return to psub --file being the default

The recent change to switch `psub` to use `argparse` caused it to use
a fifo by default because it inadvertently fixed a long standing bug in
the fish script. This changes the behavior back to `psub --file` being
the default behavior and introduces a `--fifo` flag. It also updates the
documentation to make it clearer when and why `--fifo` mode should not
be used.

Fixes #4222

share/functions/psub.fish

@@ -1,5 +1,5 @@
 function psub --description "Read from stdin into a file and output the filename. Remove the file when the command that called psub exits."
-    set -l options 'h/help' 'f/file' 's/suffix='
+    set -l options -x 'f,F' -x 'F,s' 'h/help' 'f/file' 'F/fifo' 's/suffix=' 'T-testing'
     argparse -n psub --max-args=0 $options -- $argv
     or return
 
@@ -21,10 +21,10 @@ function psub --description "Read from stdin into a file and output the filename
     set -q TMPDIR
     and set tmpdir $TMPDIR
 
-    if not set -q _flag_file
+    if set -q _flag_fifo
         # Write output to pipe. This needs to be done in the background so
         # that the command substitution exits without needing to wait for
-        # all the commands to exit
+        # all the commands to exit.
         set dirname (mktemp -d $tmpdir/.psub.XXXXXXXXXX)
         or return
         set filename $dirname/psub.fifo"$_flag_suffix"
@@ -35,13 +35,18 @@ function psub --description "Read from stdin into a file and output the filename
         cat >$filename
     else
         set dirname (mktemp -d $tmpdir/.psub.XXXXXXXXXX)
-        set filename $dirname/psub"$_flag_suffix"
+        set filename "$dirname/psub$_flag_suffix"
         cat >$filename
     end
 
     # Write filename to stdout
     echo $filename
 
+    # This flag isn't documented. It's strictly for our unit tests.
+    if set -q _flag_testing
+        return
+    end
+
     # Find unique function name
     while true
         set funcname __fish_psub_(random)
@@ -53,7 +58,7 @@ function psub --description "Read from stdin into a file and output the filename
     # Make sure we erase file when caller exits
     function $funcname --on-job-exit caller --inherit-variable filename --inherit-variable dirname --inherit-variable funcname
         command rm $filename
-        if count $dirname >/dev/null
+        if test -n "$dirname"
             command rmdir $dirname
         end
         functions -e $funcname