From afcd7c33e72666494b8f6ed354ac6ad5a6222a25 Mon Sep 17 00:00:00 2001
From: Martin Haefner <m.haefner@cab.de>
Date: Mon, 23 Aug 2021 11:47:28 +0200
Subject: [PATCH] std::ref support for FileDescriptor to reduce dup() calls

---
 include/simppl/filedescriptor.h | 13 ++++++
 src/filedescriptor.cpp          | 81 +++++++++++++++++++++++----------
 2 files changed, 71 insertions(+), 23 deletions(-)

diff --git a/include/simppl/filedescriptor.h b/include/simppl/filedescriptor.h
index 38b4e76..9ac2175 100644
--- a/include/simppl/filedescriptor.h
+++ b/include/simppl/filedescriptor.h
@@ -4,6 +4,9 @@
 
 #include "simppl/serialization.h"
 
+#include <unistd.h>
+#include <functional>
+
 
 namespace simppl
 {
@@ -21,9 +24,18 @@ class FileDescriptor
 
    FileDescriptor();
 
+   /**
+    * RAII behaviour: destroys the fd by calling close on object destruction.
+    */
    explicit
    FileDescriptor(int fd);
 
+   /**
+    * Non-RAII behaviour: keeps fd open on object destruction.
+    */
+   explicit
+   FileDescriptor(std::reference_wrapper<int> fd);
+
    ~FileDescriptor();
 
    FileDescriptor(const FileDescriptor& rhs);
@@ -38,6 +50,7 @@ class FileDescriptor
 
 private:
    int fd_;
+   int (*destructor_)(int);
 };
 
 
diff --git a/src/filedescriptor.cpp b/src/filedescriptor.cpp
index c693db4..cfef731 100644
--- a/src/filedescriptor.cpp
+++ b/src/filedescriptor.cpp
@@ -2,18 +2,30 @@
 
 #include <cassert>
 
-#include <unistd.h>
+
+namespace {
+
+    int noop(int)
+    {
+        // NOOP
+        return 0;
+    }
+}
+
+
+// ---------------------------------------------------------------------
 
 
 namespace simppl
 {
-   
+
 namespace dbus
 {
 
 
 FileDescriptor::FileDescriptor()
  : fd_(-1)
+ , destructor_(&close)
 {
    // NOOP
 }
@@ -21,23 +33,38 @@ FileDescriptor::FileDescriptor()
 
 FileDescriptor::FileDescriptor(int fd)
  : fd_(fd)
+ , destructor_(&close)
+{
+   assert(fd >= 0);
+}
+
+
+FileDescriptor::FileDescriptor(std::reference_wrapper<int> fd)
+ : fd_(fd)
+ , destructor_(&noop)
 {
    assert(fd >= 0);
 }
 
-   
+
 FileDescriptor::~FileDescriptor()
 {
    if (fd_ != -1)
-      close(fd_);
+      (*destructor_)(fd_);
 }
 
-   
+
 FileDescriptor::FileDescriptor(const FileDescriptor& rhs)
  : fd_(-1)
+ , destructor_(rhs.destructor_)
 {
    if (rhs.fd_ != -1)
-      fd_ = dup(rhs.fd_);
+   {
+       if (destructor_ == &noop)
+          fd_ = rhs.fd_;
+       else
+          fd_ = dup(rhs.fd_);
+   }
 }
 
 
@@ -46,22 +73,28 @@ FileDescriptor& FileDescriptor::operator=(const FileDescriptor& rhs)
    if (this != &rhs)
    {
       if (fd_ != -1)
-         close(fd_);
-      
+         (*destructor_)(fd_);
+
       if (rhs.fd_ != -1)
       {
-         fd_ = dup(rhs.fd_);
+         if (rhs.destructor_ == &noop)
+            fd_ = rhs.fd_;
+         else
+            fd_ = dup(rhs.fd_);
       }
       else
          fd_ = -1;
+
+      destructor_ = rhs.destructor_;
    }
-   
+
    return *this;
 }
 
 
 FileDescriptor::FileDescriptor(FileDescriptor&& rhs)
  : fd_(rhs.fd_)
+ , destructor_(rhs.destructor_)
 {
    rhs.fd_ = -1;
 }
@@ -70,13 +103,13 @@ FileDescriptor::FileDescriptor(FileDescriptor&& rhs)
 FileDescriptor& FileDescriptor::operator=(int fd)
 {
    if (fd_ != -1)
-      close(fd_);
-      
+      (*destructor_)(fd_);
+
    fd_ = fd;
-   
+
    if (fd_ < 0)
       fd_ = -1;
-      
+
    return *this;
 }
 
@@ -84,11 +117,13 @@ FileDescriptor& FileDescriptor::operator=(int fd)
 FileDescriptor& FileDescriptor::operator=(FileDescriptor&& rhs)
 {
    if (fd_ != -1)
-      close(fd_);
-      
+      (*destructor_)(fd_);
+
    fd_ = rhs.fd_;
    rhs.fd_ = -1;
-   
+
+   destructor_ = rhs.destructor_;
+
    return *this;
 }
 
@@ -103,12 +138,12 @@ int FileDescriptor::release()
 {
    int fd = fd_;
    fd_ = -1;
-   
+
    return fd;
 }
 
 
-/*static*/ 
+/*static*/
 void FileDescriptorCodec::encode(DBusMessageIter& iter, const FileDescriptor& fd)
 {
    int _fd = fd.native_handle();
@@ -116,7 +151,7 @@ void FileDescriptorCodec::encode(DBusMessageIter& iter, const FileDescriptor& fd
 }
 
 
-/*static*/ 
+/*static*/
 void FileDescriptorCodec::decode(DBusMessageIter& iter, FileDescriptor& fd)
 {
    int _fd;
@@ -125,13 +160,13 @@ void FileDescriptorCodec::decode(DBusMessageIter& iter, FileDescriptor& fd)
 }
 
 
-/*static*/ 
+/*static*/
 std::ostream& FileDescriptorCodec::make_type_signature(std::ostream& os)
 {
    return os << DBUS_TYPE_UNIX_FD_AS_STRING;
 }
-   
-   
+
+
 }   // namespace dbus
 
 }   // namespace simppl