Don't fail evolution-test when it triggers issue #344.

c++/src/capnp/compiler/evolution-test.c++

@@ -868,6 +868,18 @@ public:
   }
 
   kj::MainBuilder::Validity run() {
+    // https://github.com/sandstorm-io/capnproto/issues/344 describes an obscure bug in the layout
+    // algorithm, the fix for which breaks backwards-compatibility for any schema triggering the
+    // bug. In order to avoid silently breaking protocols, we are temporarily throwing an exception
+    // in cases where this bug would have occurred, so that people can decide what to do.
+    // However, the evolution test can occasionally trigger the bug (depending on the random path
+    // it takes). Rather than try to avoid it, we disable the exception-throwing, because the bug
+    // is actually fixed, and the exception is only there to raise awareness of the compatibility
+    // concerns.
+    //
+    // On Linux, seed 1467142714 (for example) will trigger the exception (without this env var).
+    setenv("CAPNP_IGNORE_ISSUE_344", "1", true);
+
     srand(seed);
 
     {

c++/src/capnp/compiler/node-translator.c++

@@ -30,6 +30,10 @@
 namespace capnp {
 namespace compiler {
 
+bool shouldDetectIssue344() {
+  return getenv("CAPNP_IGNORE_ISSUE_344") == nullptr;
+}
+
 class NodeTranslator::StructLayout {
   // Massive, disgusting class which implements the layout algorithm, which decides the offset
   // for each field.
@@ -411,7 +415,7 @@ public:
         // Underlying slot is big enough, so expand our size and update holes.
         if (newHoles) {
           holes.addHolesAtEnd(lgSizeUsed, 1, desiredUsage);
-        } else {
+        } else if (shouldDetectIssue344()) {
           // Unfortunately, Cap'n Proto 0.5.x and below would always call addHolesAtEnd(), which
           // was the wrong thing to do when called from tryExpand(), which itself is only called
           // in cases involving unions nested in other unions. The bug could lead to multiple
@@ -526,7 +530,11 @@ public:
         // problem and throw an exception.
         //
         // TODO(cleanup): Once sufficient time has elapsed, switch to "return false;" here.
-        mustFail = true;
+        if (shouldDetectIssue344()) {
+          mustFail = true;
+        } else {
+          return false;
+        }
       }
 
       for (uint i = 0; i < parentDataLocationUsage.size(); i++) {