Fix zero-padding bug with (format "%#08x" n)

Problem reported by Gustaf Waldemarson (Bug#29609).
* src/editfns.c (styled_format):
Put zero padding after a leading "0x", not before.
* test/src/editfns-tests.el (format-sharp-0-x): New test.

2 files changed, 22 insertions, 8 deletions

diff --git a/src/editfns.c b/src/editfns.c
index e671ba0761c..ebf6518994b 100644
--- a/src/editfns.c
+++ b/src/editfns.c
@@ -4722,11 +4722,19 @@ styled_format (ptrdiff_t nargs, Lisp_Object *args, bool message)
                   char src0 = src[0];
                   int exponent_bytes = 0;
                   bool signedp = src0 == '-' || src0 == '+' || src0 == ' ';
-                  unsigned char after_sign = src[signedp];
-                  if (zero_flag && 0 <= char_hexdigit (after_sign))
+                  int prefix_bytes = (signedp
+                                      + ((src[signedp] == '0'
+                                          && (src[signedp + 1] == 'x'
+                                              || src[signedp + 1] == 'X'))
+                                         ? 2 : 0));
+                  if (zero_flag)
                     {
-                      leading_zeros += padding;
-                      padding = 0;
+                      unsigned char after_prefix = src[prefix_bytes];
+                      if (0 <= char_hexdigit (after_prefix))
+                        {
+                          leading_zeros += padding;
+                          padding = 0;
+                        }
                     }
 
                   if (excess_precision
@@ -4745,13 +4753,13 @@ styled_format (ptrdiff_t nargs, Lisp_Object *args, bool message)
                       nchars += padding;
                     }
 
-                  *p = src0;
-                  src += signedp;
-                  p += signedp;
+                  memcpy (p, src, prefix_bytes);
+                  p += prefix_bytes;
+                  src += prefix_bytes;
                   memset (p, '0', leading_zeros);
                   p += leading_zeros;
                   int significand_bytes
-                    = sprintf_bytes - signedp - exponent_bytes;
+                    = sprintf_bytes - prefix_bytes - exponent_bytes;
                   memcpy (p, src, significand_bytes);
                   p += significand_bytes;
                   src += significand_bytes;
diff --git a/test/src/editfns-tests.el b/test/src/editfns-tests.el
index 70dc9372fad..283a642dd97 100644
--- a/test/src/editfns-tests.el
+++ b/test/src/editfns-tests.el
@@ -136,6 +136,12 @@
 (ert-deftest format-c-float ()
   (should-error (format "%c" 0.5)))
 
+;;; Test for Bug#29609.
+(ert-deftest format-sharp-0-x ()
+  (should (string-equal (format "%#08x" #x10) "0x000010"))
+  (should (string-equal (format "%#05X" #x10) "0X010"))
+  (should (string-equal (format "%#04x" 0) "0000")))
+
 ;;; Check format-time-string with various TZ settings.
 ;;; Use only POSIX-compatible TZ values, since the tests should work
 ;;; even if tzdb is not in use.