core: Fix latent @go.printf infinite loop bug

Noticed the bug when trying to run:

  time COLUMNS=27 ./go help builtins

The script would loop infinitely when trying to print the line for the
`path` builtin. When the `@go.printf` loop would reach the point where
the `prefix` variable contained:

  'script, [alias] or'

the expression:

  line="${line#$prefix}"

would cause `[alias]` to be interpreted as a set of characters to match
per standard Bash pattern matching, rather than as a literal string in
its own right. Consequently, the prefix would not get removed from
`line`, so `line` would remain constant and the loop condition
`"${#line}" -gt "$COLUMNS"` would never fail.

The fix was to use index notation and the length of `prefix` to trim
`line` instead:

  line="${line:${#prefix}}"

The included test case reproduces the bug and verifies its fix.

Author: mbland

go-core.bash

@@ -166,7 +166,9 @@ declare _GO_INJECT_MODULE_PATH="$_GO_INJECT_MODULE_PATH"
     while [[ "${#line}" -gt "$COLUMNS" ]]; do
       prefix="${line:0:$COLUMNS}"
       prefix="${prefix%[[:space:]]*}"
-      line="${line#$prefix}"
+
+      # Trim `line` using an index in case `prefix` contains pattern characters.
+      line="${line:${#prefix}}"
 
       if [[ "$prefix" =~ [[:space:]]+$ ]]; then
         prefix="${prefix%${BASH_REMATCH[0]}}"

tests/core/printf.bats

@@ -57,3 +57,18 @@ teardown() {
   run "$TEST_GO_SCRIPT"
   assert_success 'foobarbaz'
 }
+
+@test "$SUITE: don't loop infinitely if line contains pattern characters" {
+  # `[alias]` and `[builtin]` were previously interpreted as character sets, not
+  # literal strings in and of themselves, when appering in `$prefix` in the
+  # expression `${line#$prefix}`.
+  local test_string='  path       '
+  test_string+='Prints the path to the <command> script, [alias] or [builtin]'
+
+  @go.create_test_go_script "@go.printf '%s' '$test_string'"
+  COLUMNS=27 run "$TEST_GO_SCRIPT"
+  assert_success '  path       Prints the' \
+    'path to the <command>' \
+    'script, [alias] or' \
+    '[builtin]'
+}