SERVER-19977 Offer a stack based JSStringWrapper

Small string optimization for encoding utf8 strings from JS strings on
the stack. Additions to the api to use on stack buffers.

9 files changed, 82 insertions, 36 deletions

diff --git a/src/mongo/scripting/mozjs/bson.cpp b/src/mongo/scripting/mozjs/bson.cpp
index f2c30407845..9f0a6125d22 100644
--- a/src/mongo/scripting/mozjs/bson.cpp
+++ b/src/mongo/scripting/mozjs/bson.cpp
@@ -194,15 +194,16 @@ void BSONInfo::resolve(JSContext* cx, JS::HandleObject obj, JS::HandleId id, boo
     }
 
     IdWrapper idw(cx, id);
+    JSStringWrapper jsstr;
 
-    if (!holder->_readOnly && holder->_removed.count(idw.toString())) {
+    auto sname = idw.toStringData(&jsstr);
+
+    if (!holder->_readOnly && holder->_removed.find(sname.toString()) != holder->_removed.end()) {
         return;
     }
 
     ObjectWrapper o(cx, obj);
 
-    std::string sname = IdWrapper(cx, id).toString();
-
     if (holder->_obj.hasField(sname)) {
         auto elem = holder->_obj[sname];
 
diff --git a/src/mongo/scripting/mozjs/idwrapper.cpp b/src/mongo/scripting/mozjs/idwrapper.cpp
index 2c701037a22..89c24807d53 100644
--- a/src/mongo/scripting/mozjs/idwrapper.cpp
+++ b/src/mongo/scripting/mozjs/idwrapper.cpp
@@ -41,15 +41,22 @@ namespace mozjs {
 IdWrapper::IdWrapper(JSContext* cx, JS::HandleId value) : _context(cx), _value(cx, value) {}
 
 std::string IdWrapper::toString() const {
+    JSStringWrapper jsstr;
+    return toStringData(&jsstr).toString();
+}
+
+StringData IdWrapper::toStringData(JSStringWrapper* jsstr) const {
     if (JSID_IS_STRING(_value)) {
-        return JSStringWrapper(_context, JSID_TO_STRING(_value)).toString();
+        *jsstr = JSStringWrapper(_context, JSID_TO_STRING(_value));
     } else if (JSID_IS_INT(_value)) {
-        return std::to_string(JSID_TO_INT(_value));
+        *jsstr = JSStringWrapper(JSID_TO_INT(_value));
     } else {
         throwCurrentJSException(_context,
                                 ErrorCodes::TypeMismatch,
                                 "Cannot toString() non-string and non-integer jsid");
     }
+
+    return jsstr->toStringData();
 }
 
 uint32_t IdWrapper::toInt32() const {
diff --git a/src/mongo/scripting/mozjs/idwrapper.h b/src/mongo/scripting/mozjs/idwrapper.h
index 4865e5e0aec..0ba380c64cb 100644
--- a/src/mongo/scripting/mozjs/idwrapper.h
+++ b/src/mongo/scripting/mozjs/idwrapper.h
@@ -32,6 +32,7 @@
 #include <string>
 
 #include "mongo/base/string_data.h"
+#include "mongo/scripting/mozjs/jsstringwrapper.h"
 
 namespace mongo {
 namespace mozjs {
@@ -51,6 +52,7 @@ public:
      * Converts to a string.  This coerces for integers
      */
     std::string toString() const;
+    StringData toStringData(JSStringWrapper* jsstr) const;
 
     /**
      * Converts to an int.  This throws if the id is not an integer
diff --git a/src/mongo/scripting/mozjs/jsstringwrapper.cpp b/src/mongo/scripting/mozjs/jsstringwrapper.cpp
index 8261b44a2fe..2732c98607b 100644
--- a/src/mongo/scripting/mozjs/jsstringwrapper.cpp
+++ b/src/mongo/scripting/mozjs/jsstringwrapper.cpp
@@ -41,7 +41,11 @@
 namespace mongo {
 namespace mozjs {
 
-JSStringWrapper::JSStringWrapper(JSContext* cx, JSString* str) : _context(cx) {
+JSStringWrapper::JSStringWrapper(std::int32_t value) : _isSet(true) {
+    _length = sprintf(_buf, "%d", value);
+}
+
+JSStringWrapper::JSStringWrapper(JSContext* cx, JSString* str) : _isSet(true) {
     if (!str)
         throwCurrentJSException(cx, ErrorCodes::InternalError, "Cannot encode null JSString");
 
@@ -56,20 +60,21 @@ JSStringWrapper::JSStringWrapper(JSContext* cx, JSString* str) : _context(cx) {
 
     _length = JS::GetDeflatedUTF8StringLength(flat);
 
-    JS::RootedString rstr(cx, str);
-
-    JSAutoByteString abs;
-    abs.encodeUtf8(cx, rstr);
-
-    if (!abs)
-        throwCurrentJSException(cx, ErrorCodes::InternalError, "Failed to encode JSString");
+    char* out;
+    if (_length < sizeof(_buf)) {
+        out = _buf;
+    } else {
+        _str.reset(new char[_length + 1]);
+        out = _str.get();
+    }
 
-    _str.reset(new char[_length]);
-    std::memcpy(_str.get(), abs.ptr(), _length);
+    JS::DeflateStringToUTF8Buffer(flat, mozilla::RangedPtr<char>(out, _length));
+    out[_length] = '\0';
 }
 
 StringData JSStringWrapper::toStringData() const {
-    return StringData(_str.get(), _length);
+    invariant(_isSet);
+    return StringData(_str ? _str.get() : _buf, _length);
 }
 
 std::string JSStringWrapper::toString() const {
@@ -77,7 +82,7 @@ std::string JSStringWrapper::toString() const {
 }
 
 JSStringWrapper::operator bool() const {
-    return _str.get();
+    return _isSet;
 }
 
 }  // namespace mozjs
diff --git a/src/mongo/scripting/mozjs/jsstringwrapper.h b/src/mongo/scripting/mozjs/jsstringwrapper.h
index 9a0ee21e900..ec94593bd05 100644
--- a/src/mongo/scripting/mozjs/jsstringwrapper.h
+++ b/src/mongo/scripting/mozjs/jsstringwrapper.h
@@ -45,6 +45,7 @@ class JSStringWrapper {
 public:
     JSStringWrapper() = default;
     JSStringWrapper(JSContext* cx, JSString* str);
+    JSStringWrapper(std::int32_t val);
 
     StringData toStringData() const;
     std::string toString() const;
@@ -52,9 +53,10 @@ public:
     explicit operator bool() const;
 
 private:
-    JSContext* _context = nullptr;
     std::unique_ptr<char[]> _str;
     size_t _length = 0;
+    char _buf[64];
+    bool _isSet = false;
 };
 
 }  // namespace mozjs
diff --git a/src/mongo/scripting/mozjs/objectwrapper.cpp b/src/mongo/scripting/mozjs/objectwrapper.cpp
index 8e1bc5aa92e..e467dfe8b48 100644
--- a/src/mongo/scripting/mozjs/objectwrapper.cpp
+++ b/src/mongo/scripting/mozjs/objectwrapper.cpp
@@ -198,23 +198,40 @@ void ObjectWrapper::Key::del(JSContext* cx, JS::HandleObject o) {
 }
 
 std::string ObjectWrapper::Key::toString(JSContext* cx) {
-    switch (_type) {
-        case Type::Field:
-            return _field;
-        case Type::Index:
-            return std::to_string(_idx);
-        case Type::Id: {
-            JS::RootedId id(cx, _id);
-            return IdWrapper(cx, id).toString();
-        }
-        case Type::InternedString: {
-            InternedStringId id(cx, _internedString);
-            return IdWrapper(cx, id).toString();
-        }
+    JSStringWrapper jsstr;
+    return toStringData(cx, &jsstr).toString();
+}
+
+StringData ObjectWrapper::Key::toStringData(JSContext* cx, JSStringWrapper* jsstr) {
+    if (_type == Type::Field) {
+        return _field;
     }
 
-    throwCurrentJSException(
-        cx, ErrorCodes::InternalError, "Failed to toString a ObjectWrapper::Key");
+    if (_type == Type::Index) {
+        *jsstr = JSStringWrapper(_idx);
+        return jsstr->toStringData();
+    }
+
+    JS::RootedId rid(cx);
+
+    if (_type == Type::Id) {
+        rid.set(_id);
+    } else {
+        InternedStringId id(cx, _internedString);
+        rid.set(id);
+    }
+
+    if (JSID_IS_INT(rid)) {
+        *jsstr = JSStringWrapper(JSID_TO_INT(rid));
+        return jsstr->toStringData();
+    }
+
+    if (JSID_IS_STRING(rid)) {
+        *jsstr = JSStringWrapper(cx, JSID_TO_STRING(rid));
+        return jsstr->toStringData();
+    }
+
+    uasserted(ErrorCodes::BadValue, "Couldn't convert key to String");
 }
 
 ObjectWrapper::ObjectWrapper(JSContext* cx, JS::HandleObject obj)
@@ -500,7 +517,9 @@ void ObjectWrapper::_writeField(BSONObjBuilder* b,
     ValueWriter x(_context, value);
     x.setOriginalBSON(originalParent);
 
-    x.writeThis(b, key.toString(_context), frames);
+    JSStringWrapper jsstr;
+
+    x.writeThis(b, key.toStringData(_context, &jsstr), frames);
 }
 
 std::string ObjectWrapper::getClassName() {
diff --git a/src/mongo/scripting/mozjs/objectwrapper.h b/src/mongo/scripting/mozjs/objectwrapper.h
index efc8039bf26..4463cd99c98 100644
--- a/src/mongo/scripting/mozjs/objectwrapper.h
+++ b/src/mongo/scripting/mozjs/objectwrapper.h
@@ -35,6 +35,7 @@
 #include "mongo/platform/decimal128.h"
 #include "mongo/scripting/mozjs/exception.h"
 #include "mongo/scripting/mozjs/internedstring.h"
+#include "mongo/scripting/mozjs/jsstringwrapper.h"
 #include "mongo/scripting/mozjs/lifetimestack.h"
 
 namespace mongo {
@@ -85,6 +86,7 @@ public:
         void define(JSContext* cx, JS::HandleObject o, JS::HandleValue value, unsigned attrs);
         void del(JSContext* cx, JS::HandleObject o);
         std::string toString(JSContext* cx);
+        StringData toStringData(JSContext* cx, JSStringWrapper* jsstr);
 
         union {
             const char* _field;
diff --git a/src/mongo/scripting/mozjs/valuewriter.cpp b/src/mongo/scripting/mozjs/valuewriter.cpp
index 6f57d48c460..33097e99e18 100644
--- a/src/mongo/scripting/mozjs/valuewriter.cpp
+++ b/src/mongo/scripting/mozjs/valuewriter.cpp
@@ -121,7 +121,13 @@ BSONObj ValueWriter::toBSON() {
 }
 
 std::string ValueWriter::toString() {
-    return JSStringWrapper(_context, JS::ToString(_context, _value)).toString();
+    JSStringWrapper jsstr;
+    return toStringData(&jsstr).toString();
+}
+
+StringData ValueWriter::toStringData(JSStringWrapper* jsstr) {
+    *jsstr = JSStringWrapper(_context, JS::ToString(_context, _value));
+    return jsstr->toStringData();
 }
 
 double ValueWriter::toNumber() {
@@ -192,7 +198,8 @@ void ValueWriter::writeThis(BSONObjBuilder* b,
             (std::string::npos == sd.find('\0')));
 
     if (_value.isString()) {
-        b->append(sd, toString());
+        JSStringWrapper jsstr;
+        b->append(sd, toStringData(&jsstr));
     } else if (_value.isNumber()) {
         double val = toNumber();
 
diff --git a/src/mongo/scripting/mozjs/valuewriter.h b/src/mongo/scripting/mozjs/valuewriter.h
index 65f6f418d09..9947f61d741 100644
--- a/src/mongo/scripting/mozjs/valuewriter.h
+++ b/src/mongo/scripting/mozjs/valuewriter.h
@@ -54,6 +54,7 @@ public:
      * toNumber()
      */
     std::string toString();
+    StringData toStringData(JSStringWrapper* jsstr);
     int type();
     double toNumber();
     int32_t toInt32();