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.]

src/basic/env-util.c

@@ -525,12 +525,16 @@ char *replace_env_n(const char *format, size_t n, char **env, unsigned flags) {
                 CURLY,
                 VARIABLE,
                 VARIABLE_RAW,
+                TEST,
+                DEFAULT_VALUE,
+                ALTERNATE_VALUE,
         } state = WORD;
 
-        const char *e, *word = format;
+        const char *e, *word = format, *test_value;
         char *k;
         _cleanup_free_ char *r = NULL;
-        size_t i;
+        size_t i, len;
+        int nest = 0;
 
         assert(format);
 
@@ -554,7 +558,7 @@ char *replace_env_n(const char *format, size_t n, char **env, unsigned flags) {
 
                                 word = e-1;
                                 state = VARIABLE;
-
+                                nest++;
                         } else if (*e == '$') {
                                 k = strnappend(r, word, e-word);
                                 if (!k)
@@ -594,6 +598,63 @@ char *replace_env_n(const char *format, size_t n, char **env, unsigned flags) {
                                 free(r);
                                 r = k;
 
+                                word = e+1;
+                                state = WORD;
+                        } else if (*e == ':') {
+                                if (!(flags & REPLACE_ENV_ALLOW_EXTENDED))
+                                        /* Treat this as unsupported syntax, i.e. do no replacement */
+                                        state = WORD;
+                                else {
+                                        len = e-word-2;
+                                        state = TEST;
+                                }
+                        }
+                        break;
+
+                case TEST:
+                        if (*e == '-')
+                                state = DEFAULT_VALUE;
+                        else if (*e == '+')
+                                state = ALTERNATE_VALUE;
+                        else {
+                                state = WORD;
+                                break;
+                        }
+
+                        test_value = e+1;
+                        break;
+
+                case DEFAULT_VALUE: /* fall through */
+                case ALTERNATE_VALUE:
+                        assert(flags & REPLACE_ENV_ALLOW_EXTENDED);
+
+                        if (*e == '{') {
+                                nest++;
+                                break;
+                        }
+
+                        if (*e != '}')
+                                break;
+
+                        nest--;
+                        if (nest == 0) { // || !strchr(e+1, '}')) {
+                                const char *t;
+                                _cleanup_free_ char *v = NULL;
+
+                                t = strv_env_get_n(env, word+2, len, flags);
+
+                                if (t && state == ALTERNATE_VALUE)
+                                        t = v = replace_env_n(test_value, e-test_value, env, flags);
+                                else if (!t && state == DEFAULT_VALUE)
+                                        t = v = replace_env_n(test_value, e-test_value, env, flags);
+
+                                k = strappend(r, t);
+                                if (!k)
+                                        return NULL;
+
+                                free(r);
+                                r = k;
+
                                 word = e+1;
                                 state = WORD;
                         }

src/basic/env-util.h

@@ -32,6 +32,7 @@ bool env_assignment_is_valid(const char *e);
 enum {
         REPLACE_ENV_USE_ENVIRONMENT = 1u,
         REPLACE_ENV_ALLOW_BRACELESS = 2u,
+        REPLACE_ENV_ALLOW_EXTENDED  = 4u,
 };
 
 char *replace_env_n(const char *format, size_t n, char **env, unsigned flags);

src/basic/fileio.c

@@ -784,7 +784,9 @@ static int merge_env_file_push(
         }
 
         expanded_value = replace_env(value, *env,
-                                     REPLACE_ENV_USE_ENVIRONMENT|REPLACE_ENV_ALLOW_BRACELESS);
+                                     REPLACE_ENV_USE_ENVIRONMENT|
+                                     REPLACE_ENV_ALLOW_BRACELESS|
+                                     REPLACE_ENV_ALLOW_EXTENDED);
         if (!expanded_value)
                 return -ENOMEM;
 
@@ -799,7 +801,7 @@ int merge_env_file(
                 const char *fname) {
 
         /* NOTE: this function supports braceful and braceless variable expansions,
-         * unlike other exported parsing functions.
+         * plus "extended" substitutions, unlike other exported parsing functions.
          */
 
         return parse_env_file_internal(f, fname, NEWLINE, merge_env_file_push, env, NULL);