journal: move valid_user_field() to journal-util.[ch] and rename it → journal_field_valid()

Being able to validate journal field names is useful outside of the
journal itself.

src/shared/journal-util.c

@@ -149,3 +149,41 @@ int journal_access_check_and_warn(sd_journal *j, bool quiet) {
 
         return r;
 }
+
+bool journal_field_valid(const char *p, size_t l, bool allow_protected) {
+        const char *a;
+
+        /* We kinda enforce POSIX syntax recommendations for
+           environment variables here, but make a couple of additional
+           requirements.
+
+           http://pubs.opengroup.org/onlinepubs/000095399/basedefs/xbd_chap08.html */
+
+        if (l == (size_t) -1)
+                l = strlen(p);
+
+        /* No empty field names */
+        if (l <= 0)
+                return false;
+
+        /* Don't allow names longer than 64 chars */
+        if (l > 64)
+                return false;
+
+        /* Variables starting with an underscore are protected */
+        if (!allow_protected && p[0] == '_')
+                return false;
+
+        /* Don't allow digits as first character */
+        if (p[0] >= '0' && p[0] <= '9')
+                return false;
+
+        /* Only allow A-Z0-9 and '_' */
+        for (a = p; a < p + l; a++)
+                if ((*a < 'A' || *a > 'Z') &&
+                    (*a < '0' || *a > '9') &&
+                    *a != '_')
+                        return false;
+
+        return true;
+}

src/shared/journal-util.h

@@ -19,7 +19,10 @@
 ***/
 
 #include <stdbool.h>
+#include <sys/types.h>
 
 #include "sd-journal.h"
 
+bool journal_field_valid(const char *p, size_t l, bool allow_protected);
+
 int journal_access_check_and_warn(sd_journal *j, bool quiet);