Act like POSIX sh if $HOME is relative

POSIX says sh ~/foo should act like $HOME/foo even if $HOME is
relative, so be consistent with that (Bug#33255).
* admin/merge-gnulib (GNULIB_MODULES): Add dosname.
* src/buffer.c (init_buffer): Use emacs_wd to get
initial working directory with slash appended if needed.
(default-directory): Say it must be absolute.
* src/emacs.c (emacs_wd): New global variable.
(init_cmdargs): Dir arg is now char const *.
(main): Set emacs_wd.
* src/emacs.c (main) [NS_IMPL_COCOA]:
* src/fileio.c (Fexpand_file_name):
Use get_homedir instead of egetenv ("HOME").
* src/fileio.c: Include dosname.h, for IS_ABSOLUTE_FILE_NAME.
(splice_dir_file, get_homedir): New functions.
* src/xrdb.c (gethomedir): Remove.  All callers changed
to use get_homedir and splice_dir_file.
* test/src/fileio-tests.el (fileio-tests--relative-HOME): New test.

1 files changed, 58 insertions, 4 deletions

diff --git a/src/fileio.c b/src/fileio.c
index 7fb865809f5..e178c39fc18 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -96,6 +96,7 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
 #include <acl.h>
 #include <allocator.h>
 #include <careadlinkat.h>
+#include <dosname.h>
 #include <fsusage.h>
 #include <stat-time.h>
 #include <tempname.h>
@@ -1093,8 +1094,7 @@ the root directory.  */)
         {
           Lisp_Object tem;
 
-          if (!(newdir = egetenv ("HOME")))
-            newdir = newdirlim = "";
+          newdir = get_homedir ();
           nm++;
 #ifdef WINDOWSNT
           if (newdir[0])
@@ -1109,7 +1109,7 @@ the root directory.  */)
 #endif
             tem = build_string (newdir);
           newdirlim = newdir + SBYTES (tem);
-          /* `egetenv' may return a unibyte string, which will bite us
+          /* get_homedir may return a unibyte string, which will bite us
              if we expect the directory to be multibyte.  */
           if (multibyte && !STRING_MULTIBYTE (tem))
             {
@@ -1637,7 +1637,6 @@ See also the function `substitute-in-file-name'.")
 }
 #endif
 
-/* If /~ or // appears, discard everything through first slash.  */
 static bool
 file_name_absolute_p (const char *filename)
 {
@@ -1650,6 +1649,61 @@ file_name_absolute_p (const char *filename)
      );
 }
 
+/* Put into BUF the concatenation of DIR and FILE, with an intervening
+   directory separator if needed.  Return a pointer to the null byte
+   at the end of the concatenated string.  */
+char *
+splice_dir_file (char *buf, char const *dir, char const *file)
+{
+  char *e = stpcpy (buf, dir);
+  *e = DIRECTORY_SEP;
+  e += ! (buf < e && IS_DIRECTORY_SEP (e[-1]));
+  return stpcpy (e, file);
+}
+
+/* Get the home directory, an absolute file name.  Return the empty
+   string on failure.  The returned value does not survive garbage
+   collection, calls to this function, or calls to the getpwnam class
+   of functions.  */
+char const *
+get_homedir (void)
+{
+  char const *home = egetenv ("HOME");
+  if (!home)
+    {
+      static char const *userenv[] = {"LOGNAME", "USER"};
+      struct passwd *pw = NULL;
+      for (int i = 0; i < ARRAYELTS (userenv); i++)
+        {
+          char *user = egetenv (userenv[i]);
+          if (user)
+            {
+              pw = getpwnam (user);
+              if (pw)
+                break;
+            }
+        }
+      if (!pw)
+        pw = getpwuid (getuid ());
+      if (pw)
+        home = pw->pw_dir;
+      if (!home)
+        return "";
+    }
+  if (IS_ABSOLUTE_FILE_NAME (home))
+    return home;
+  if (!emacs_wd)
+    error ("$HOME is relative to unknown directory");
+  static char *ahome;
+  static ptrdiff_t ahomesize;
+  ptrdiff_t ahomelenbound = strlen (emacs_wd) + 1 + strlen (home) + 1;
+  if (ahomesize <= ahomelenbound)
+    ahome = xpalloc (ahome, &ahomesize, ahomelenbound + 1 - ahomesize, -1, 1);
+  splice_dir_file (ahome, emacs_wd, home);
+  return ahome;
+}
+
+/* If /~ or // appears, discard everything through first slash.  */
 static char *
 search_embedded_absfilename (char *nm, char *endp)
 {