basic: support default and alternate values for env expansion

Sometimes it's useful to provide a default value during an environment
expansion, if the environment variable isn't already set.

For instance $XDG_DATA_DIRS is suppose to default to:

/usr/local/share/:/usr/share/

if it's not yet set. That means callers wishing to augment
XDG_DATA_DIRS need to manually add those two values.

This commit changes replace_env to support the following shell
compatible default value syntax:

XDG_DATA_DIRS=/foo:${XDG_DATA_DIRS:-/usr/local/share/:/usr/share}

Likewise, it's useful to provide an alternate value during an
environment expansion, if the environment variable isn't already set.

For instance, $LD_LIBRARY_PATH will inadvertently search the current
working directory if it starts or ends with a colon, so the following
is usually wrong:

LD_LIBRARY_PATH=/foo/lib:${LD_LIBRARY_PATH}

To address that, this changes replace_env to support the following
shell compatible alternate value syntax:

LD_LIBRARY_PATH=/foo/lib${LD_LIBRARY_PATH:+:${LD_LIBRARY_PATH}}

[zj: gate the new syntax under REPLACE_ENV_ALLOW_EXTENDED switch, so
existing callers are not modified.]

1 files changed, 13 insertions, 1 deletions

diff --git a/src/test/test-env-util.c b/src/test/test-env-util.c
index 77a5219d82..dfcd9cb724 100644
--- a/src/test/test-env-util.c
+++ b/src/test/test-env-util.c
@@ -185,6 +185,12 @@ static void test_replace_env_argv(void) {
                 "${FOO",
                 "FOO$$${FOO}",
                 "$$FOO${FOO}",
+                "${FOO:-${BAR}}",
+                "${QUUX:-${FOO}}",
+                "${FOO:+${BAR}}",
+                "${QUUX:+${BAR}}",
+                "${FOO:+|${BAR}|}}",
+                "${FOO:+|${BAR}{|}",
                 NULL
         };
         _cleanup_strv_free_ char **r = NULL;
@@ -202,7 +208,13 @@ static void test_replace_env_argv(void) {
         assert_se(streq(r[8], "${FOO"));
         assert_se(streq(r[9], "FOO$BAR BAR"));
         assert_se(streq(r[10], "$FOOBAR BAR"));
-        assert_se(strv_length(r) == 11);
+        assert_se(streq(r[11], "${FOO:-waldo}"));
+        assert_se(streq(r[12], "${QUUX:-BAR BAR}"));
+        assert_se(streq(r[13], "${FOO:+waldo}"));
+        assert_se(streq(r[14], "${QUUX:+waldo}"));
+        assert_se(streq(r[15], "${FOO:+|waldo|}}"));
+        assert_se(streq(r[16], "${FOO:+|waldo{|}"));
+        assert_se(strv_length(r) == 17);
 }
 
 static void test_env_clean(void) {