[PATCH] filenamecat: make base a suffix of result

[Paul Eggert]

* lib/filenamecat-lgpl.c (longest_relative_suffix): Remove.
(mfile_name_concat): Always make BASE a suffix of the result, as
cp expects this.  To implement this, separate with '.' instead of
'/' in some rare cases.  Clarify spec to say ./BASE not BASE.
* tests/test-filenamecat.c (main): Adjust tests to match
current behavior.  Check that BASE_IN_RESULT points to
a copy of BASE and is a suffix of the resultk, and that DIR
is a prefix of the result that is no longer than the prefix
indicated by BASE_IN_RESULT.
---
 ChangeLog                | 13 +++++++++++
 lib/filenamecat-lgpl.c   | 57 ++++++++++++++++++++++++------------------------
 lib/filenamecat.c        |  4 ++--
 tests/test-filenamecat.c | 27 ++++++++++++++++++++---
 4 files changed, 67 insertions(+), 34 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 13e868aca..e4801bd17 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,16 @@
+2018-01-14  Paul Eggert  <address@hidden>
+
+       filenamecat: make base a suffix of result
+       * lib/filenamecat-lgpl.c (longest_relative_suffix): Remove.
+       (mfile_name_concat): Always make BASE a suffix of the result, as
+       cp expects this.  To implement this, separate with '.' instead of
+       '/' in some rare cases.  Clarify spec to say ./BASE not BASE.
+       * tests/test-filenamecat.c (main): Adjust tests to match
+       current behavior.  Check that BASE_IN_RESULT points to
+       a copy of BASE and is a suffix of the resultk, and that DIR
+       is a prefix of the result that is no longer than the prefix
+       indicated by BASE_IN_RESULT.
+
 2018-01-04  Mathieu Lirzin  <address@hidden>
 
        update-copyright: Handle use of ©
diff --git a/lib/filenamecat-lgpl.c b/lib/filenamecat-lgpl.c
index 452d3b95d..f50ecc138 100644
--- a/lib/filenamecat-lgpl.c
+++ b/lib/filenamecat-lgpl.c
@@ -31,55 +31,54 @@
 # define mempcpy(D, S, N) ((void *) ((char *) memcpy (D, S, N) + (N)))
 #endif
 
-/* Return the longest suffix of F that is a relative file name.
-   If it has no such suffix, return the empty string.  */
-
-static char const * _GL_ATTRIBUTE_PURE
-longest_relative_suffix (char const *f)
-{
-  for (f += FILE_SYSTEM_PREFIX_LEN (f); ISSLASH (*f); f++)
-    continue;
-  return f;
-}
-
-/* Concatenate two file name components, DIR and ABASE, in
+/* Concatenate two file name components, DIR and BASE, in
    newly-allocated storage and return the result.
    The resulting file name F is such that the commands "ls F" and "(cd
-   DIR; ls BASE)" refer to the same file, where BASE is ABASE with any
-   file system prefixes and leading separators removed.
-   Arrange for a directory separator if necessary between DIR and BASE
-   in the result, removing any redundant separators.
+   DIR; ls ./BASE)" refer to the same file.  If necessary, put
+   a separator between DIR and BASE in the result.  Typically this
+   separator is "/", but in rare cases it might be ".".
    In any case, if BASE_IN_RESULT is non-NULL, set
-   *BASE_IN_RESULT to point to the copy of ABASE in the returned
-   concatenation.  However, if ABASE begins with more than one slash,
-   set *BASE_IN_RESULT to point to the sole corresponding slash that
-   is copied into the result buffer.
+   *BASE_IN_RESULT to point to the copy of BASE at the end of the
+   returned concatenation.
 
    Return NULL if malloc fails.  */
 
 char *
-mfile_name_concat (char const *dir, char const *abase, char **base_in_result)
+mfile_name_concat (char const *dir, char const *base, char **base_in_result)
 {
   char const *dirbase = last_component (dir);
   size_t dirbaselen = base_len (dirbase);
   size_t dirlen = dirbase - dir + dirbaselen;
-  size_t needs_separator = (dirbaselen && ! ISSLASH (dirbase[dirbaselen - 1]));
-
-  char const *base = longest_relative_suffix (abase);
   size_t baselen = strlen (base);
-
-  char *p_concat = malloc (dirlen + needs_separator + baselen + 1);
+  char sep = '\0';
+  if (dirbaselen)
+    {
+      /* DIR is not a file system root, so separate with / if needed.  */
+      if (! ISSLASH (dir[dirlen - 1]) && ! ISSLASH (*base))
+        sep = '/';
+    }
+  else if (ISSLASH (*base))
+    {
+      /* DIR is a file system root and BASE begins with a slash, so
+         separate with ".".  For example, if DIR is "/" and BASE is
+         "/foo" then return "/./foo", as "//foo" would be wrong on
+         some POSIX systems.  A fancier algorithm could omit "." in
+         some cases but is not worth the trouble.  */
+      sep = '.';
+    }
+
+  char *p_concat = malloc (dirlen + (sep != '\0')  + baselen + 1);
   char *p;
 
   if (p_concat == NULL)
     return NULL;
 
   p = mempcpy (p_concat, dir, dirlen);
-  *p = DIRECTORY_SEPARATOR;
-  p += needs_separator;
+  *p = sep;
+  p += sep != '\0';
 
   if (base_in_result)
-    *base_in_result = p - IS_ABSOLUTE_FILE_NAME (abase);
+    *base_in_result = p;
 
   p = mempcpy (p, base, baselen);
   *p = '\0';
diff --git a/lib/filenamecat.c b/lib/filenamecat.c
index 5f5e24465..cad459586 100644
--- a/lib/filenamecat.c
+++ b/lib/filenamecat.c
@@ -32,9 +32,9 @@
    "memory exhausted" condition and exit.  */
 
 char *
-file_name_concat (char const *dir, char const *abase, char **base_in_result)
+file_name_concat (char const *dir, char const *base, char **base_in_result)
 {
-  char *p = mfile_name_concat (dir, abase, base_in_result);
+  char *p = mfile_name_concat (dir, base, base_in_result);
   if (p == NULL)
     xalloc_die ();
   return p;
diff --git a/tests/test-filenamecat.c b/tests/test-filenamecat.c
index f9f121712..17ac670da 100644
--- a/tests/test-filenamecat.c
+++ b/tests/test-filenamecat.c
@@ -22,6 +22,7 @@
 #include "filenamecat.h"
 
 #include <stdbool.h>
+#include <stddef.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -38,11 +39,11 @@ main (int argc _GL_UNUSED, char *argv[])
       {"a", "/b",  "a/b"},
 
       {"/", "b",  "/b"},
-      {"/", "/b", "/b"},
-      {"/", "/",  "/"},
+      {"/", "/b", "/./b"}, /* This result could be shorter.  */
+      {"/", "/",  "/./"},  /* This result could be shorter.  */
       {"a", "/",  "a/"},   /* this might deserve a diagnostic */
       {"/a", "/", "/a/"},  /* this might deserve a diagnostic */
-      {"a", "//b",  "a/b"},
+      {"a", "//b",  "a//b"},
       {"", "a", "a"},  /* this might deserve a diagnostic */
     };
   unsigned int i;
@@ -53,11 +54,31 @@ main (int argc _GL_UNUSED, char *argv[])
       char *base_in_result;
       char const *const *t = tests[i];
       char *res = file_name_concat (t[0], t[1], &base_in_result);
+      ptrdiff_t prefixlen = base_in_result - res;
+      size_t t0len = strlen (t[0]);
+      size_t reslen = strlen (res);
       if (strcmp (res, t[2]) != 0)
         {
           fprintf (stderr, "test #%u: got %s, expected %s\n", i, res, t[2]);
           fail = true;
         }
+      if (strcmp (t[1], base_in_result) != 0)
+        {
+          fprintf (stderr, "test #%u: base %s != base_in_result %s\n",
+                   i, t[1], base_in_result);
+          fail = true;
+        }
+      if (! (0 <= prefixlen && prefixlen <= reslen))
+        {
+          fprintf (stderr, "test #%u: base_in_result is not in result\n", i);
+          fail = true;
+        }
+      if (reslen < t0len || memcmp (res, t[0], t0len) != 0)
+        {
+          fprintf (stderr, "test #%u: %s is not a prefix of %s\n",
+                   i, t[0], res);
+          fail = true;
+        }
     }
   exit (fail ? EXIT_FAILURE : EXIT_SUCCESS);
 }
-- 
2.14.3