Make the homedir overridable with the NSS_UNKNOWN_HOME env var

By default `libnss-unknown` violates the assumption that the result of
calling `getpwent()`/`getpwuid()` does not change when the environment
changes as the value for the `pw_dir` field depends on the `$HOME` var.

For instance the `posix/wordexp-test` test in the glibc testsuite
breaks with `libnss-unknown`:

* the call to `getpwuid()` with `HOME=/dummy/home` returns
  `/dummy/home` due to `libnss-unknown`
* `unsetenv ("HOME")` is called
* any call to `getpwuid()` will now return `/` due to
  `libnss-unknown` as no `$HOME` is set
* test fails because results don't match

Setting `NSS_UNKNOWN_HOME` before running the test will get rid of the
dependency between `getpwuid()` and `$HOME`.
Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

src/nss-unknown.c

@@ -31,7 +31,11 @@ static char * determine_homedir (uid_t uid, char *buffer, size_t buflen)
   if (uid != getuid())
     return UNKNOWN_USER_DIR;
 
-  home = getenv ("HOME");
+  home = getenv ("NSS_UNKNOWN_HOME");
+
+  if (home == NULL)
+    home = getenv ("HOME");
+
   if (home == NULL)
     return UNKNOWN_USER_DIR;
 

tests/resolve-unknown-current-user.t

@@ -17,6 +17,7 @@ get_passwd_entry () {
   env -i \
     ${HOME+HOME=$HOME} \
     ${NSS_WRAPPER_DEBUGLEVEL+NSS_WRAPPER_DEBUGLEVEL=$NSS_WRAPPER_DEBUGLEVEL} \
+    ${NSS_UNKNOWN_HOME+NSS_UNKNOWN_HOME=$NSS_UNKNOWN_HOME} \
     LD_PRELOAD=libnss_wrapper.so \
     NSS_WRAPPER_PASSWD=/dev/null \
     NSS_WRAPPER_GROUP=/dev/null \
@@ -56,4 +57,13 @@ test_debug "echo home = '$home'"
 test_expect_success 'Fallback to "/" if $HOME is not set' \
   'test "$home" = "/"'
 
+export NSS_UNKNOWN_HOME="/unknownhome"
+export HOME="/fakehome"
+entry=$(get_passwd_entry $(id -u))
+home=$(echo "$entry" | cut -f 6 -d ":")
+test_debug "echo entry = '$entry'"
+test_debug "echo home = '$home'"
+test_expect_success 'Check that NSS_UNKNOWN_HOME overrides HOME' \
+  'test "$home" = "$NSS_UNKNOWN_HOME"'
+
 test_done