Allow braceless variables to be expanded

(Only in environment.d files.)

We have only basic compatibility with shell syntax, but specifying variables
without using braces is probably more common, and I think a lot of people would
be surprised if this didn't work.

src/basic/env-util.c

@@ -523,7 +523,8 @@ char *replace_env(const char *format, char **env, unsigned flags) {
         enum {
                 WORD,
                 CURLY,
-                VARIABLE
+                VARIABLE,
+                VARIABLE_RAW,
         } state = WORD;
 
         const char *e, *word = format;
@@ -563,6 +564,18 @@ char *replace_env(const char *format, char **env, unsigned flags) {
 
                                 word = e+1;
                                 state = WORD;
+
+                        } else if (flags & REPLACE_ENV_ALLOW_BRACELESS && strchr(VALID_CHARS_ENV_NAME, *e)) {
+                                k = strnappend(r, word, e-word-1);
+                                if (!k)
+                                        return NULL;
+
+                                free(r);
+                                r = k;
+
+                                word = e-1;
+                                state = VARIABLE_RAW;
+
                         } else
                                 state = WORD;
                         break;
@@ -584,10 +597,38 @@ char *replace_env(const char *format, char **env, unsigned flags) {
                                 state = WORD;
                         }
                         break;
+
+                case VARIABLE_RAW:
+                        assert(flags & REPLACE_ENV_ALLOW_BRACELESS);
+
+                        if (!strchr(VALID_CHARS_ENV_NAME, *e)) {
+                                const char *t;
+
+                                t = strv_env_get_n(env, word+1, e-word-1, flags);
+
+                                k = strappend(r, t);
+                                if (!k)
+                                        return NULL;
+
+                                free(r);
+                                r = k;
+
+                                word = e--;
+                                state = WORD;
+                        }
+                        break;
                 }
         }
 
-        return strnappend(r, word, e-word);
+        if (state == VARIABLE_RAW) {
+                const char *t;
+
+                assert(flags & REPLACE_ENV_ALLOW_BRACELESS);
+
+                t = strv_env_get_n(env, word+1, e-word-1, flags);
+                return strappend(r, t);
+        } else
+                return strnappend(r, word, e-word);
 }
 
 char **replace_env_argv(char **argv, char **env) {

src/basic/env-util.h

@@ -31,6 +31,7 @@ bool env_assignment_is_valid(const char *e);
 
 enum {
         REPLACE_ENV_USE_ENVIRONMENT = 1u,
+        REPLACE_ENV_ALLOW_BRACELESS = 2u,
 };
 
 char *replace_env(const char *format, char **env, unsigned flags);

src/basic/fileio.c

@@ -773,7 +773,8 @@ static int merge_env_file_push(
 
         assert(env);
 
-        expanded_value = replace_env(value, *env, REPLACE_ENV_USE_ENVIRONMENT);
+        expanded_value = replace_env(value, *env,
+                                     REPLACE_ENV_USE_ENVIRONMENT|REPLACE_ENV_ALLOW_BRACELESS);
         if (!expanded_value)
                 return -ENOMEM;
 
@@ -787,6 +788,10 @@ int merge_env_file(
                 FILE *f,
                 const char *fname) {
 
+        /* NOTE: this function supports braceful and braceless variable expansions,
+         * unlike other exported parsing functions.
+         */
+
         return parse_env_file_internal(f, fname, NEWLINE, merge_env_file_push, env, NULL);
 }