Fix potential concurrency edge case (#21)

The issue with the select check read and write is that they previously werent wrapped in an atomic block. What can happen is that the check read/write returns true, then the thread gets context switch and another thread violates the invariant that the channel to operate on. By wrapping up all checks and channel operations in atomic blocks, we ensure that this cannot happen.

src/vm/oogavm-compiler.ts

@@ -343,6 +343,8 @@ const compileComp = {
                 instrs[wc++] = { tag: Opcodes.ENTER_SCOPE, num: declarations.length };
                 ce = compileTimeEnvironmentExtend(declarations, ce);
                 compile(compCase.operation.expression.channel, ce);
+                // start atomic needed to ensure that no illegal read from channel if context switched
+                instrs[wc++] = { tag: Opcodes.START_ATOMIC };
                 instrs[wc++] = { tag: Opcodes.CHECK_READ };
                 // this pushes either true or false depending on if channel is ready to be read from
                 // if channel is not ready to be read from, should jump to the next case
@@ -351,6 +353,7 @@ const compileComp = {
                 // now if we reach this instruction, channel could be read, so read from channel and assign to
                 // variable declaration if there was one, or pop
                 compile(compCase.operation, ce); // this handles assignment
+                instrs[wc++] = { tag: Opcodes.END_ATOMIC };
                 compile(compCase.body, ce);
                 instrs[wc++] = { tag: Opcodes.EXIT_SCOPE };
                 // same strategy as switch, all these GOTOs will go to the end
@@ -361,6 +364,7 @@ const compileComp = {
             } else if (compCase.tag === 'SelectReadCase') {
                 // This is a ChannelReadExpression that does not have a variable declaration
                 compile(compCase.operation.channel, ce);
+                instrs[wc++] = { tag: Opcodes.START_ATOMIC };
                 instrs[wc++] = { tag: Opcodes.CHECK_READ };
                 // this pushes either true or false depending on if channel is ready to be read from
                 // if channel is not ready to be read from, should jump to the next case
@@ -370,6 +374,7 @@ const compileComp = {
                 // variable declaration if there was one, or pop
                 compile(compCase.operation, ce); // this will read value and then pop
                 instrs[wc++] = { tag: Opcodes.POP };
+                instrs[wc++] = { tag: Opcodes.END_ATOMIC };
                 compile(compCase.body, ce);
                 // same strategy as switch, all these GOTOs will go to the end
                 jumps.push(wc);
@@ -380,12 +385,14 @@ const compileComp = {
                 // a write expression differs from a read in that no variable declaration
                 // push channel value and CHECK_WRITE
                 compile(compCase.operation.channel, ce);
+                instrs[wc++] = { tag: Opcodes.START_ATOMIC };
                 instrs[wc++] = { tag: Opcodes.CHECK_WRITE };
                 // this pushes either true or false depending on if channel is ready to be write to
                 // if channel is not ready to be write to, should jump to the next case
                 let jof = { tag: Opcodes.JOF, addr: 0 };
                 instrs[wc++] = jof;
                 compile(compCase.operation, ce);
+                instrs[wc++] = { tag: Opcodes.END_ATOMIC };
                 compile(compCase.body, ce);
                 // same strategy as switch, all these GOTOs will go to the end
                 jumps.push(wc);
@@ -396,12 +403,14 @@ const compileComp = {
                 console.log("Default case");
                 hasDefault = true;
                 compile(compCase.body, ce);
+                instrs[wc++] = { tag: Opcodes.END_ATOMIC };
                 jumps.push(wc);
                 instrs[wc++] = { tag: Opcodes.GOTO, addr: 0};
             } else {
                 throw new CompilerError('Unsupported select case in SelectStatement');
             }
         }
+        instrs[wc++] = { tag: Opcodes.END_ATOMIC };
         // If none of the cases were handled and there is no default, this is when we reach this compile section
         // we will push a BLOCK_THREAD instruction followed by a GOTO to the start of the case
         if (!hasDefault) {