From 5ed76afa814812edaeaff2ea7b3227c18d5de5a6 Mon Sep 17 00:00:00 2001
From: Dmitry Vyukov <dvyukov@google.com>
Date: Tue, 4 Aug 2020 12:29:59 +0200
Subject: pkg/compiler: check for unused resources

If a resource is never used as an input, it is not useful.
It's effectively the same as using an integer.
Detect such cases, they are quite confusing.
Fix all existing errors in descriptions.
This uncovered some interesting bugs as well,
e.g. use of a completely unrelated fd subtype after copy-paste
(while the resource that was supposed to be used there is completely unused).
---
 pkg/compiler/check.go | 32 +++++++++++++++++++++++---------
 1 file changed, 23 insertions(+), 9 deletions(-)

(limited to 'pkg/compiler/check.go')

diff --git a/pkg/compiler/check.go b/pkg/compiler/check.go
index e8abc574e..9bb8c1c0c 100644
--- a/pkg/compiler/check.go
+++ b/pkg/compiler/check.go
@@ -547,16 +547,17 @@ type structDir struct {
 }
 
 func (comp *compiler) checkConstructors() {
-	ctors := make(map[string]bool) // resources for which we have ctors
+	ctors := make(map[string]bool)  // resources for which we have ctors
+	inputs := make(map[string]bool) // resources which are used as inputs
 	checked := make(map[structDir]bool)
 	for _, decl := range comp.desc.Nodes {
 		switch n := decl.(type) {
 		case *ast.Call:
 			for _, arg := range n.Args {
-				comp.checkTypeCtors(arg.Type, prog.DirIn, true, ctors, checked)
+				comp.checkTypeCtors(arg.Type, prog.DirIn, true, ctors, inputs, checked)
 			}
 			if n.Ret != nil {
-				comp.checkTypeCtors(n.Ret, prog.DirOut, true, ctors, checked)
+				comp.checkTypeCtors(n.Ret, prog.DirOut, true, ctors, inputs, checked)
 			}
 		}
 	}
@@ -564,17 +565,23 @@ func (comp *compiler) checkConstructors() {
 		switch n := decl.(type) {
 		case *ast.Resource:
 			name := n.Name.Name
-			if !ctors[name] && comp.used[name] {
+			if !comp.used[name] {
+				continue
+			}
+			if !ctors[name] {
 				comp.error(n.Pos, "resource %v can't be created"+
-					" (never mentioned as a syscall return value or output argument/field)",
-					name)
+					" (never mentioned as a syscall return value or output argument/field)", name)
+			}
+			if !inputs[name] {
+				comp.error(n.Pos, "resource %v is never used as an input"+
+					"(such resources are not useful)", name)
 			}
 		}
 	}
 }
 
 func (comp *compiler) checkTypeCtors(t *ast.Type, dir prog.Dir, isArg bool,
-	ctors map[string]bool, checked map[structDir]bool) {
+	ctors, inputs map[string]bool, checked map[structDir]bool) {
 	desc := comp.getTypeDesc(t)
 	if desc == typeResource {
 		// TODO(dvyukov): consider changing this to "dir == prog.DirOut".
@@ -589,6 +596,13 @@ func (comp *compiler) checkTypeCtors(t *ast.Type, dir prog.Dir, isArg bool,
 				r = comp.resources[r.Base.Ident]
 			}
 		}
+		if dir != prog.DirOut {
+			r := comp.resources[t.Ident]
+			for r != nil && !inputs[r.Name.Name] {
+				inputs[r.Name.Name] = true
+				r = comp.resources[r.Base.Ident]
+			}
+		}
 		return
 	}
 	if desc == typeStruct {
@@ -600,7 +614,7 @@ func (comp *compiler) checkTypeCtors(t *ast.Type, dir prog.Dir, isArg bool,
 		}
 		checked[key] = true
 		for _, fld := range s.Fields {
-			comp.checkTypeCtors(fld.Type, dir, false, ctors, checked)
+			comp.checkTypeCtors(fld.Type, dir, false, ctors, inputs, checked)
 		}
 		return
 	}
@@ -610,7 +624,7 @@ func (comp *compiler) checkTypeCtors(t *ast.Type, dir prog.Dir, isArg bool,
 	_, args, _ := comp.getArgsBase(t, isArg)
 	for i, arg := range args {
 		if desc.Args[i].Type == typeArgType {
-			comp.checkTypeCtors(arg, dir, desc.Args[i].IsArg, ctors, checked)
+			comp.checkTypeCtors(arg, dir, desc.Args[i].IsArg, ctors, inputs, checked)
 		}
 	}
 }
-- 
cgit mrf-deployment