Windows: factor out mutex lock/unlock from sharing.c.

This will let me reuse them easily in Pageant setup.

windows/CMakeLists.txt

@@ -13,6 +13,7 @@ add_sources_from_current_dir(utils
   utils/getdlgitemtext_alloc.c
   utils/get_system_dir.c
   utils/get_username.c
+  utils/interprocess_mutex.c
   utils/is_console_handle.c
   utils/load_system32_dll.c
   utils/ltime.c

windows/platform.h

@@ -740,4 +740,7 @@ void plug_closing_winsock_error(Plug *plug, DWORD error);
 
 SeatPromptResult make_spr_sw_abort_winerror(const char *prefix, DWORD error);
 
+HANDLE lock_interprocess_mutex(const char *mutexname, char **error);
+void unlock_interprocess_mutex(HANDLE mutex);
+
 #endif /* PUTTY_WINDOWS_PLATFORM_H */

windows/sharing.c

@@ -36,8 +36,6 @@ int platform_ssh_share(const char *pi_name, Conf *conf,
     char *name, *mutexname, *pipename;
     HANDLE mutex;
     Socket *retsock;
-    PSECURITY_DESCRIPTOR psd;
-    PACL acl;
 
     /*
      * Transform the platform-independent version of the connection
@@ -57,39 +55,12 @@ int platform_ssh_share(const char *pi_name, Conf *conf,
      * Make a mutex name out of the connection identifier, and lock it
      * while we decide whether to be upstream or downstream.
      */
-    {
+    mutexname = make_name(CONNSHARE_MUTEX_PREFIX, name);
-        SECURITY_ATTRIBUTES sa;
+    mutex = lock_interprocess_mutex(mutexname, logtext);
-
+    if (!mutex) {
-        mutexname = make_name(CONNSHARE_MUTEX_PREFIX, name);
-        if (!make_private_security_descriptor(MUTEX_ALL_ACCESS,
-                                              &psd, &acl, logtext)) {
-            sfree(mutexname);
-            sfree(name);
-            return SHARE_NONE;
-        }
-
-        memset(&sa, 0, sizeof(sa));
-        sa.nLength = sizeof(sa);
-        sa.lpSecurityDescriptor = psd;
-        sa.bInheritHandle = false;
-
-        mutex = CreateMutex(&sa, false, mutexname);
-
-        if (!mutex) {
-            *logtext = dupprintf("CreateMutex(\"%s\") failed: %s",
-                                 mutexname, win_strerror(GetLastError()));
-            sfree(mutexname);
-            sfree(name);
-            LocalFree(psd);
-            LocalFree(acl);
-            return SHARE_NONE;
-        }
-
         sfree(mutexname);
-        LocalFree(psd);
+        sfree(name);
-        LocalFree(acl);
+        return SHARE_NONE;
-
-        WaitForSingleObject(mutex, INFINITE);
     }
 
     pipename = make_name(CONNSHARE_PIPE_PREFIX, name);
@@ -103,8 +74,7 @@ int platform_ssh_share(const char *pi_name, Conf *conf,
             *logtext = pipename;
             *sock = retsock;
             sfree(name);
-            ReleaseMutex(mutex);
+            unlock_interprocess_mutex(mutex);
-            CloseHandle(mutex);
             return SHARE_DOWNSTREAM;
         }
         sfree(*ds_err);

windows/utils/interprocess_mutex.c

@@ -0,0 +1,48 @@
+/*
+ * Lock and unlock a mutex with a globally visible name. Used to
+ * synchronise between unrelated processes, such as two
+ * connection-sharing PuTTYs deciding which will be the upstream.
+ */
+
+#include "putty.h"
+#include "security-api.h"
+
+HANDLE lock_interprocess_mutex(const char *mutexname, char **error)
+{
+    PSECURITY_DESCRIPTOR psd = NULL;
+    PACL acl = NULL;
+    HANDLE mutex = NULL;
+
+    if (!make_private_security_descriptor(MUTEX_ALL_ACCESS,
+                                          &psd, &acl, error))
+        goto out;
+
+    SECURITY_ATTRIBUTES sa;
+    memset(&sa, 0, sizeof(sa));
+    sa.nLength = sizeof(sa);
+    sa.lpSecurityDescriptor = psd;
+    sa.bInheritHandle = false;
+
+    mutex = CreateMutex(&sa, false, mutexname);
+    if (!mutex) {
+        *error = dupprintf("CreateMutex(\"%s\") failed: %s",
+                           mutexname, win_strerror(GetLastError()));
+        goto out;
+    }
+
+    WaitForSingleObject(mutex, INFINITE);
+
+  out:
+    if (psd)
+        LocalFree(psd);
+    if (acl)
+        LocalFree(acl);
+
+    return mutex;
+}
+
+void unlock_interprocess_mutex(HANDLE mutex)
+{
+    ReleaseMutex(mutex);
+    CloseHandle(mutex);
+}