read: discard IFS delimiters before the last token

Do this only when splitting on IFS characters which usually contains
whitespace characters --- read --delimiter is unchanged; it still
consumes no more than one delimiter per variable. This seems better,
because it allows arbitrary delimiters in the last field.

Fixes #6406

src/builtin_read.cpp

@@ -608,6 +608,14 @@ int builtin_read(parser_t &parser, io_streams_t &streams, wchar_t **argv) {
                     wcstring substr;
                     loc = wcstring_tok(buff, (vars_left() > 1) ? opts.delimiter : wcstring(), loc);
                     if (loc.first != wcstring::npos) {
+                        if (vars_left() == 1) {  // Discard trailing delimiters, see #6406
+                            loc.first =
+                                std::find_if(buff.begin() + loc.first, buff.end(),
+                                             [&opts](wchar_t c) {
+                                                 return opts.delimiter.find(c) == wcstring::npos;
+                                             }) -
+                                buff.begin();
+                        }
                         substr = wcstring(buff, loc.first, loc.second);
                     }
                     vars.set_one(*var_ptr++, opts.place, substr);

tests/checks/read.fish

@@ -12,3 +12,11 @@ set -l
 # CHECK: a 'afoo barb'
 # CHECK: b
 # CHECK: c
+
+echo "a  b b" | read a b; string escape $a $b
+# CHECK: a
+# CHECK: 'b b'
+
+echo 'a<><>b<>b' | read -d '<>' a b; printf %s\n $a $b
+# CHECK: a
+# CHECK: <>b<>b

tests/read.out

@@ -27,7 +27,7 @@ two
 1 ''
 1 '' 1 ''
 1 'test' 1 '' 1 ''
-1 'foo' 1 'bar' 1 '  baz'
+1 'foo' 1 'bar' 1 'baz'
 0 a
 
 ####################
@@ -129,6 +129,6 @@ c
 Multi-char delimiters with IFS
 a
 b
-..c
+c
 
 banana sausage