wincred: do not write creds if nothing has changed

Do no bother writing out credentials if there is already an existing
credential for the same account, with the same secret/password value.

Author: mjcheetham

src/shared/Core/Interop/Windows/Native/Advapi32.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Runtime.InteropServices;
+using System.Text;
 using FILETIME = System.Runtime.InteropServices.ComTypes.FILETIME;
 
 namespace GitCredentialManager.Interop.Windows.Native
@@ -93,5 +94,16 @@ public struct Win32Credential
         public string TargetAlias;
         [MarshalAs(UnmanagedType.LPWStr)]
         public string UserName;
+
+        public string GetCredentialBlobAsString()
+        {
+            if (CredentialBlobSize != 0 && CredentialBlob != IntPtr.Zero)
+            {
+                byte[] passwordBytes = InteropUtils.ToByteArray(CredentialBlob, CredentialBlobSize);
+                return Encoding.Unicode.GetString(passwordBytes);
+            }
+
+            return null;
+        }
     }
 }

src/shared/Core/Interop/Windows/WindowsCredentialManager.cs

@@ -13,8 +13,6 @@ public class WindowsCredentialManager : ICredentialStore
 
         private readonly string _namespace;
 
-        #region Constructors
-
         /// <summary>
         /// Open the Windows Credential Manager vault for the current user.
         /// </summary>
@@ -26,10 +24,6 @@ public WindowsCredentialManager(string @namespace = null)
             _namespace = @namespace;
         }
 
-        #endregion
-
-        #region ICredentialStore
-
         public ICredential Get(string service, string account)
         {
             return Enumerate(service, account).FirstOrDefault();
@@ -66,6 +60,15 @@ public void AddOrUpdate(string service, string account, string secret)
                         // Create new entry with the account in the target name
                         targetName = CreateTargetName(service, account);
                     }
+                    else
+                    {
+                        // No need to write out credential if the account and secret/password are the same
+                        string existingSecret = existingCred.GetCredentialBlobAsString();
+                        if (StringComparer.Ordinal.Equals(existingSecret, secret))
+                        {
+                            return;
+                        }
+                    }
                 }
 
                 byte[] secretBytes = Encoding.Unicode.GetBytes(secret);
@@ -129,8 +132,6 @@ public bool Remove(string service, string account)
             return false;
         }
 
-        #endregion
-
         /// <summary>
         /// Check if we can persist credentials to for the current process and logon session.
         /// </summary>
@@ -205,14 +206,7 @@ private IEnumerable<WindowsCredential> Enumerate(string service, string account)
 
         private WindowsCredential CreateCredentialFromStructure(Win32Credential credential)
         {
-            string password = null;
-            if (credential.CredentialBlobSize != 0 && credential.CredentialBlob != IntPtr.Zero)
-            {
-                byte[] passwordBytes = InteropUtils.ToByteArray(
-                    credential.CredentialBlob,
-                    credential.CredentialBlobSize);
-                password = Encoding.Unicode.GetString(passwordBytes);
-            }
+            string password = credential.GetCredentialBlobAsString();
 
             // Recover the target name we gave from the internal (raw) target name
             string targetName = credential.TargetName.TrimUntilIndexOf(TargetNameLegacyGenericPrefix);