Re: [PATCH v4 26/27] t/lib-unicode-nfc-nfd: helper prereqs for testing unicode nfc/nfd

> From: Jeff Hostetler <jeffhost@microsoft.com>
> 
> Create a set of prereqs to help understand how file names
> are handled by the filesystem when they contain NFC and NFD
> Unicode characters.
> 
> Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
> ---
>   t/lib-unicode-nfc-nfd.sh | 167 +++++++++++++++++++++++++++++++++++++++
>   1 file changed, 167 insertions(+)
>   create mode 100755 t/lib-unicode-nfc-nfd.sh
> 
> diff --git a/t/lib-unicode-nfc-nfd.sh b/t/lib-unicode-nfc-nfd.sh
> new file mode 100755
> index 00000000000..cf9c26d1e22
> --- /dev/null
> +++ b/t/lib-unicode-nfc-nfd.sh
> @@ -0,0 +1,167 @@
> +# Help detect how Unicode NFC and NFD are handled on the filesystem.
> +
> +# A simple character that has a NFD form.
> +#
> +# NFC:       U+00e9 LATIN SMALL LETTER E WITH ACUTE
> +# UTF8(NFC): \xc3 \xa9
> +#
> +# NFD:       U+0065 LATIN SMALL LETTER E
> +#            U+0301 COMBINING ACUTE ACCENT
> +# UTF8(NFD): \x65  +  \xcc \x81
> +#
> +utf8_nfc=$(printf "\xc3\xa9")
> +utf8_nfd=$(printf "\x65\xcc\x81")
> +

> +# Is the OS or the filesystem "Unicode composition sensitive"?
> +#
> +# That is, does the OS or the filesystem allow files to exist with
> +# both the NFC and NFD spellings?  Or, does the OS/FS lie to us and
> +# tell us that the NFC and NFD forms are equivalent.
> +#
> +# This is or may be independent of what type of filesystem we have,
> +# since it might be handled by the OS at a layer above the FS.
> +# Testing shows on MacOS using APFS, HFS+, and FAT32 reports a
> +# collision, for example.
> +#
> +# This does not tell us how the Unicode pathname will be spelled
> +# on disk, but rather only that the two spelling "collide".  We
> +# will examine the actual on disk spelling in a later prereq.
> +#
> +test_lazy_prereq UNICODE_COMPOSITION_SENSITIVE '
> +	mkdir trial_${utf8_nfc} &&
> +	mkdir trial_${utf8_nfd}
> +'
> +
> +# Is the spelling of an NFC pathname preserved on disk?
> +#
> +# On MacOS with HFS+ and FAT32, NFC paths are converted into NFD
> +# and on APFS, NFC paths are preserved.  As we have established
> +# above, this is independent of "composition sensitivity".
> +#
> +# 0000000 63 5f c3 a9
> +#
> +# (/usr/bin/od output contains different amount of whitespace
> +# on different platforms, so we need the wildcards here.)
> +#
> +test_lazy_prereq UNICODE_NFC_PRESERVED '
> +	mkdir c_${utf8_nfc} &&
> +	ls | od -t x1 | grep "63 *5f *c3 *a9"
> +'
> +
> +# Is the spelling of an NFD pathname preserved on disk?
> +#
> +# 0000000 64 5f 65 cc 81
> +#
> +test_lazy_prereq UNICODE_NFD_PRESERVED '
> +	mkdir d_${utf8_nfd} &&
> +	ls | od -t x1 | grep "64 *5f *65 *cc *81"
> +'
> +	mkdir c_${utf8_nfc} &&
> +	mkdir d_${utf8_nfd} &&
> +
> +# The following _DOUBLE_ forms are more for my curiosity,
> +# but there may be quirks lurking when there are multiple
> +# combining characters in non-canonical order.
> +
> +# Unicode also allows multiple combining characters
> +# that can be decomposed in pieces.
> +#
> +# NFC:        U+1f67 GREEK SMALL LETTER OMEGA WITH DASIA AND PERISPOMENI
> +# UTF8(NFC):  \xe1 \xbd \xa7
> +#
> +# NFD1:       U+1f61 GREEK SMALL LETTER OMEGA WITH DASIA
> +#             U+0342 COMBINING GREEK PERISPOMENI
> +# UTF8(NFD1): \xe1 \xbd \xa1  +  \xcd \x82
> +#
> +# But U+1f61 decomposes into
> +# NFD2:       U+03c9 GREEK SMALL LETTER OMEGA
> +#             U+0314 COMBINING REVERSED COMMA ABOVE
> +# UTF8(NFD2): \xcf \x89  +  \xcc \x94
> +#
> +# Yielding:   \xcf \x89  +  \xcc \x94  +  \xcd \x82
> +#
> +# Note that I've used the canonical ordering of the
> +# combinining characters.  It is also possible to
> +# swap them.  My testing shows that that non-standard
> +# ordering also causes a collision in mkdir.  However,
> +# the resulting names don't draw correctly on the
> +# terminal (implying that the on-disk format also has
> +# them out of order).
> +#
> +greek_nfc=$(printf "\xe1\xbd\xa7")
> +greek_nfd1=$(printf "\xe1\xbd\xa1\xcd\x82")
> +greek_nfd2=$(printf "\xcf\x89\xcc\x94\xcd\x82")
> +

> +# See if a double decomposition also collides.
> +#
> +test_lazy_prereq UNICODE_DOUBLE_COMPOSITION_SENSITIVE '
> +	mkdir trial_${greek_nfc} &&
> +	mkdir trial_${greek_nfd2}
> +'
> +
> +# See if the NFC spelling appears on the disk.
> +#
> +test_lazy_prereq UNICODE_DOUBLE_NFC_PRESERVED '
> +	mkdir c_${greek_nfc} &&
> +	ls | od -t x1 | grep "63 *5f *e1 *bd *a7"
> +'
> +
> +# See if the NFD spelling appears on the disk.
> +#
> +test_lazy_prereq UNICODE_DOUBLE_NFD_PRESERVED '
> +	mkdir d_${greek_nfd2} &&
> +	ls | od -t x1 | grep "64 *5f *cf *89 *cc *94 *cd *82"
> +'
> +
> +# The following is for debugging. I found it useful when
> +# trying to understand the various (OS, FS) quirks WRT
> +# Unicode and how composition/decomposition is handled.
> +# For example, when trying to understand how (macOS, APFS)
> +# and (macOS, HFS) and (macOS, FAT32) compare.
> +#
> +# It is rather noisy, so it is disabled by default.
> +#
> +if test "$unicode_debug" = "true"
> +then
> +	if test_have_prereq UNICODE_COMPOSITION_SENSITIVE
> +	then
> +		echo NFC and NFD are distinct on this OS/filesystem.
> +	else
> +		echo NFC and NFD are aliases on this OS/filesystem.
> +	fi
> +
> +	if test_have_prereq UNICODE_NFC_PRESERVED
> +	then
> +		echo NFC maintains original spelling.
> +	else
> +		echo NFC is modified.
> +	fi
> +
> +	if test_have_prereq UNICODE_NFD_PRESERVED
> +	then
> +		echo NFD maintains original spelling.
> +	else
> +		echo NFD is modified.
> +	fi
> +
> +	if test_have_prereq UNICODE_DOUBLE_COMPOSITION_SENSITIVE
> +	then
> +		echo DOUBLE NFC and NFD are distinct on this OS/filesystem.
> +	else
> +		echo DOUBLE NFC and NFD are aliases on this OS/filesystem.
> +	fi
> +
> +	if test_have_prereq UNICODE_DOUBLE_NFC_PRESERVED
> +	then
> +		echo Double NFC maintains original spelling.
> +	else
> +		echo Double NFC is modified.
> +	fi
> +
> +	if test_have_prereq UNICODE_DOUBLE_NFD_PRESERVED
> +	then
> +		echo Double NFD maintains original spelling.
> +	else
> +		echo Double NFD is modified.
> +	fi
> +fi