The Boston Diaries

The ongoing saga of a programmer who doesn't live in Boston, nor does he even like Boston, but yet named his weblog/journal “The Boston Diaries.”

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Tuesday, May 03, 2022

The legality of double slashes in URIs

Martin Chang replied to my musings on processing malformed Gemini requests, saying that double slashes in URIs are illegal, and pointed out the ABNF grammar from the URI specification to back up his claim:

path          = path-absolute   ; begins with "/" but not "//"
path-absolute = "/" [ segment-nz *( "/" segment ) ]
segment-nz    = 1*pchar
pchar         = unreserved / pct-encoded / sub-delims / ":" / "@"

But he didn't quote the segment rule:

segment       = *pchar

which translated says, “0 or more pchar rules.”

So the ABNF he quoted does indeed rule out //­boston/­2018/­07/­04.2. It doesn't rule out /­boston//­2018/­07/­04.2, since by the time we hit the double slash, we're in the *( "/" segment ) part of the path-absolute rule, and segment can have 0 characters. But what he quoted only applies to relative links, what I receive is an abolute link. If you follow the ABNF from that perspective:

URI-reference = URI / relative-ref
URI           = scheme ":" hier-part [ "?" query ] [ "#" fragment ]
hier-part     = "//" authority path-abempty
                 / path-absolute
                 / path-rootless
                 / path-empty

path-abempty  = *( "/" segment )

; other rules omitted

not only does this allow gemini://­­/­boston/­2018/­07/­04.2 but gemini://­­/­/­/­/­/­/­/­/­/­/­boston/­2018/­07/­04.2.

I can understand why this was done—to simplify the grammar as the various path- rules generally end with *( "/" segment ) allows one to end a URI with a trailing slash or not. I don't think the intent was to allow long strings of slashes, but that's the end result of a lax grammar. Martin is also correct that multiple slashes are treated as a single slash on POSIX (basically, any Unix system), that's not the case across all operating systems. One exception I can think of AmigaOS, where each slash represents a parent directory. This command, cd /// on AmigaOS is the same as cd ‥/‥/‥ on a POSIX system. Crazy, I know. And maybe not even relevant these days, but I thought I should mention it.

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