The additional code is trivial, but making sure we get the priority of
various options correct takes a few tests.
---
 lib/open.cc            |  4 +++
 test/T590-libconfig.sh | 67 +++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 70 insertions(+), 1 deletion(-)

diff --git a/lib/open.cc b/lib/open.cc
index 92661271..acc9f7a4 100644
--- a/lib/open.cc
+++ b/lib/open.cc
@@ -162,6 +162,10 @@ notmuch_database_open_with_config (const char *database_path,
     GKeyFile *key_file = NULL;
     static int initialized = 0;
 
+    if (! database_path) {
+	database_path = getenv ("NOTMUCH_DATABASE");
+    }
+
     if ((status = _pre_open (config_path, profile, &key_file, &database_path, &message)))
 	goto DONE;
 
diff --git a/test/T590-libconfig.sh b/test/T590-libconfig.sh
index 97f8fdc7..7f187d36 100755
--- a/test/T590-libconfig.sh
+++ b/test/T590-libconfig.sh
@@ -27,7 +27,7 @@ int main (int argc, char** argv)
                                               &db,
                                               &msg);
    if (stat != NOTMUCH_STATUS_SUCCESS) {
-     fprintf (stderr, "error opening database: %d %s\n", stat, msg ? msg : "");
+     fprintf (stderr, "error opening database\n%s\n%s\n", notmuch_status_to_string (stat), msg ? msg : "");
      exit (1);
    }
 EOF
@@ -473,4 +473,69 @@ EOF
 test_expect_equal_file EXPECTED OUTPUT
 restore_database
 
+test_begin_subtest "no config, fail to open database"
+old_NOTMUCH_CONFIG=${NOTMUCH_CONFIG}
+unset NOTMUCH_CONFIG
+cat c_head - c_tail <<'EOF' | test_C %NULL% '' %NULL%
+{
+   printf("NOT RUN");
+}
+EOF
+NOTMUCH_CONFIG=${old_NOTMUCH_CONFIG}
+cat <<'EOF' >EXPECTED
+== stdout ==
+== stderr ==
+error opening database
+Erroneous NULL pointer
+Error: Cannot open a database for a NULL path.
+
+EOF
+test_expect_equal_file EXPECTED OUTPUT
+
+test_begin_subtest "open database from NOTMUCH_DATABASE"
+old_NOTMUCH_CONFIG=${NOTMUCH_CONFIG}
+unset NOTMUCH_CONFIG
+export NOTMUCH_DATABASE=${MAIL_DIR}
+cat c_head - c_tail <<'EOF' | test_C %NULL% '' %NULL%
+{
+   EXPECT0(notmuch_database_get_config (db, "test.key1", &val));
+   printf("test.key1 = %s\n", val);
+   EXPECT0(notmuch_database_get_config (db, "test.key2", &val));
+   printf("test.key2 = %s\n", val);
+}
+EOF
+NOTMUCH_CONFIG=${old_NOTMUCH_CONFIG}
+unset NOTMUCH_DATABASE
+cat <<'EOF' >EXPECTED
+== stdout ==
+test.key1 = testvalue1
+test.key2 = testvalue2
+== stderr ==
+EOF
+test_expect_equal_file EXPECTED OUTPUT
+
+test_begin_subtest "NOTMUCH_DATABASE overrides config"
+old_path=$(notmuch config get database.path)
+notmuch config set database.path /nonexistent
+export NOTMUCH_DATABASE=${MAIL_DIR}
+cat c_head - c_tail <<'EOF' | test_C %NULL% '' %NULL%
+{
+   EXPECT0(notmuch_database_get_config (db, "test.key1", &val));
+   printf("test.key1 = %s\n", val);
+   EXPECT0(notmuch_database_get_config (db, "test.key2", &val));
+   printf("test.key2 = %s\n", val);
+}
+EOF
+NOTMUCH_CONFIG=${old_NOTMUCH_CONFIG}
+unset NOTMUCH_DATABASE
+cat <<'EOF' >EXPECTED
+== stdout ==
+test.key1 = testvalue1
+test.key2 = testvalue2
+== stderr ==
+EOF
+notmuch config set database.path "${old_path}"
+test_expect_equal_file EXPECTED OUTPUT
+
+
 test_done
-- 
2.29.2
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