka, koobs, pablogsal, nanjekyejoannah, kevans
2019-01-17 00:38:47izbyshevsetmessageid: <1547685527.73.0.658746608699.issue35537@roundup.psfhosted.org>
2019-01-17 00:38:47izbyshevlinkissue35537 messages
2019-01-17 00:38:47izbyshevcreate