Add strtcat[_a](), and use it instead of its pattern

[alejandro-colomar]

Queued after #1148

Revisions:

---

v1b

• Rebase

$ git rd 
1:  34bbd688 = 1:  b70d59b7 lib/string/strcpy/: strtcat(), STRTCAT(): Add APIs
2:  79541620 = 2:  1b15cf5d lib/salt.c: gensalt(): Use STRTCPY() instead of its pattern

v2

• Reimplement in terms of stpecpy(). This is much simpler, and thus safer. It also allows us using size_t instead of ssize_t in the input, which is more common (it was weird to need ssize_t in the input, just to avoid compiler diangostics).

$ git range-diff shadow/master gh/strtcat strtcat 
1:  b70d59b7 ! 1:  3f2d3a8e lib/string/strcpy/: strtcat(), STRTCAT(): Add APIs
    @@ lib/string/strcpy/strtcat.c (new)
     +
     +#include "string/strcpy/strtcat.h"
     +
    -+#include <sys/types.h>
    ++#include <stddef.h>
     +
     +
     +extern inline ssize_t strtcat(char *restrict dst, const char *restrict src,
    -+    ssize_t dsize);
    ++    size_t dsize);
     
      ## lib/string/strcpy/strtcat.h (new) ##
     @@
    @@ lib/string/strcpy/strtcat.h (new)
     +
     +#include "config.h"
     +
    -+#include <string.h>
    -+#include <sys/types.h>
    ++#include <stddef.h>
     +
     +#include "attr.h"
     +#include "sizeof.h"
    -+#include "string/strcpy/strtcpy.h"
    ++#include "string/strchr/strnul.h"
    ++#include "string/strcpy/stpecpy.h"
     +
     +
     +#define STRTCAT(dst, src)  strtcat(dst, src, countof(dst))
    @@ lib/string/strcpy/strtcat.h (new)
     +
     +ATTR_STRING(2)
     +inline ssize_t strtcat(char *restrict dst, const char *restrict src,
    -+    ssize_t dsize);
    ++    size_t dsize);
     +
     +
     +// string truncate catenate
     +// Returns new length, or -1 on truncation.
     +inline ssize_t
    -+strtcat(char *restrict dst, const char *restrict src, ssize_t dsize)
    ++strtcat(char *restrict dst, const char *restrict src, size_t dsize)
     +{
    -+  ssize_t  oldlen, n;
    ++  char  *p, *end;
     +
    -+  oldlen = strlen(dst);
    ++  end = dst + dsize;
     +
    -+  n = strtcpy(dst + oldlen, src, dsize - oldlen);
    -+  if (n == -1)
    ++  p = stpecpy(strnul(dst), end, src);
    ++  if (p == NULL)
     +          return -1;
     +
    -+  return oldlen + n;
    ++  return p - dst;
     +}
     +
     +
2:  1b15cf5d = 2:  ae41236f lib/salt.c: gensalt(): Use STRTCPY() instead of its pattern

v2b

• Update lib/string/README.

$ git range-diff shadow/master gh/strtcat strtcat 
1:  3f2d3a8e ! 1:  8fd61b28 lib/string/strcpy/: strtcat(), STRTCAT(): Add APIs
    @@ lib/Makefile.am: libshadow_la_SOURCES = \
        string/strcpy/strtcpy.h \
        string/strdup/strndupa.c \
     
    + ## lib/string/README ##
    +@@ lib/string/README: strcpy/ - String copying
    +     STRTCPY()
    +   Like strtcpy(), but takes an array.
    + 
    +-    strtcat()  // Unimplemented
    ++    strtcat()
    +   Catenate a string after an existing string, with truncation.
    +   Rarely useful.  Consider using stpecpy() instead.
    +-    STRTCAT()  // Unimplemented
    ++    STRTCAT()
    +   Like strtcat(), but takes an array.
    + 
    +     stpecpy()
    +
      ## lib/string/strcpy/strtcat.c (new) ##
     @@
     +// SPDX-FileCopyrightText: 2025, Alejandro Colomar <[email protected]>
2:  ae41236f = 2:  72e56458 lib/salt.c: gensalt(): Use STRTCPY() instead of its pattern

v2c

• Rebase

$ git rd 
1:  8fd61b28 = 1:  f229c84c lib/string/strcpy/: strtcat(), STRTCAT(): Add APIs
2:  72e56458 = 2:  7700f0fc lib/salt.c: gensalt(): Use STRTCPY() instead of its pattern

v2d

• Rebase

$ git rd 
1:  f229c84c ! 1:  e25872f9 lib/string/strcpy/: strtcat(), STRTCAT(): Add APIs
    @@ lib/Makefile.am: libshadow_la_SOURCES = \
     +  string/strcpy/strtcat.h \
        string/strcpy/strtcpy.c \
        string/strcpy/strtcpy.h \
    -   string/strdup/strndupa.c \
    +   string/strdup/strdup.c \
     
      ## lib/string/README ##
     @@ lib/string/README: strcpy/ - String copying
2:  7700f0fc = 2:  bf414636 lib/salt.c: gensalt(): Use STRTCPY() instead of its pattern

v3

• s/STRTCAT/strtcat_a/

$ git rd 
1:  e25872f90 ! 1:  01e32cebf lib/string/strcpy/: strtcat(), STRTCAT(): Add APIs
    @@ Metadata
     Author: Alejandro Colomar <[email protected]>
     
      ## Commit message ##
    -    lib/string/strcpy/: strtcat(), STRTCAT(): Add APIs
    +    lib/string/strcpy/: strtcat[_a](): Add APIs
     
         Signed-off-by: Alejandro Colomar <[email protected]>
     
    @@ lib/string/README: strcpy/ - String copying
        Catenate a string after an existing string, with truncation.
        Rarely useful.  Consider using stpecpy() instead.
     -    STRTCAT()  // Unimplemented
    -+    STRTCAT()
    ++    strtcat_a()
        Like strtcat(), but takes an array.
      
          stpecpy()
    @@ lib/string/strcpy/strtcat.h (new)
     +#include "string/strcpy/stpecpy.h"
     +
     +
    -+#define STRTCAT(dst, src)  strtcat(dst, src, countof(dst))
    ++// strtcat_a - string truncate catenate array
    ++#define strtcat_a(dst, src)  strtcat(dst, src, countof(dst))
     +
     +
     +ATTR_STRING(2)
    @@ lib/string/strcpy/strtcat.h (new)
     +    size_t dsize);
     +
     +
    -+// string truncate catenate
    ++// strtcat - string truncate catenate
     +// Returns new length, or -1 on truncation.
     +inline ssize_t
     +strtcat(char *restrict dst, const char *restrict src, size_t dsize)
2:  bf4146369 ! 2:  b1906dffc lib/salt.c: gensalt(): Use STRTCPY() instead of its pattern
    @@ lib/salt.c: static /*@observer@*/const char *gensalt (size_t salt_size)
     -  /* Concatenate a pseudo random salt. */
     -  strncat (result, gensalt (salt_len),
     -           GENSALT_SETTING_SIZE - strlen (result) - 1);
    -+  assert(STRTCAT(result, gensalt(salt_len)) != -1);
    ++  assert(strtcat_a(result, gensalt(salt_len)) != -1);
      
        return result;
      #endif /* USE_XCRYPT_GENSALT */