Fix reading of /etc/paths on OS X

(and /etc/paths.d/*)

Do so by emulating the behavior of /usr/libexec/path_helper for login
shells, matching the behavior in /etc/profile.

Also add a path_helper command to reproduce the behavior of
/usr/libexec/path_helper for fish.

This also handles setting MANPATH if necessary.

Fixes issue fish-shell#4336

share/config.fish

@@ -126,59 +126,7 @@ if test -d /usr/xpg4/bin
     end
 end
 
-# OS X-ism: Load the path files out of /etc/paths and /etc/paths.d/*
-set -g __fish_tmp_path $PATH
-function __fish_load_path_helper_paths
-    # We want to rearrange the path to reflect this order. Delete that path component if it exists and then prepend it.
-    # Since we are prepending but want to preserve the order of the input file, we reverse the array, append, and then reverse it again
-    set __fish_tmp_path $__fish_tmp_path[-1..1]
-    while read -l new_path_comp
-        if test -d $new_path_comp
-            set -l where (contains -i -- $new_path_comp $__fish_tmp_path)
-            and set -e __fish_tmp_path[$where]
-            set __fish_tmp_path $new_path_comp $__fish_tmp_path
-        end
-    end
-    set __fish_tmp_path $__fish_tmp_path[-1..1]
-end
-test -r /etc/paths
-and __fish_load_path_helper_paths </etc/paths
-for pathfile in /etc/paths.d/*
-    __fish_load_path_helper_paths <$pathfile
-end
-set -xg PATH $__fish_tmp_path
-set -e __fish_tmp_path
-functions -e __fish_load_path_helper_paths
-
-# OS X-ism: Load the manpath files out of /etc/manpaths and /etc/manpaths.d/*
-if set -q MANPATH
-    set -g __fish_tmp_manpath $MANPATH
-    function __fish_load_manpath_helper_manpaths
-        # We want to rearrange the manpath to reflect this order. Delete that manpath component if it exists and then prepend it.
-        # Since we are prepending but want to preserve the order of the input file, we reverse the array, append, and then reverse it again
-        set __fish_tmp_manpath $__fish_tmp_manpath[-1..1]
-        while read -l new_manpath_comp
-            if test -d $new_manpath_comp
-                set -l where (contains -i -- $new_manpath_comp $__fish_tmp_manpath)
-                and set -e __fish_tmp_manpath[$where]
-                set __fish_tmp_manpath $new_manpath_comp $__fish_tmp_manpath
-            end
-        end
-        set __fish_tmp_manpath $__fish_tmp_manpath[-1..1]
-    end
-    test -r /etc/manpaths
-    and __fish_load_manpath_helper_manpaths </etc/manpaths
-    for manpathfile in /etc/manpaths.d/*
-        __fish_load_manpath_helper_manpaths <$manpathfile
-    end
-    set -xg MANPATH $__fish_tmp_manpath
-    set -e __fish_tmp_manpath
-    functions -e __fish_load_manpath_helper_manpaths
-end
-
-
 # Add a handler for when fish_user_path changes, so we can apply the same changes to PATH
-# Invoke it immediately to apply the current value of fish_user_path
 function __fish_reconstruct_path -d "Update PATH when fish_user_paths changes" --on-variable fish_user_paths
     set -l local_path $PATH
 
@@ -202,8 +150,6 @@ function __fish_reconstruct_path -d "Update PATH when fish_user_paths changes" -
     set -xg PATH $local_path
 end
 
-__fish_reconstruct_path
-
 #
 # Launch debugger on SIGTRAP
 #
@@ -259,7 +205,40 @@ end
 # This used to be in etc/config.fish - keep it here to keep the semantics
 #
 
+# Adapt construct_path from the macOS /usr/libexec/path_helper
+# executable for fish; see
+# https://opensource.apple.com/source/shell_cmds/shell_cmds-203/path_helper/path_helper.c.auto.html .
+function __fish_macos_set_env -d "set an environment variable like path_helper does (macOS only)"
+    set -l result
+
+    for path_file in $argv[2] $argv[3]/*
+        if test -f $path_file
+            while read -la entry
+                if not contains $entry $result
+                    set result $result $entry
+                end
+            end <$path_file
+        end
+    end
+
+    for entry in $$argv[1]
+        if not contains $entry $result
+            set result $result $entry
+        end
+    end
+
+    set -xg $argv[1] $result
+end
+
 if status --is-login
+    # macOS-ism: Emulate calling path_helper.
+    if command -sq /usr/libexec/path_helper
+        __fish_macos_set_env 'PATH' '/etc/paths' '/etc/paths.d'
+        if [ -n "$MANPATH" ]
+            __fish_macos_set_env 'MANPATH' '/etc/manpaths' '/etc/manpaths.d'
+        end
+    end
+
     #
     # Put linux consoles in unicode mode.
     #
@@ -271,3 +250,7 @@ if status --is-login
         end
     end
 end
+
+# Invoke this here to apply the current value of fish_user_path after
+# PATH is possibly set above.
+__fish_reconstruct_path