pwd: Clean up and adopt POSIX semantics

[des]

Summary

According to POSIX, the default should be -L. Based on code history,
whoever first wrote BSD pwd(1) could not figure out how to implement
-L and therefore made -P the default (and only) option. Support for -L
was later added, but the default was never changed.

Clean up the code, make -L the default, and rewrite getcwd_logical() to
reject paths that contain dot or dot-dot, as required by POSIX.

MFC after: 1 week

[olce]

(Small point: Last if currently looks slightly weird as it is calling getcwd_physical() even on !physical, would use a logical variable instead of physical.)

[ziaee]

Why shouldn't we relnotes yes/mfc no/warn everyone for a subtle change in behavior?

bin/pwd/pwd.c

98

Note pwd -? used to say:

pwd: Illegal option -?

Now says:

pwd: illegal option -- ?
usage: pwd [-L | -P]

[des]

bin/pwd/pwd.c

98

Nothing has changed. The first example is from our sh's builtin pwd, the second is from /sbin/pwd.

[des]

In D55146#1260826, @olce wrote:

(Small point: Last if currently looks slightly weird as it is calling getcwd_physical() even on !physical, would use a logical variable instead of physical.)

I agree but the diff was starting to look like I was changing code just for the sake of change so I reverted that part...

[des]

rf

[des]

implement dot and dotdot checks

[olce]

bin/pwd/pwd.c

55–56

Detection of .. is wrong (any two-character path component ending with . passes the test).

[des]

bin/pwd/pwd.c

55–56

No.

[olce]

bin/pwd/pwd.c

55–56

Oh, right.

This form is obscure though. Additionally, it does twice the == '.' test if the first fails, and the second operand of the disjunction increments p for nothing if the second '.' matched.

So I strongly suggest changing that to a logical formula that does not use side-effects and is exactly to the point.

[olce]

Looks fine (with pointed-out test changed).