Name the malformed domain name in the error message about that domain.

Author: arnt

core/src/main/java/org/apache/james/core/Domain.java

@@ -55,15 +55,26 @@ public static Domain of(String domain) {
         Preconditions.checkArgument(domain.length() <= MAXIMUM_DOMAIN_LENGTH,
             "Domain name length should not exceed %s characters", MAXIMUM_DOMAIN_LENGTH);
 
-        String domainWithoutBrackets = IDN.toASCII(removeBrackets(domain), IDN.ALLOW_UNASSIGNED);
+        String domainWithoutBrackets;
+        try {
+            domainWithoutBrackets = IDN.toASCII(removeBrackets(domain), IDN.ALLOW_UNASSIGNED);
+        } catch (IllegalArgumentException e) {
+            // IDN.toASCII's own message can be cryptic ("Empty label is not
+            // a legal name", "A prohibited code point was found in the
+            // input..."). Let's save wear and tear on the poor developer's
+            // brain.
+            throw new IllegalArgumentException(
+                "Domain '" + domain + "' is invalid according to IDNA: " + e.getMessage(), e);
+        }
         Preconditions.checkArgument(PART_CHAR_MATCHER.matchesAllOf(domainWithoutBrackets),
                                     "Domain parts ASCII chars must be a-z A-Z 0-9 - or _ in %s", domain);
 
         if (domainWithoutBrackets.startsWith("xn--") ||
             domainWithoutBrackets.contains(".xn--")) {
             domainWithoutBrackets = IDN.toUnicode(domainWithoutBrackets);
             Preconditions.checkArgument(!domainWithoutBrackets.startsWith("xn--") &&
-                                        !domainWithoutBrackets.contains(".xn--"));
+                                        !domainWithoutBrackets.contains(".xn--"),
+                                        "A-label could not be decoded to Unicode in %s", domain);
         }
 
         int pos = 0;

core/src/test/java/org/apache/james/core/DomainTest.java

@@ -76,6 +76,16 @@ void testMalformedDomains(String malformed) {
             .as("rejecting malformed domain " + malformed)
             .isInstanceOf(IllegalArgumentException.class);
     }
+
+    @ParameterizedTest
+    @MethodSource("malformedDomains")
+    void exceptionForMalformedDomainShouldNameTheOffendingInput(String malformed) {
+        // Without the offending input in the message, "Domain invalid
+        // according to IDNA" leaves a future debugger guessing what
+        // string actually triggered it.
+        assertThatThrownBy(() -> Domain.of(malformed))
+            .hasMessageContaining(malformed);
+    }
 }