exc: Contextf

A shorthand for when exception context is just formatted string.
Simplifies

	func doSomething(path string) {
		defer exc.Context(func() interface{} {
			return fmt.Sprintf("doing something %s", path)
		})()
		...

to

	func doSomething(path string) {
		defer exc.Contextf("doing something %s", path)
		...

exc/error.go

-// Copyright (C) 2015-2019  Nexedi SA and Contributors.
+// Copyright (C) 2015-2020  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -22,7 +22,7 @@
 // Raise and Catch allow to raise and catch exceptions.
 //
 // By default the error caught is the same error that was raised. However with
-// Context functions can arrange for context related to what they are doing to
+// Context* functions can arrange for context related to what they are doing to
 // be added to raised error as prefix, for example
 //
 //	func doSomething(path string) {
@@ -30,6 +30,11 @@
 //			return fmt.Sprintf("doing something %s", path)
 //		})()
 //
+// or
+//
+//	func doSomething(path string) {
+//		defer exc.Contextf("doing something %s", path)
+//		...
 //
 // Lacking such Context annotations Addcallingcontext allows to add function
 // names up to the exception point as the calling context. However this way
@@ -175,6 +180,20 @@ func Context(f func() interface{}) {
 	panic(Addcontext(e, arg))
 }
 
+// Contextf provides formatted string to be added to error as context on unwinding.
+//
+// It is shorthand for Context wrapping fmt.Sprintf.
+//
+// Must be called under defer.
+func Contextf(format string, argv ...interface{}) {
+	e := _errcatch(recover())
+	if e == nil {
+		return
+	}
+
+	panic(Addcontext(e, fmt.Sprintf(format, argv...)))
+}
+
 // Addcontext adds "prefix" context to error.
 func Addcontext(e *Error, arg interface{}) *Error {
 	return &Error{arg, e}

exc/error_test.go

-// Copyright (C) 2015-2017  Nexedi SA and Contributors.
+// Copyright (C) 2015-2020  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -104,6 +104,25 @@ func TestErrContext(t *testing.T) {
 	t.Fatal("error not caught")
 }
 
+func do_contextf1() {
+	defer Contextf("must not be added")
+	return // no exception
+}
+
+func do_contextf2() {
+	defer Contextf("hello %d world", 123)
+	do_raise1()
+}
+
+func TestErrContextf(t *testing.T) {
+	defer Catch(func(e *Error) {
+		verifyErrChain(t, e, "hello 123 world", 1)
+	})
+	do_contextf1()
+	do_contextf2()
+	t.Fatal("error not caught")
+}
+
 func do_raise11() {
 	do_raise1()
 }