AK: Add a ScopeGuard helper that invokes a callback when destroyed.

This is useful when you want to ensure some little thing happens when you
exit a certain scope.

This patch makes use of it in LibC's netdb code to make sure we close the
connection to the LookupServer.

AK/ScopeGuard.h

@@ -0,0 +1,24 @@
+#pragma once
+
+namespace AK {
+
+template<typename Callback>
+class ScopeGuard {
+public:
+ ScopeGuard(Callback callback)
+ : m_callback(move(callback))
+ {
+ }
+
+ ~ScopeGuard()
+ {
+ m_callback();
+ }
+
+private:
+ Callback m_callback;
+};
+
+}
+
+using AK::ScopeGuard;

LibC/netdb.cpp

@@ -1,12 +1,13 @@
+#include <AK/AKString.h>
+#include <AK/Assertions.h>
+#include <AK/ScopeGuard.h>
+#include <Kernel/Net/IPv4.h>
+#include <arpa/inet.h>
 #include <netdb.h>
+#include <netinet/in.h>
 #include <stdio.h>
 #include <string.h>
-#include <arpa/inet.h>
-#include <netinet/in.h>
 #include <unistd.h>
-#include <AK/Assertions.h>
-#include <AK/AKString.h>
-#include <Kernel/Net/IPv4.h>
 
 extern "C" {
 
@@ -73,11 +74,15 @@ hostent* gethostbyname(const char* name)
  if (fd < 0)
  return nullptr;
 
+ auto close_fd_on_exit = ScopeGuard([fd] {
+ dbgprintf("closing fd\n");
+ close(fd);
+ });
+
  auto line = String::format("L%s\n", name);
  int nsent = write(fd, line.characters(), line.length());
  if (nsent < 0) {
  perror("write");
- close(fd);
  return nullptr;
  }
 
@@ -87,11 +92,9 @@ hostent* gethostbyname(const char* name)
  int nrecv = read(fd, buffer, sizeof(buffer) - 1);
  if (nrecv < 0) {
  perror("recv");
- close(fd);
  return nullptr;
  }
  buffer[nrecv] = '\0';
- close(fd);
 
  if (!memcmp(buffer, "Not found.", sizeof("Not found.") - 1))
  return nullptr;
@@ -129,18 +132,20 @@ hostent* gethostbyaddr(const void* addr, socklen_t addr_size, int type)
  if (fd < 0)
  return nullptr;
 
+ auto close_fd_on_exit = ScopeGuard([fd] {
+ close(fd);
+ });
+
  IPv4Address ipv4_address((const byte*)&((const in_addr*)addr)->s_addr);
 
  auto line = String::format("R%d.%d.%d.%d.in-addr.arpa\n",
  ipv4_address[3],
  ipv4_address[2],
  ipv4_address[1],
- ipv4_address[0]
- );
+ ipv4_address[0]);
  int nsent = write(fd, line.characters(), line.length());
  if (nsent < 0) {
  perror("write");
- close(fd);
  return nullptr;
  }
 
@@ -150,15 +155,13 @@ hostent* gethostbyaddr(const void* addr, socklen_t addr_size, int type)
  int nrecv = read(fd, buffer, sizeof(buffer) - 1);
  if (nrecv < 0) {
  perror("recv");
- close(fd);
  return nullptr;
  }
  if (nrecv > 1) {
  // Strip newline.
  buffer[nrecv - 1] = '\0';
  }
  buffer[nrecv] = '\0';
- close(fd);
 
  if (!memcmp(buffer, "Not found.", sizeof("Not found.") - 1))
  return nullptr;
@@ -175,5 +178,4 @@ hostent* gethostbyaddr(const void* addr, socklen_t addr_size, int type)
 
  return &__gethostbyaddr_buffer;
 }
-
 }