Function to check a UTF-8 string for unknown characters.

So we can reject things we don't know how to NFC yet.

utils/CMakeLists.txt

@@ -65,6 +65,7 @@ add_sources_from_current_dir(utils
   stripctrl.c
   tempseat.c
   tree234.c
+  unicode-known.c
   unicode-norm.c
   validate_manual_hostkey.c
   version.c

utils/unicode-known.c

@@ -0,0 +1,53 @@
+/*
+ * Check a UTF-8 string to ensure every character in it is part of the
+ * version of Unicode that we understand.
+ *
+ * (If it isn't, then we don't know what combining properties it has,
+ * so we can't safely NFC it and rely on the result not changing when
+ * we later update our Unicode version.)
+ */
+
+#include "misc.h"
+#include "unicode/version.h"
+
+static bool known(unsigned c)
+{
+    struct range {
+        unsigned start, end;
+    };
+    static const struct range ranges[] = {
+        #include "unicode/known_chars.h"
+    };
+
+    const struct range *start = ranges, *end = start + lenof(ranges);
+
+    while (end > start) {
+        const struct range *curr = start + (end-start) / 2;
+        if (c < curr->start)
+            end = curr;
+        else if (c > curr->end)
+            start = curr + 1;
+        else
+            return true;
+    }
+
+    return false;
+};
+
+char *utf8_unknown_char(ptrlen input)
+{
+    BinarySource src[1];
+    BinarySource_BARE_INIT_PL(src, input);
+
+    for (size_t nchars = 0; get_avail(src); nchars++) {
+        unsigned c = decode_utf8(src);
+        if (!known(c))
+            return dupprintf(
+                "cannot stably normalise this string: code point %04X "
+                "(at character position %"SIZEu", byte position %"SIZEu") "
+                "is not in Unicode %s", c, nchars, src->pos,
+                UNICODE_VERSION_SHORT);
+    }
+
+    return NULL;
+}