[PATCH] ZSH_EXEPATH: improve documentation, add test

i haven't confirmed myself but by all accounts this should work on
solaris via /proc/self/path/a.out and under cygwin via /proc/self/exe,
so i've documented that

also added a test. i only tried it on macos so lmk if it's bad for
anyone

dana


diff --git a/Doc/Zsh/params.yo b/Doc/Zsh/params.yo
index ecedc3862..8fdc7353f 100644
--- a/Doc/Zsh/params.yo
+++ b/Doc/Zsh/params.yo
@@ -1175,6 +1175,9 @@ the argument passed to the option.  Otherwise it is not set.
 vindex(ZSH_EXEPATH)
 item(tt(ZSH_EXEPATH))(
 Full pathname of the executable file of the current zsh process.
+This value should be accurate on Linux, macOS, FreeBSD, DragonflyBSD,
+NetBSD, Solaris, Cygwin, and their derivatives.  On other systems, it is
+determined from tt(argv[0]), which is not always accurate.
 )
 vindex(ZSH_NAME)
 item(tt(ZSH_NAME))(
diff --git a/NEWS b/NEWS
index 952071dbe..50f358f49 100644
--- a/NEWS
+++ b/NEWS
@@ -20,8 +20,8 @@ The parameter expansion flag ! was added for the referred-to name of a
 named reference.
 
 The parameter ZSH_EXEPATH expands to the full path of the current zsh
-process, reliably on Linux, Mac, {Net,Free,Dragonfly}BSD and falls
-back to argv[0] elsewhere.
+process, reliably on Linux, Mac, {Net,Free,Dragonfly}BSD, Solaris, and
+Cygwin, and falls back to argv[0] elsewhere.
 
 The default keymap is now emacs. Users of the vi keymaps please check
 README to see if you are affected.
diff --git a/Test/D04parameter.ztst b/Test/D04parameter.ztst
index ed25fd7a9..0399faa8e 100644
--- a/Test/D04parameter.ztst
+++ b/Test/D04parameter.ztst
@@ -2864,3 +2864,24 @@ F:behavior, see http://austingroupbugs.net/view.php?id=888
 0:regression for workers/53179 unicode ZDOTDIR
 F:output ignorable as long as not an error
 *>*
+
+  argv0=/not/a/real/path
+  exepath=$(
+    ARGV0=$argv0 $ZTST_testdir/../Src/zsh -fc 'print -r - $ZSH_EXEPATH'
+  )
+  if
+    [[ $OSTYPE == (darwin|dragonfly|freebsd|netbsd)* ]] || # via library call
+    [[ $OSTYPE == (cygwin|linux|solaris)* && -e /proc/self ]] # via procfs
+  then
+    [[ $exepath == (/*)#/Src/zsh ]] && print -rn 'match: '
+    print -r - $exepath
+  # on any other system without procfs, we should have argv[0] verbatim since
+  # it's already absolute
+  elif [[ ! -e /proc/$$ ]]; then
+    [[ $exepath == $argv0 ]] && print -rn 'match: '
+    print -r - $exepath
+  else
+    ZTST_skip='ZSH_EXEPATH not reliable'
+  fi
+-:ZSH_EXEPATH
+*>match: /*