From e18f650fc9598a2abe63cec499da2b46c0d4d169 Mon Sep 17 00:00:00 2001
From: Nicholas Ng <nickng@nickng.io>
Date: Fri, 12 Oct 2018 16:11:49 +0100
Subject: [PATCH 1/2] Detect and panic if final statetype has error.

---
 .../core/codegen/statetype3/RPCoreSTSessionApiBuilder.java  | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/scribble-go/src/main/java/org/scribble/ext/go/core/codegen/statetype3/RPCoreSTSessionApiBuilder.java b/scribble-go/src/main/java/org/scribble/ext/go/core/codegen/statetype3/RPCoreSTSessionApiBuilder.java
index 0db11d832..39f6a68db 100644
--- a/scribble-go/src/main/java/org/scribble/ext/go/core/codegen/statetype3/RPCoreSTSessionApiBuilder.java
+++ b/scribble-go/src/main/java/org/scribble/ext/go/core/codegen/statetype3/RPCoreSTSessionApiBuilder.java
@@ -569,7 +569,11 @@ public int compare(Entry<Integer, String> o1, Entry<Integer, String> o2)
 												// cf. state chan builder  // FIXME: chan struct reuse
 										//"ini._" + init + ".id = 1\n" +
 												"return f(ini._" + init + ")\n")*/
-							+ "return f(ini.Init())\n"
+							+ "end := f(ini.Init())\n"
+							+ "if end." + RPCoreSTApiGenConstants.GO_MPCHAN_ERR + " != nil {\n"
+							+ "panic(end." + RPCoreSTApiGenConstants.GO_MPCHAN_ERR + ")\n"
+							+ "}\n"
+							+ "return end\n"
 
 							+ "}";
 					

From 062522ae2caa7ce3ef39c17f95fe0542261468bd Mon Sep 17 00:00:00 2001
From: Nicholas Ng <nickng@nickng.io>
Date: Fri, 12 Oct 2018 17:44:31 +0100
Subject: [PATCH 2/2] Patch error handling for Send functions.

---
 .../statetype3/RPCoreSTSendActionBuilder.java | 28 +++++++++++++++----
 1 file changed, 22 insertions(+), 6 deletions(-)

diff --git a/scribble-go/src/main/java/org/scribble/ext/go/core/codegen/statetype3/RPCoreSTSendActionBuilder.java b/scribble-go/src/main/java/org/scribble/ext/go/core/codegen/statetype3/RPCoreSTSendActionBuilder.java
index 24b4cd687..9712de0e4 100644
--- a/scribble-go/src/main/java/org/scribble/ext/go/core/codegen/statetype3/RPCoreSTSendActionBuilder.java
+++ b/scribble-go/src/main/java/org/scribble/ext/go/core/codegen/statetype3/RPCoreSTSendActionBuilder.java
@@ -42,6 +42,24 @@ public String buildArgs(STStateChanApiBuilder api, EAction a)
 		}
 	}
 
+	/**
+	 * checkError helper function surrounds the given expression expr with Go's error check
+	 * and assigns errorField with err if it exists, i.e.
+	 *
+	 * <pre>{@code
+	 * if err := $expression; err != nil {
+	 * 	 $errField = err
+	 * }
+	 * }</pre>
+	 *
+	 * @param expr Expression that returns error
+	 * @param errField Location to store the error
+	 * @return code fragment of expr with error check
+	 */
+	private String checkError(String expr, String errField) {
+		return "if err := " + expr + "; err != nil {\n" + errField + " = err\n" + "}\n";
+	}
+
 	@Override
 	public String buildBody(STStateChanApiBuilder api, EState curr, EAction a, EState succ)
 	{
@@ -131,23 +149,21 @@ public String buildBody(STStateChanApiBuilder api, EState curr, EAction a, EStat
 						throw new RuntimeException("[rp-core] [param-api] TODO: " + a);
 					}
 				
-					res += errorField + " = "
-							+ sEpWrite
+					res +=  checkError(sEpWrite
 												+ "."
 												+ (a.mid.isOp() ? RPCoreSTApiGenConstants.GO_MPCHAN_ISEND : RPCoreSTApiGenConstants.GO_MPCHAN_MSEND)
 												+ "(\"" + r.getName() + "\", i" 
 												+ IntStream.range(0, a.payload.elems.size()).mapToObj(i -> ", &arg" + i + "[j]").collect(Collectors.joining(""))
-												+ ")\n";
+												+ ")", errorField);
 				}
 			}
 			else //(a.mid.isMessageSigName)
 			{
 				// FIXME: factor out with above
-				res += errorField + " = "
-						+ sEpWrite
+				res += checkError( sEpWrite
 											+ "."
 											+ (a.mid.isOp() ? RPCoreSTApiGenConstants.GO_MPCHAN_ISEND : RPCoreSTApiGenConstants.GO_MPCHAN_MSEND)
-											+ "(\"" + r.getName() + "\", i, &arg0[j])\n";
+											+ "(\"" + r.getName() + "\", i, &arg0[j])", errorField);
 			}
 		}