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, 264 insertions, 2 deletions

diff --git a/process.c b/process.c
index ee5b164dd7..e8a786d55e 100644
--- a/process.c
+++ b/process.c
@@ -179,12 +179,40 @@ static int exec_async_signal_safe(const struct rb_execarg *, char *, size_t);
 #define p_gid_from_name p_gid_from_name
 #endif
 
+#if defined(HAVE_UNISTD_H)
+# if defined(HAVE_GETLOGIN_R)
+#  define USE_GETLOGIN_R 1
+#  define GETLOGIN_R_SIZE_DEFAULT   0x100
+#  define GETLOGIN_R_SIZE_LIMIT    0x1000
+#  if defined(_SC_LOGIN_NAME_MAX)
+#    define GETLOGIN_R_SIZE_INIT sysconf(_SC_LOGIN_NAME_MAX)
+#  else
+#    define GETLOGIN_R_SIZE_INIT GETLOGIN_R_SIZE_DEFAULT
+#  endif
+# elif defined(HAVE_GETLOGIN)
+#  define USE_GETLOGIN 1
+# endif
+#endif
+
 #if defined(HAVE_PWD_H)
-# if defined(HAVE_GETPWNAM_R) && defined(_SC_GETPW_R_SIZE_MAX)
+# if defined(HAVE_GETPWUID_R)
+#  define USE_GETPWUID_R 1
+# elif defined(HAVE_GETPWUID)
+#  define USE_GETPWUID 1
+# endif
+# if defined(HAVE_GETPWNAM_R)
 #  define USE_GETPWNAM_R 1
-#  define GETPW_R_SIZE_INIT sysconf(_SC_GETPW_R_SIZE_MAX)
+# elif defined(HAVE_GETPWNAM)
+#  define USE_GETPWNAM 1
+# endif
+# if defined(HAVE_GETPWNAM_R) || defined(HAVE_GETPWUID_R)
 #  define GETPW_R_SIZE_DEFAULT 0x1000
 #  define GETPW_R_SIZE_LIMIT  0x10000
+#  if defined(_SC_GETPW_R_SIZE_MAX)
+#   define GETPW_R_SIZE_INIT sysconf(_SC_GETPW_R_SIZE_MAX)
+#  else
+#   define GETPW_R_SIZE_INIT GETPW_R_SIZE_DEFAULT
+#  endif
 # endif
 # ifdef USE_GETPWNAM_R
 #   define PREPARE_GETPWNAM \
@@ -5531,6 +5559,240 @@ check_gid_switch(void)
 }
 
 
+#if defined(HAVE_PWD_H)
+/**
+ * Best-effort attempt to obtain the name of the login user, if any,
+ * associated with the process. Processes not descended from login(1) (or
+ * similar) may not have a logged-in user; returns Qnil in that case.
+ */
+VALUE
+rb_getlogin(void)
+{
+#if ( !defined(USE_GETLOGIN_R) && !defined(USE_GETLOGIN) )
+    return Qnil;
+#else
+    char MAYBE_UNUSED(*login) = NULL;
+
+# ifdef USE_GETLOGIN_R
+
+    long loginsize = GETLOGIN_R_SIZE_INIT;  /* maybe -1 */
+
+    if (loginsize < 0)
+        loginsize = GETLOGIN_R_SIZE_DEFAULT;
+
+    VALUE maybe_result = rb_str_buf_new(loginsize);
+
+    login = RSTRING_PTR(maybe_result);
+    loginsize = rb_str_capacity(maybe_result);
+    rb_str_set_len(maybe_result, loginsize);
+
+    int gle;
+    errno = 0;
+    while ((gle = getlogin_r(login, loginsize)) != 0) {
+
+        if (gle == ENOTTY || gle == ENXIO || gle == ENOENT) {
+            rb_str_resize(maybe_result, 0);
+            return Qnil;
+        }
+
+        if (gle != ERANGE || loginsize >= GETLOGIN_R_SIZE_LIMIT) {
+            rb_str_resize(maybe_result, 0);
+            rb_syserr_fail(gle, "getlogin_r");
+        }
+
+        rb_str_modify_expand(maybe_result, loginsize);
+        login = RSTRING_PTR(maybe_result);
+        loginsize = rb_str_capacity(maybe_result);
+    }
+
+    if (login == NULL) {
+        rb_str_resize(maybe_result, 0);
+        return Qnil;
+    }
+
+    return maybe_result;
+
+# elif USE_GETLOGIN
+
+    errno = 0;
+    login = getlogin();
+    if (errno) {
+        if (errno == ENOTTY || errno == ENXIO || errno == ENOENT) {
+            return Qnil;
+        }
+        rb_syserr_fail(errno, "getlogin");
+    }
+
+    return login ? rb_str_new_cstr(login) : Qnil;
+# endif
+
+#endif
+}
+
+VALUE
+rb_getpwdirnam_for_login(VALUE login_name)
+{
+#if ( !defined(USE_GETPWNAM_R) && !defined(USE_GETPWNAM) )
+    return Qnil;
+#else
+
+    if (NIL_P(login_name)) {
+        /* nothing to do; no name with which to query the password database */
+        return Qnil;
+    }
+
+    char *login = RSTRING_PTR(login_name);
+
+    struct passwd *pwptr;
+
+# ifdef USE_GETPWNAM_R
+
+    struct passwd pwdnm;
+    char *bufnm;
+    long bufsizenm = GETPW_R_SIZE_INIT;  /* maybe -1 */
+
+    if (bufsizenm < 0)
+        bufsizenm = GETPW_R_SIZE_DEFAULT;
+
+    VALUE getpwnm_tmp = rb_str_tmp_new(bufsizenm);
+
+    bufnm = RSTRING_PTR(getpwnm_tmp);
+    bufsizenm = rb_str_capacity(getpwnm_tmp);
+    rb_str_set_len(getpwnm_tmp, bufsizenm);
+
+    int enm;
+    errno = 0;
+    while ((enm = getpwnam_r(login, &pwdnm, bufnm, bufsizenm, &pwptr)) != 0) {
+
+        if (enm == ENOENT || enm== ESRCH || enm == EBADF || enm == EPERM) {
+            /* not found; non-errors */
+            rb_str_resize(getpwnm_tmp, 0);
+            return Qnil;
+        }
+
+        if (enm != ERANGE || bufsizenm >= GETPW_R_SIZE_LIMIT) {
+            rb_str_resize(getpwnm_tmp, 0);
+            rb_syserr_fail(enm, "getpwnam_r");
+        }
+
+        rb_str_modify_expand(getpwnm_tmp, bufsizenm);
+        bufnm = RSTRING_PTR(getpwnm_tmp);
+        bufsizenm = rb_str_capacity(getpwnm_tmp);
+    }
+
+    if (pwptr == NULL) {
+        /* no record in the password database for the login name */
+        rb_str_resize(getpwnm_tmp, 0);
+        return Qnil;
+    }
+
+    /* found it */
+    VALUE result = rb_str_new_cstr(pwptr->pw_dir);
+    rb_str_resize(getpwnm_tmp, 0);
+    return result;
+
+# elif USE_GETPWNAM
+
+    errno = 0;
+    pwptr = getpwnam(login);
+    if (pwptr) {
+        /* found it */
+        return rb_str_new_cstr(pwptr->pw_dir);
+    }
+    if (errno
+        /*   avoid treating as errors errno values that indicate "not found" */
+        && ( errno != ENOENT && errno != ESRCH && errno != EBADF && errno != EPERM)) {
+        rb_syserr_fail(errno, "getpwnam");
+    }
+
+    return Qnil;  /* not found */
+# endif
+
+#endif
+}
+
+/**
+ * Look up the user's dflt home dir in the password db, by uid.
+ */
+VALUE
+rb_getpwdiruid(void)
+{
+# if !defined(USE_GETPWUID_R) && !defined(USE_GETPWUID)
+    /* Should never happen... </famous-last-words> */
+    return Qnil;
+# else
+    uid_t ruid = getuid();
+
+    struct passwd *pwptr;
+
+# ifdef USE_GETPWUID_R
+
+    struct passwd pwdid;
+    char *bufid;
+    long bufsizeid = GETPW_R_SIZE_INIT;  /* maybe -1 */
+
+    if (bufsizeid < 0)
+        bufsizeid = GETPW_R_SIZE_DEFAULT;
+
+    VALUE getpwid_tmp = rb_str_tmp_new(bufsizeid);
+
+    bufid = RSTRING_PTR(getpwid_tmp);
+    bufsizeid = rb_str_capacity(getpwid_tmp);
+    rb_str_set_len(getpwid_tmp, bufsizeid);
+
+    int eid;
+    errno = 0;
+    while ((eid = getpwuid_r(ruid, &pwdid, bufid, bufsizeid, &pwptr)) != 0) {
+
+        if (eid == ENOENT || eid== ESRCH || eid == EBADF || eid == EPERM) {
+            /* not found; non-errors */
+            rb_str_resize(getpwid_tmp, 0);
+            return Qnil;
+        }
+
+        if (eid != ERANGE || bufsizeid >= GETPW_R_SIZE_LIMIT) {
+            rb_str_resize(getpwid_tmp, 0);
+            rb_syserr_fail(eid, "getpwuid_r");
+        }
+
+        rb_str_modify_expand(getpwid_tmp, bufsizeid);
+        bufid = RSTRING_PTR(getpwid_tmp);
+        bufsizeid = rb_str_capacity(getpwid_tmp);
+    }
+
+    if (pwptr == NULL) {
+        /* no record in the password database for the uid */
+        rb_str_resize(getpwid_tmp, 0);
+        return Qnil;
+    }
+
+    /* found it */
+    VALUE result = rb_str_new_cstr(pwptr->pw_dir);
+    rb_str_resize(getpwid_tmp, 0);
+    return result;
+
+# elif defined(USE_GETPWUID)
+
+    errno = 0;
+    pwptr = getpwuid(ruid);
+    if (pwptr) {
+        /* found it */
+        return rb_str_new_cstr(pwptr->pw_dir);
+    }
+    if (errno
+        /*   avoid treating as errors errno values that indicate "not found" */
+        && ( errno == ENOENT || errno == ESRCH || errno == EBADF || errno == EPERM)) {
+        rb_syserr_fail(errno, "getpwuid");
+    }
+
+    return Qnil;  /* not found */
+# endif
+
+#endif /* !defined(USE_GETPWUID_R) && !defined(USE_GETPWUID) */
+}
+#endif /* HAVE_PWD_H */
+
+
 /*********************************************************************
  * Document-class: Process::Sys
  *