[#1680] Bugfix: disallow ArgGroups with multiplicity=0 to prevent Sta…

…ckOverflowError during parsing

Closes #1680

RELEASE-NOTES.md

@@ -95,6 +95,7 @@ Picocli 4.7.0 introduced a `sortSynopsis = false` attribute to let the synopsis
 * [#1602] Enhancement: Fix incorrect debug output for add/removeAlias.
 * [#1603] Enhancement: Improve debug tracing information for help requests and command execution.
 * [#1629] Enhancement: Omit empty braces in standard prompt for interactive options without description. Thanks to [Andreas Deininger](https://github.com/deining) for raising this.
+* [#1680] Bugfix: ArgGroups with `multiplicity="0"` are now disallowed at construction time and no longer throw a `StackOverflowError` while parsing. Thanks to [ARNOLD Somogyi](https://github.com/zappee) for raising this.
 * [#1615][#1616] Bugfix: `getCJKAdjustedLength()` no longer miscalculates for supplementary code points. Thanks to [gwalbran](https://github.com/gwalbran) for the pull request.
 * [#1575] Bugfix: Synopsis should not cluster boolean options if `posixClusteredShortOptionsAllowed` is set to false.
 * [#1642] Bugfix: Negatable options should negate explicit values. Thanks to [Nat Burns](https://github.com/burnnat) for raising this.

src/main/java/picocli/CommandLine.java

@@ -10339,6 +10339,8 @@ public static class ArgGroupSpec implements IOrdered {
                 headingKey       = NO_HEADING_KEY.equals(builder.headingKey) ? null : builder.headingKey;
                 exclusive        = builder.exclusive && builder.validate; // non-validating groups cannot be exclusive: https://github.com/remkop/picocli/issues/810
                 multiplicity     = builder.multiplicity;
+                if (multiplicity.max() <= 0) { throw new InitializationException("ArgGroup must have multiplicity that allows at least one occurrence, but had multiplicity=" + multiplicity); }
+
                 validate         = builder.validate;
                 order            = builder.order;
                 typeInfo         = builder.typeInfo;

src/test/java/picocli/ArgGroupTest.java

@@ -4331,4 +4331,24 @@ public void testIssue1409ModInitializeGroup2() {
         assertEquals(errorB2,"z", obj.optXAndGroupOneOrGroupTwo.oneORtwo.two.b2);
     }
 
+    static class Issue1680MultiplicityZeroCausesInfiniteLoop {
+        static class MyGroup {
+            @Option(names = "-x") String x;
+        }
+
+        @ArgGroup(multiplicity = "0")
+        MyGroup myGroup;
+    }
+
+    @Test
+    public void testIssue1680MultiplicityZeroCausesInfiniteLoop() {
+        final Object obj = new Issue1680MultiplicityZeroCausesInfiniteLoop();
+        try {
+            new CommandLine(obj).parseArgs("-x", "abc");
+            fail("Expected exception");
+        } catch (InitializationException good) {
+            assertEquals("ArgGroup must have multiplicity that allows at least one occurrence, but had multiplicity=0", good.getMessage());
+        }
+    }
+
 }