Added support for stderr param for expectation handlers

This is unfortunately one of the cleanest ways to implement this, since
stdout is piped to stdin of the handler; I did not want to rely on using a
file descriptor, because then you have to deal with issues of rewinding the
pointer for possible future uses, and other considerations.

src/expect-core

@@ -34,6 +34,23 @@ _bail_clause()
 }
 
 
+##
+# Asserts that at least the given argument shift count was provided
+#
+# The shift count is used to determine where shspec's arguments end and
+# where the remainder clause begins; this ensures that shspec can continue
+# to evolve in the future without BC breaks in properly designed expection
+# handlers.
+__chk-nshift()
+{
+  local -ri expect="$1"
+  local -ri given="$2"
+
+  test "$given" -ge "$expect" || _bail \
+    "internal: expected shift of at least $expect, but given $given"
+}
+
+
 _expect--succeed() { test "$1" -eq 0; }
 _expect--fail()    { test "$1" -ne 0; }
 

src/spec

@@ -168,11 +168,11 @@ to()
     || _bail "unknown expectation: \`$type'" $__spec_caller
 
   # first argument is exit code, second is the number of arguments to shift
-  # to place $1 at the remainder clause, and all remaining arguments are
-  # said remainder clause; the shift argument allows the implementation to
-  # vary without breaking BC so long as the meaning of the shifted arguments
-  # do not change
-  $assert "$__spec_rexit" 2 "$@" <<< "$__spec_result" \
+  # to place $1 at the remainder clause, third is the path to the stderr
+  # output file, and all remaining arguments are said remainder clause; the
+  # shift argument allows the implementation to vary without breaking BC so
+  # long as the meaning of the shifted arguments do not change
+  $assert "$__spec_rexit" 3 "$__spec_errpath" "$@" <<< "$__spec_result" \
     || fail "$expect_full"
 
   __spec_caller=