libgolang: _selcase: Introduce accessors to get pointers to for-send and for-recv data

In the next patch we are going to teach _selcase to support both
external and inplace data. Before that let's do a couple of preparatory
things.

This patch: introduces .ptx() and .prx() accessors to get to
corresponding data buffer associated with _selcase. Convert _selcase
users to use .ptx() and .prx() instead of doing direct .ptxrx access.
This way when we'll add inplace support to _selcase, we'll need to adapt
only accessors, not clients.

The only place that is left using .ptxrx directly is one tricky place in
_chanselect2<onstack=false> where gevent-related code needs to carefully
deal with proxying tx/rx buffers due to STACK_DEAD_WHILE_PARKED.

NOTE even though new accessors may panic, libgolang.cpp always calls them
after checking that the conditions for ptx/prx calls are valid.

golang/_golang.pxd

@@ -103,10 +103,14 @@ cdef extern from "golang/libgolang.h" namespace "golang" nogil:
         _CHANSEND
         _CHANRECV
         _DEFAULT
-    struct _selcase:
+    cppclass _selcase:
         _chanop op
         void    *ptxrx
         cbool   *rxok
+
+        const void *ptx() const
+        void *prx() const
+
     const _selcase default "golang::_default"
 
     int select(_selcase casev[])

golang/_golang.pyx

@@ -331,7 +331,7 @@ def pyselect(*pycasev):
         # decref not sent tx (see ^^^ send prepare)
         for i in range(n):
             if casev[i].op == _CHANSEND and (i != selected):
-                p_tx = <PyObject **>casev[i].ptxrx
+                p_tx = <PyObject **>casev[i].ptx()
                 _tx  = p_tx[0]
                 tx   = <object>_tx
                 Py_DECREF(tx)

golang/libgolang.h

@@ -156,6 +156,16 @@ typedef struct _selcase {
     enum _chanop    op;     // chansend/chanrecv/default
     void            *ptxrx; // chansend: ptx; chanrecv: prx
     bool            *rxok;  // chanrecv: where to save ok if !NULL; otherwise not used
+
+#ifdef __cplusplus
+    // ptx returns pointer to data to send for this case.
+    // .op must be _CHANSEND.
+    LIBGOLANG_API const void *ptx() const;
+
+    // prx returns pointer for this case to receive data into.
+    // .op must be _CHANRECV.
+    LIBGOLANG_API void *prx() const;
+#endif
 } _selcase;
 
 LIBGOLANG_API int _chanselect(const _selcase *casev, int casec);

golang/runtime/libgolang.cpp

@@ -863,6 +863,21 @@ const _selcase _default = {
     .rxok   = NULL,
 };
 
+const void *_selcase::ptx() const {
+    const _selcase *cas = this;
+    if (cas->op != _CHANSEND)
+        panic("_selcase: ptx: op != send");
+    return cas->ptxrx;
+}
+
+void *_selcase::prx() const {
+    const _selcase *cas = this;
+    if (cas->op != _CHANRECV)
+        panic("_selcase: prx: op != recv");
+    return cas->ptxrx;
+}
+
+
 static const _RecvSendWaiting _sel_txrx_prepoll_won;
 template<bool onstack> static int _chanselect2(const _selcase *, int, const vector<int>&);
 template<> int _chanselect2</*onstack=*/true> (const _selcase *, int, const vector<int>&);
@@ -916,7 +931,7 @@ int _chanselect(const _selcase *casev, int casec) {
             if (ch != NULL) {   // nil chan is never ready
                 ch->_mu.lock();
                 if (1) {
-                    bool done = ch->_trysend(cas->ptxrx);
+                    bool done = ch->_trysend(cas->ptx());
                     if (done)
                         return n;
                 }
@@ -930,7 +945,7 @@ int _chanselect(const _selcase *casev, int casec) {
             if (ch != NULL) {   // nil chan is never ready
                 ch->_mu.lock();
                 if (1) {
-                    bool ok, done = ch->_tryrecv(cas->ptxrx, &ok);
+                    bool ok, done = ch->_tryrecv(cas->prx(), &ok);
                     if (done) {
                         if (cas->rxok != NULL)
                             *cas->rxok = ok;
@@ -1084,7 +1099,7 @@ static int __chanselect2(const _selcase *casev, int casec, const vector<int>& nv
 
             // send
             if (cas->op == _CHANSEND) {
-                bool done = ch->_trysend(cas->ptxrx);
+                bool done = ch->_trysend(cas->ptx());
                 if (done) {
                     g->which = &_sel_txrx_prepoll_won; // !NULL not to let already queued cases win
                     return n;
@@ -1095,7 +1110,7 @@ static int __chanselect2(const _selcase *casev, int casec, const vector<int>& nv
                 _RecvSendWaiting *w = &waitv[waitc++];
 
                 w->init(g, ch);
-                w->pdata = cas->ptxrx;
+                w->pdata = (void *)cas->ptx();
                 w->ok    = false;
                 w->sel_n = n;
 
@@ -1104,7 +1119,7 @@ static int __chanselect2(const _selcase *casev, int casec, const vector<int>& nv
 
             // recv
             else if (cas->op == _CHANRECV) {
-                bool ok, done = ch->_tryrecv(cas->ptxrx, &ok);
+                bool ok, done = ch->_tryrecv(cas->prx(), &ok);
                 if (done) {
                     g->which = &_sel_txrx_prepoll_won; // !NULL not to let already queued cases win
                     if (cas->rxok != NULL)
@@ -1117,7 +1132,7 @@ static int __chanselect2(const _selcase *casev, int casec, const vector<int>& nv
                 _RecvSendWaiting *w = &waitv[waitc++];
 
                 w->init(g, ch);
-                w->pdata = cas->ptxrx;
+                w->pdata = cas->prx();
                 w->ok    = false;
                 w->sel_n = n;