#5487 A #rrggbb color with a bad last digit, such as "#00000g", is accepted

[David Scherfgen]

This bug was discovered by Claude Opus 5.5 and reproduced on Maxima Git commit c62924c (2026-09-26) with SBCL 2.2.9.

Where: src/plotcolors.lisp, PLOTCOLORP, RGB-COLOR

PLOTCOLORP and RGB-COLOR parse (subseq color 1 6), which is only five characters. So the sixth hex digit is never checked, and an invalid color is passed to gnuplot.

Steps

1. Verify that the existing behavior is incorrect: run the reproducer below; it gives the result under "Before the fix". Judge for yourself that this is wrong instead of relying on the ticket: where the bug allows it, try some related inputs of your own, or check the result another way, for example numerically or against the documentation.
2. Edit the file src/plotcolors.lisp as described under "Fix".
3. Rebuild Maxima.
4. Verify that the behavior is now correct: the reproducer gives the result under "Correct result". Judge for yourself that this is right, not only that it matches: where the bug allows it, try some related inputs of your own, including some that the fix should leave unchanged.
5. Add the test case(s) under "Test cases" to tests/rtest_plotoptions.mac, at the end of the file.
6. Run the complete test suite, share tests included (make check, or run_testsuite(share_tests=true); in ./maxima-local), and check that the change breaks no test. Failures that also occur without the change (a run on the unchanged build shows which) are pre-existing and can be ignored.
7. Add the line * \#5487 A #rrggbb color with a bad last digit, such as "#00000g", is accepted to ChangeLog, under "Bug fixes for numbered bugs:".
8. Commit the fix; a proposed Git commit message is under "Commit message".
9. Push the commit. Then set this ticket's status to "closed" and add a comment naming the commit, for example Fixed by commit [1a2b3c]., with the first six characters of the commit's SHA in the brackets (git rev-parse HEAD | cut -c1-6 prints them). SourceForge links this to the commit.

Reproducer

set_plot_option([color,"#00000g"]);

Before the fix:

accepted: the options now contain [color,"#00000g"]

Correct result:

Wrong argument for plot option "color". Expecting a color but found "#00000g".

Fix

In src/plotcolors.lisp (PLOTCOLORP, line "(ignore-errors (parse-integer (subseq color 1 6) :radix 16)))"), replace

                (= (length color) 7)
                (ignore-errors (parse-integer (subseq color 1 6) :radix 16)))

with

                (= (length color) 7)
                (ignore-errors (parse-integer (subseq color 1 7) :radix 16)))

In src/plotcolors.lisp (RGB-COLOR, line "(ignore-errors (parse-integer (subseq color 1 6) :radix 16)))"), replace

             (= (length color) 7)
             (ignore-errors (parse-integer (subseq color 1 6) :radix 16)))

with

             (= (length color) 7)
             (ignore-errors (parse-integer (subseq color 1 7) :radix 16)))

Test cases

Add these lines to tests/rtest_plotoptions.mac, separated from the tests before them by two empty lines:

/* Bug #5487: "A #rrggbb color with a bad last digit, such as "#00000g", is accepted" */

block ([r],
  reset_plot_options (),
  r: [errcatch (set_plot_option ([color, "#00000g"])),
      (set_plot_option ([color, "#00000f"]), get_plot_option (color))],
  set_plot_option ([color, blue, red, green, magenta, black, cyan]),
  reset_plot_options (),
  r);
[[], [color, "#00000f"]]$

Commit message

plotcolors.lisp: check all six digits of #rrggbb colors

PLOTCOLORP and RGB-COLOR parsed (SUBSEQ COLOR 1 6), which leaves out the
last digit, so a color such as "#00000g" was accepted and passed to the
graphic program. Parse all six digits.

This fixes bug #5487.

Found and fixed by Claude Opus 5.5.