refactor: strlcat instead of str{n}cat.

Add xstrlcat function.

Closes #3042
References #988
References #1069

coverity: 71530, 71531, 71532

src/nvim/ex_cmds.c

@@ -4977,8 +4977,12 @@ static void helptags_one(char_u *dir, char_u *ext, char_u *tagfname,
 
   // Find all *.txt files.
   size_t dirlen = STRLCPY(NameBuff, dir, sizeof(NameBuff));
-  STRCAT(NameBuff, "/**/*");  // NOLINT
-  STRCAT(NameBuff, ext);
+  if (dirlen >= MAXPATHL
+      || STRLCAT(NameBuff, "/**/*", sizeof(NameBuff)) >= MAXPATHL  // NOLINT
+      || STRLCAT(NameBuff, ext, sizeof(NameBuff)) >= MAXPATHL) {
+    EMSG(_(e_pathtoolong));
+    return;
+  }
 
   // Note: We cannot just do `&NameBuff` because it is a statically sized array
   //       so `NameBuff == &NameBuff` according to C semantics.
@@ -4995,9 +4999,12 @@ static void helptags_one(char_u *dir, char_u *ext, char_u *tagfname,
    * Open the tags file for writing.
    * Do this before scanning through all the files.
    */
-  STRLCPY(NameBuff, dir, sizeof(NameBuff));
+  memcpy(NameBuff, dir, dirlen + 1);
   add_pathsep((char *)NameBuff);
-  STRNCAT(NameBuff, tagfname, sizeof(NameBuff) - dirlen - 2);
+  if (STRLCAT(NameBuff, tagfname, sizeof(NameBuff)) >= MAXPATHL) {
+    EMSG(_(e_pathtoolong));
+    return;
+  }
   fd_tags = mch_fopen((char *)NameBuff, "w");
   if (fd_tags == NULL) {
     EMSG2(_("E152: Cannot open %s for writing"), NameBuff);
@@ -5172,7 +5179,11 @@ static void do_helptags(char_u *dirname, bool add_help_tags)
   // Get a list of all files in the help directory and in subdirectories.
   STRLCPY(NameBuff, dirname, sizeof(NameBuff));
   add_pathsep((char *)NameBuff);
-  STRCAT(NameBuff, "**");
+  if (STRLCAT(NameBuff, "**", MAXPATHL) >= MAXPATHL) {
+    EMSG(_(e_pathtoolong));
+    xfree(dirname);
+    return;
+  }
 
   // Note: We cannot just do `&NameBuff` because it is a statically sized array
   //       so `NameBuff == &NameBuff` according to C semantics.

src/nvim/file_search.c

@@ -191,9 +191,6 @@ typedef struct ff_search_ctx_T {
 # include "file_search.c.generated.h"
 #endif
 
-static char_u e_pathtoolong[] = N_("E854: path too long for completion");
-
-
 /*
  * Initialization routine for vim_findfile().
  *

src/nvim/globals.h

@@ -1215,6 +1215,7 @@ EXTERN char_u e_invalidreg[] INIT(= N_("E850: Invalid register name"));
 EXTERN char_u e_dirnotf[] INIT(= N_(
     "E919: Directory not found in '%s': \"%s\""));
 EXTERN char_u e_unsupportedoption[] INIT(= N_("E519: Option not supported"));
+EXTERN char_u e_pathtoolong[] INIT(= N_("E854: path too long for completion"));
 
 
 EXTERN char top_bot_msg[] INIT(= N_("search hit TOP, continuing at BOTTOM"));

src/nvim/memory.c

@@ -389,6 +389,33 @@ size_t xstrlcpy(char *restrict dst, const char *restrict src, size_t size)
   return ret;
 }
 
+/// xstrlcat - Appends string src to the end of dst.
+///
+/// Compatible with *BSD strlcat: Appends at most (dstsize - strlen(dst) - 1)
+/// characters. dst will be NUL-terminated.
+///
+/// Note: Replaces `vim_strcat`.
+///
+/// @param dst      Where to copy the string to
+/// @param src      Where to copy the string from
+/// @param dstsize  Size of destination buffer, must be greater than 0
+/// @return         strlen(src) + MIN(dstsize, strlen(initial dst)).
+///                 If retval >= dstsize, truncation occurs.
+size_t xstrlcat(char *restrict dst, const char *restrict src, size_t dstsize)
+  FUNC_ATTR_NONNULL_ALL
+{
+  assert(dstsize > 0);
+  size_t srclen = strlen(src);
+  size_t dstlen = strlen(dst);
+  size_t ret = srclen + dstlen;  // Total string length (excludes NUL)
+  if (srclen) {
+    size_t len = (ret >= dstsize) ? dstsize - 1 : ret;
+    memcpy(dst + dstlen, src, len - dstlen);
+    dst[len] = '\0';
+  }
+  return ret;  // Does not include NUL.
+}
+
 /// strdup() wrapper
 ///
 /// @see {xmalloc}

src/nvim/os/os_defs.h

@@ -16,7 +16,7 @@
 #define BASENAMELEN (NAME_MAX - 5)
 
 // Use the system path length if it makes sense.
-#if defined(PATH_MAX) && (PATH_MAX > 1000)
+#if defined(PATH_MAX) && (PATH_MAX > 1024)
 # define MAXPATHL PATH_MAX
 #else
 # define MAXPATHL 1024

src/nvim/path.c

@@ -398,8 +398,12 @@ char *concat_fnames_realloc(char *fname1, const char *fname2, bool sep)
 void add_pathsep(char *p)
   FUNC_ATTR_NONNULL_ALL
 {
-  if (*p != NUL && !after_pathsep(p, p + strlen(p)))
-    strcat(p, PATHSEPSTR);
+  const size_t len = strlen(p);
+  const size_t pathsep_len = sizeof(PATHSEPSTR);
+  assert(len < MAXPATHL - pathsep_len);
+  if (*p != NUL && !after_pathsep(p, p + len)) {
+    memcpy(p + len, PATHSEPSTR, pathsep_len);
+  }
 }
 
 /// Get an allocated copy of the full path to a file.

src/nvim/vim.h

@@ -269,6 +269,7 @@ enum {
 
 #define STRCAT(d, s)        strcat((char *)(d), (char *)(s))
 #define STRNCAT(d, s, n)    strncat((char *)(d), (char *)(s), (size_t)(n))
+#define STRLCAT(d, s, n)    xstrlcat((char *)(d), (char *)(s), (size_t)(n))
 
 # define vim_strpbrk(s, cs) (char_u *)strpbrk((char *)(s), (char *)(cs))