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Use relocatable shebang with space-in-path robustness #395

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merged 3 commits into from
Dec 3, 2024
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Use relocatable shebang with space-in-path robustness #395

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Use relocatable shebang with space-in-path robustness
charliermarsh Nov 24, 2024
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charliermarsh Nov 25, 2024
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charliermarsh Dec 2, 2024
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3 changes: 2 additions & 1 deletion cpython-unix/build-cpython.sh
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Original file line number Diff line number Diff line change
Expand Up @@ -858,7 +858,8 @@ def fix_shebang(full):
lines.extend([
b"#!/bin/sh\n",
b'"exec" "\$(dirname \$0)/python${PYTHON_MAJMIN_VERSION}${PYTHON_BINARY_SUFFIX}" "\$0" "\$@"\n',
b"'''exec' \"$(dirname -- \"$(realpath -- \"$0\")\")\"/'python${PYTHON_MAJMIN_VERSION}${PYTHON_BINARY_SUFFIX}' \"$0\" \"$@\"\n",
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@paveldikov paveldikov Nov 24, 2024 •
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Are you sure ${PYTHON_MAJMIN_VERSION}${PYTHON_BINARY_SUFFIX} will be interpolated within the single quotes?

(I don't have good knowledge of the control flow in here, so genuinely unsure. If the interpolation is indeed broken, you might want to use implicit concatenation e.g. 'foo'"$BAR". I.e. just flip the pythonXY bit to double-quoted.)

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Oh good point. Mentally I assumed this was an f-string because in the written-out shebang, it seems to use the expanded versions (but clearly it's not). I'll review, modify, and test prior to merging.

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Ok, so the variables did expand, but the rest of the expressions were also evaluated...

#!/bin/sh
'''exec' "."/'python3.9d' "/var/folders/p1/44pzfl0j2m301zzfb0fv0p380000gn/T/tmppqyxz114/build-cpython.sh" ""
' '''

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(Still not doing quite the right thing, will revisit soon.)

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Success!

#!/bin/sh
'''exec' "$(dirname -- "$(realpath -- "$0")")/python3.11d" "$0" "$@"
' '''

import pydoc
if __name__ == '__main__':
    pydoc.cli()

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Wouldn't it make more sense to have this complicated launcher only if the Python path contains a space?

Because the Python startup time is already slow enough, I don't think having two extra processes (per startup) (dirname and realpath) would help much. (Also it would break if one intends to use this in a minimal container / environment, where the coreutils or equivalent aren't present.)

Now I don't exactly know if this launcher should be relocatable, but if so, I think there can be a way to detect in pure POSIX sh if the $0 contains a space, and if so do this, else just leaving it alone.

Also, according to the POSIX specification https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_02 one could use something like the following (not tested, but could be worked out):

#!/bin/sh
''':' ## this is an alias for `true`
_path="${0}"
_path="${_path%/*}" ## remove the smallest suffix like `/*`
exec -- "${_path}/python3.11d" "${0}" "${@}"
' '''

This, should work, and should remove the need for any other additional processes, except the /bin/sh.

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I defer to @paveldikov on the shebang itself.

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Note however that we can't know if the path will contain a space, since the whole motivation here is that the path is relocatable. The user could put it anywhere!

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[...] we can't know if the path will contain a space [...]

One could use the following simple check (that I believe is POSIX compliant):

## replace all spaces and check if the value differs from the original
_path="${0}"
if test "${_path}" == "${_path// }" ; then
    ## the path does not contain a space
else
    ## the path contains a space
fi

However, the other snippet I've given should work, (and after some extra thought about it) perhaps an extra check should be done to see if the path did contain a / or not, as in:

#!/bin/sh
''':'
_path_0="${0}"
_path="${_path_0%/*}"
if test "${_path_0}" == "${_path}" ; then
    ## there was no `/` fould in `_path_0`
    _path='.'
fi
exec -- "${_path}/python3.11d" "${0}" "${@}"
' '''

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@zanieb zanieb Dec 3, 2024 •
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Happy to track this as a performance improvement

#397

b"' '''\n",
])
lines.extend(fh)
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