Allow Dir.home to work for non-login procs when $HOME not set

Allow the 'Dir.home' method to reliably locate the user's home directory when
all three of the following are true at the same time:

    1. Ruby is running on a Unix-like OS
    2. The $HOME environment variable is not set
    3. The process is not a descendant of login(1) (or a work-alike)

The prior behavior was that the lookup could only work for login-descended
processes.

This is accomplished by looking up the user's record in the password database
by uid (getpwuid_r(3)) as a fallback to the lookup by name (getpwname_r(3))
which is still attempted first (based on the name, if any, returned by
getlogin_r(3)).

If getlogin_r(3), getpwnam_r(3), and/or getpwuid_r(3) is not available at
compile time, will fallback on using their respective non-*_r() variants:
getlogin(3), getpwnam(3), and/or getpwuid(3).

The rationale for attempting to do the lookup by name prior to doing it by uid
is to accommodate the possibility of multiple login names (each with its own
record in the password database, so each with a potentially different home
directory) being mapped to the same uid (as is explicitly allowed for by
POSIX; see getlogin(3posix)).

Preserves the existing behavior for login-descended processes, and adds the
new capability of having Dir.home being able to find the user's home directory
for non-login-descended processes.

Fixes [Bug #16787]

Related discussion:
    https://bugs.ruby-lang.org/issues/16787
    https://github.com/ruby/ruby/pull/3034

1 files changed, 36 insertions, 15 deletions

diff --git a/file.c b/file.c
index 424e8522e6..976e730813 100644
--- a/file.c
+++ b/file.c
@@ -3603,21 +3603,42 @@ rb_default_home_dir(VALUE result)
 
 #if defined HAVE_PWD_H
     if (!dir) {
-	const char *login = getlogin();
-	if (login) {
-	    struct passwd *pw = getpwnam(login);
-	    if (pw) {
-		copy_home_path(result, pw->pw_dir);
-		endpwent();
-		return result;
-	    }
-	    endpwent();
-	    rb_raise(rb_eArgError, "couldn't find HOME for login `%s' -- expanding `~'",
-		     login);
-	}
-	else {
-	    rb_raise(rb_eArgError, "couldn't find login name -- expanding `~'");
-	}
+        /* We'll look up the user's default home dir in the password db by
+         * login name, if possible, and failing that will fall back to looking
+         * the information up by uid (as would be needed for processes that
+         * are not a descendant of login(1) or a work-alike).
+         *
+         * While the lookup by uid is more likely to succeed (since we always
+         * have a uid, but may or may not have a login name), we prefer first
+         * looking up by name to accommodate the possibility of multiple login
+         * names (each with its own record in the password database, so each
+         * with a potentially different home directory) being mapped to the
+         * same uid (as explicitly allowed for by POSIX; see getlogin(3posix)).
+         */
+        VALUE login_name = rb_getlogin();
+
+# if !defined(HAVE_GETPWUID_R) && !defined(HAVE_GETPWUID)
+        /* This is a corner case, but for backward compatibility reasons we
+         * want to emit this error if neither the lookup by login name nor
+         * lookup by getuid() has a chance of succeeding.
+         */
+        if (NIL_P(login_name)) {
+            rb_raise(rb_eArgError, "couldn't find login name -- expanding `~'");
+        }
+# endif
+
+        VALUE pw_dir = rb_getpwdirnam_for_login(login_name);
+        if (NIL_P(pw_dir)) {
+            pw_dir = rb_getpwdiruid();
+            if (NIL_P(pw_dir)) {
+                rb_raise(rb_eArgError, "couldn't find home for uid `%ld'", (long)getuid());
+            }
+        }
+
+        /* found it */
+        copy_home_path(result, RSTRING_PTR(pw_dir));
+        rb_str_resize(pw_dir, 0);
+        return result;
     }
 #endif
     if (!dir) {