library: Add new warning: ignoredReturnErrorCode (#2877)

* library: Add optional "type" attribute to "use-retval"

Added an optional "type" attribute to "use-retval" nodes in the
configuration. When the return type of a function configured with
`<use-retval type="error-code"\>` node does not used, the new style
error "ignoredReturnErrorCode" will be generated.

* Fix and improve patch after the initial review

* Fixed severity level and [[nodiscard]] attribute

* Fix incorrect condition

* Remove redundant condition

lib/checkfunctions.cpp

@@ -192,7 +192,7 @@ void CheckFunctions::invalidFunctionArgStrError(const Token *tok, const std::str
 //---------------------------------------------------------------------------
 void CheckFunctions::checkIgnoredReturnValue()
 {
-    if (!mSettings->isEnabled(Settings::WARNING))
+    if (!mSettings->isEnabled(Settings::WARNING) && !mSettings->isEnabled(Settings::STYLE))
         return;
 
     const SymbolDatabase *symbolDatabase = mTokenizer->getSymbolDatabase();
@@ -216,9 +216,15 @@ void CheckFunctions::checkIgnoredReturnValue()
             }
 
             if ((!tok->function() || !Token::Match(tok->function()->retDef, "void %name%")) &&
-                (mSettings->library.isUseRetVal(tok) || (tok->function() && tok->function()->isAttributeNodiscard())) &&
                  !WRONG_DATA(!tok->next()->astOperand1(), tok)) {
+                const Library::UseRetValType retvalTy = mSettings->library.getUseRetValType(tok);
+                if (mSettings->isEnabled(Settings::WARNING) &&
+                    ((retvalTy == Library::UseRetValType::DEFAULT) ||
+                    (tok->function() && tok->function()->isAttributeNodiscard())))
                   ignoredReturnValueError(tok, tok->next()->astOperand1()->expressionString());
+                else if (mSettings->isEnabled(Settings::STYLE) &&
+                         retvalTy == Library::UseRetValType::ERROR_CODE)
+                  ignoredReturnErrorCode(tok, tok->next()->astOperand1()->expressionString());
             }
         }
     }
@@ -230,6 +236,11 @@ void CheckFunctions::ignoredReturnValueError(const Token* tok, const std::string
                 "$symbol:" + function + "\nReturn value of function $symbol() is not used.", CWE252, false);
 }
 
+void CheckFunctions::ignoredReturnErrorCode(const Token* tok, const std::string& function)
+{
+    reportError(tok, Severity::style, "ignoredReturnErrorCode",
+                "$symbol:" + function + "\nError code from the return value of function $symbol() is not used.", CWE252, false);
+}
 
 //---------------------------------------------------------------------------
 // Detect passing wrong values to <cmath> functions like atan(0, x);

lib/checkfunctions.h

@@ -109,6 +109,7 @@ private:
     void invalidFunctionArgBoolError(const Token *tok, const std::string &functionName, int argnr);
     void invalidFunctionArgStrError(const Token *tok, const std::string &functionName, nonneg int argnr);
     void ignoredReturnValueError(const Token* tok, const std::string& function);
+    void ignoredReturnErrorCode(const Token* tok, const std::string& function);
     void mathfunctionCallWarning(const Token *tok, const nonneg int numParam = 1);
     void mathfunctionCallWarning(const Token *tok, const std::string& oldexp, const std::string& newexp);
     void memsetZeroBytesError(const Token *tok);

lib/library.cpp

@@ -655,9 +655,12 @@ Library::Error Library::loadFunction(const tinyxml2::XMLElement * const node, co
             func.isconst = true; // a constant function is pure
         } else if (functionnodename == "leak-ignore")
             func.leakignore = true;
-        else if (functionnodename == "use-retval")
+        else if (functionnodename == "use-retval") {
-            func.useretval = true;
+            func.useretval = Library::UseRetValType::DEFAULT;
-        else if (functionnodename == "returnValue") {
+            if (const char *type = functionnode->Attribute("type"))
+                if (std::strcmp(type, "error-code") == 0)
+                    func.useretval = Library::UseRetValType::ERROR_CODE;
+        } else if (functionnodename == "returnValue") {
             if (const char *expr = functionnode->GetText())
                 mReturnValue[name] = expr;
             if (const char *type = functionnode->Attribute("type"))
@@ -1278,14 +1281,14 @@ bool Library::formatstr_secure(const Token* ftok) const
     return functions.at(getFunctionName(ftok)).formatstr_secure;
 }
 
-bool Library::isUseRetVal(const Token* ftok) const
+Library::UseRetValType Library::getUseRetValType(const Token *ftok) const
 {
     if (isNotLibraryFunction(ftok))
-        return false;
+        return Library::UseRetValType::NONE;
     const std::map<std::string, Function>::const_iterator it = functions.find(getFunctionName(ftok));
     if (it != functions.cend())
         return it->second.useretval;
-    return false;
+    return Library::UseRetValType::NONE;
 }
 
 const std::string& Library::returnValue(const Token *ftok) const

lib/library.h

@@ -179,7 +179,8 @@ public:
     bool isNotLibraryFunction(const Token *ftok) const;
     bool matchArguments(const Token *ftok, const std::string &functionName) const;
 
-    bool isUseRetVal(const Token* ftok) const;
+    enum class UseRetValType { NONE, DEFAULT, ERROR_CODE };
+    UseRetValType getUseRetValType(const Token* ftok) const;
 
     const std::string& returnValue(const Token *ftok) const;
     const std::string& returnValueType(const Token *ftok) const;
@@ -306,12 +307,12 @@ public:
         bool leakignore;
         bool isconst;
         bool ispure;
-        bool useretval;
+        UseRetValType useretval;
         bool ignore;  // ignore functions/macros from a library (gtk, qt etc)
         bool formatstr;
         bool formatstr_scan;
         bool formatstr_secure;
-        Function() : use(false), leakignore(false), isconst(false), ispure(false), useretval(false), ignore(false), formatstr(false), formatstr_scan(false), formatstr_secure(false) {}
+        Function() : use(false), leakignore(false), isconst(false), ispure(false), useretval(UseRetValType::NONE), ignore(false), formatstr(false), formatstr_scan(false), formatstr_secure(false) {}
     };
 
     const Function *getFunction(const Token *ftok) const;

test/testfunctions.cpp

@@ -76,6 +76,7 @@ private:
 
         // Ignored return value
         TEST_CASE(checkIgnoredReturnValue);
+        TEST_CASE(checkIgnoredErrorCode);
 
         // memset..
         TEST_CASE(memsetZeroBytes);
@@ -1232,6 +1233,27 @@ private:
         ASSERT_EQUALS("", errout.str());
     }
 
+    void checkIgnoredErrorCode() {
+        Settings settings2;
+        settings2.addEnabled("style");
+        const char xmldata[] = "<?xml version=\"1.0\"?>\n"
+                               "<def version=\"2\">\n"
+                               "  <function name=\"mystrcmp\">\n"
+                               "    <use-retval type=\"error-code\"/>\n"
+                               "    <arg nr=\"1\"/>\n"
+                               "    <arg nr=\"2\"/>\n"
+                               "  </function>\n"
+                               "</def>";
+        tinyxml2::XMLDocument doc;
+        doc.Parse(xmldata, sizeof(xmldata));
+        settings2.library.load(doc);
+
+        check("void foo() {\n"
+              "  mystrcmp(a, b);\n"
+              "}", "test.cpp", &settings2);
+        ASSERT_EQUALS("[test.cpp:2]: (style) Error code from the return value of function mystrcmp() is not used.\n", errout.str());
+    }
+
     void memsetZeroBytes() {
         check("void f() {\n"
               "    memset(p, 10, 0x0);\n"