From 3963bea16d6c74acdba9366ced3fbcfb37911f0b Mon Sep 17 00:00:00 2001
From: sugarme <thangtran@hotmail.com>
Date: Wed, 3 Jun 2020 11:03:38 +1000
Subject: [PATCH] feat(wrapper/util): TorchErr

---
 libtch/README.md  | 13 ++++++++
 libtch/tensor.go  |  4 +++
 wrapper/error.go  | 84 ++++++++++++++++++++++++++---------------------
 wrapper/tensor.go |  6 +++-
 4 files changed, 69 insertions(+), 38 deletions(-)

diff --git a/libtch/README.md b/libtch/README.md
index 345372e..cf16a09 100644
--- a/libtch/README.md
+++ b/libtch/README.md
@@ -95,5 +95,18 @@ then in the return of function body
 
 ```
 
+### C type pointers e.g. `char *FUNCTION()` --> `*C.char`
+
+then just return the C function call. 
+
+```c
+char *get_and_reset_last_err(); // thread-local
+```
+
+```go
+func GetAndResetLastErr() *C.char{
+   return C.get_and_reset_last_err()
+}
+```
 
 
diff --git a/libtch/tensor.go b/libtch/tensor.go
index c8a66de..f227977 100644
--- a/libtch/tensor.go
+++ b/libtch/tensor.go
@@ -58,3 +58,7 @@ func AtScalarType(t *C_tensor) int32 {
 	c_result := C.at_scalar_type(c_tensor)
 	return *(*int32)(unsafe.Pointer(&c_result))
 }
+
+func GetAndResetLastErr() *C.char {
+	return C.get_and_reset_last_err()
+}
diff --git a/wrapper/error.go b/wrapper/error.go
index 01c1459..2a59523 100644
--- a/wrapper/error.go
+++ b/wrapper/error.go
@@ -1,39 +1,49 @@
 package wrapper
 
-/*
- * import "C"
- *
- * import (
- *   "fmt"
- * )
- *
- * // ptrToString returns nil on the null pointer. If not null,
- * // the pointer gets freed.
- * // NOTE: C does not have exception design. C++ throws exception
- * // to stderr. This code to check stderr for any err message,
- * // if it exists, takes it and frees up C pointer.
- * func ptrToString(ptr *C.c_char) string {
- *   var str string
- *   if !ptr.is_null() {
- *     // TODO: implement this
- *     // str := GET_ERROR_FROM C std::err
- *     C.free(ptr)
- *     return str
- *   } else {
- *     return ""
- *   }
- * }
- *
- * // readAndCleanError wraps error handling and C memory free up
- * func UnsafeTorch(f func()) (retF func(), err error) {
- *
- *   var str string
- *   // TODO: implement this
- *   // str := ptrToString(torch_sys.get_and_reset_last_err())
- *   if str != "" {
- *     err = fmt.Errorf("Unsafe error: %v\n", err.Error())
- *     return nil, err
- *   } else {
- *     return f, nil
- *   }
- * } */
+// #include <stdlib.h>
+import "C"
+
+import (
+	"fmt"
+	"unsafe"
+
+	lib "github.com/sugarme/gotch/libtch"
+)
+
+// ptrToString returns nil on the null pointer. If not null,
+// the pointer gets freed.
+// NOTE: C does not have exception design. C++ throws exception
+// to stderr. This code to check stderr for any err message,
+// if it exists, takes it and frees up C pointer.
+func ptrToString(cptr *C.char) string {
+	var str string
+
+	str = *(*string)(unsafe.Pointer(&cptr))
+	fmt.Printf("Err Msg from C: %v\n", str)
+	if str != "" {
+		// Free up C memory
+		C.free(unsafe.Pointer(cptr))
+		return str
+	} else {
+		return ""
+	}
+}
+
+// TorchErr checks and retrieves last error message from
+// C `thread_local` if existing and frees up C memory the C pointer
+// points to.
+//
+// NOTE: Go language atm does not have generic function something
+// similar to `macro` in Rust language, does it? So we have to
+// wrap this function to any Libtorch C function call to check error
+// instead of doing the other way around.
+// See Go2 proposal: https://github.com/golang/go/issues/32620
+func TorchErr() error {
+	cptr := (*C.char)(lib.GetAndResetLastErr())
+	errStr := ptrToString(cptr)
+	if errStr != "" {
+		return fmt.Errorf("Libtorch API Err: %v\n", errStr)
+	}
+
+	return nil
+}
diff --git a/wrapper/tensor.go b/wrapper/tensor.go
index 2a2817c..163cc6d 100644
--- a/wrapper/tensor.go
+++ b/wrapper/tensor.go
@@ -26,7 +26,11 @@ func NewTensor() Tensor {
 }
 
 func (ts Tensor) Dim() uint64 {
-	return lib.AtDim(ts.ctensor)
+	retVal := lib.AtDim(ts.ctensor)
+	if err := TorchErr(); err != nil {
+		log.Fatal(err)
+	}
+	return retVal
 }
 
 // Size return shape of the tensor