[Strings] Escape strings printed by fuzz-exec (#6441)

Previously we printed strings as WTF-8 in the output of fuzz-exec, but this
could produce invalid unicode output and did not make unprintable characters
visible. Fix both these problems by escaping the output, using the JSON string
escape procedure since the string to be escaped is WTF-16. Reimplement the same
escaping procedure in fuzz_shell.js so that the way we print strings when
running on a real JS engine matches the way we print them in our own fuzz-exec
interpreter.

Fixes #6435.

1 files changed, 5 insertions, 6 deletions

diff --git a/src/wasm/literal.cpp b/src/wasm/literal.cpp
index afdc14c72..887c777ec 100644
--- a/src/wasm/literal.cpp
+++ b/src/wasm/literal.cpp
@@ -639,7 +639,7 @@ std::ostream& operator<<(std::ostream& o, Literal literal) {
           if (!data) {
             o << "nullstring";
           } else {
-            o << "string(\"";
+            o << "string(";
             // Convert WTF-16 literals to WTF-16 string.
             std::stringstream wtf16;
             for (auto c : data->values) {
@@ -648,12 +648,11 @@ std::ostream& operator<<(std::ostream& o, Literal literal) {
               wtf16 << uint8_t(u & 0xFF);
               wtf16 << uint8_t(u >> 8);
             }
-            // Convert to WTF-8 for printing.
+            // Escape to ensure we have valid unicode output and to make
+            // unprintable characters visible.
             // TODO: Use wtf16.view() once we have C++20.
-            [[maybe_unused]] bool valid =
-              String::convertWTF16ToWTF8(o, wtf16.str());
-            assert(valid);
-            o << "\")";
+            String::printEscapedJSON(o, wtf16.str());
+            o << ")";
           }
           break;
         }