version_is_valid() tweaks (#43025)

We have two validators, and neither really makes sense. Let's unify and
clean things up.

src/basic/string-util.c

@@ -1501,28 +1501,47 @@ char* find_line_after_internal(const char *haystack, const char *needle) {
         return NULL;
 }
 
-bool version_is_valid(const char *s) {
-        if (isempty(s))
+bool version_is_valid(const char *s, VersionFlags flags) {
+
+        /* Validates a version string superficially. This does not proces the version string in any
+         * semantical way, it mostly just validates that its charset is reasonable. */
+
+        if (FLAGS_SET(flags, VERSION_ALLOW_EMPTY) ? !s : isempty(s))
                 return false;
 
         if (!filename_part_is_valid(s))
                 return false;
 
-        /* This is a superset of the characters used by semver. We additionally allow "," and "_". */
-        if (!in_charset(s, ALPHANUMERICAL ".,_-+"))
-                return false;
+        /* We always allow all characters specified by the UAPI.10 Version Specification, i.e. 0-9, a-z, A-Z,
+         * ".", "-", "~", "^".
+         *
+         * If the relevant flags are set we'll also allow "+" and "_" separators.
+         *
+         * Note that with SemVer allows 0-9, a-z, A-Z, "+", "-", ".", hence with VERSION_ALLOW_PLUS we
+         * implement a superset of it.
+         *
+         * If you wonder when to use which flags: when validating foreign versions (e.g. distribution
+         * versions in /etc/os-release or so) validate liberally, i.e. add
+         * VERSION_ALLOW_UNDERSCORE|VERSION_ALLOW_PLUS. When validating our own versioned objects (e.g. vpick
+         * or so) validate more strictly, and in particular refuse characters such as "_" and "+" that may be
+         * used for separating component names or boot attempt counters. Also: first – if appropriate – split
+         * the string into individual components. For example, if the string consists of a name and a
+         * version, separated by some character, only pass the version part to this function. The name part
+         * may pass verification, but it's cleaner to not rely on that.
+         *
+         * For details about UAPI.10 see:
+         *
+         * → https://uapi-group.org/specifications/specs/version_format_specification/ */
 
-        return true;
-}
+        char charset[] = ALPHANUMERICAL ".-~^" /* plus room for the two chars below: */ "\0\0";
+        size_t l = strlen(charset);
 
-bool version_is_valid_versionspec(const char *s) {
-        if (!filename_part_is_valid(s))
-                return false;
+        if (FLAGS_SET(flags, VERSION_ALLOW_UNDERSCORE))
+                charset[l++] = '_';
+        if (FLAGS_SET(flags, VERSION_ALLOW_PLUS))
+                charset[l++] = '+';
 
-        if (!in_charset(s, ALPHANUMERICAL "-.~^"))
-                return false;
-
-        return true;
+        return in_charset(s, charset);
 }
 
 ssize_t strlevenshtein(const char *x, const char *y) {