settings: cache Git config values across process

When enumerating all Git configuration values in `Settings`, we
currently cache a copy of all values for the lifetime of the
`GetSettingValues()` method. This saves us some calls to Git, but we can
do better.

Pull-up the cache of Git configuration entries to the lifetime of the
`Settings` object, meaning all calls to `GetSettingValues()` over the
lifetime of the process can benefit from the cache! The savings here can
be significant as each `ISettings` property typically results in a call
to `GetSettingValues()`.

Author: mjcheetham

src/shared/Core/Settings.cs

@@ -12,6 +12,7 @@ namespace GitCredentialManager
 {
     /// <summary>
     /// Component that represents settings for Git Credential Manager as found from the environment and Git configuration.
+    /// Setting values from Git configuration may be cached for performance reasons.
     /// </summary>
     public interface ISettings : IDisposable
     {
@@ -26,7 +27,6 @@ public interface ISettings : IDisposable
         /// <returns>True if a setting value was found, false otherwise.</returns>
         bool TryGetSetting(string envarName, string section, string property, out string value);
 
-
         /// <summary>
         /// Try and get the value of a specified setting as specified in the environment and Git configuration,
         /// with the environment taking precedence over Git. If the value is pulled from the Git configuration,
@@ -291,6 +291,8 @@ public class Settings : ISettings
         private readonly IEnvironment _environment;
         private readonly IGit _git;
 
+        private Dictionary<string,string> _configEntries;
+
         public Settings(IEnvironment environment, IGit git)
         {
             EnsureArgument.NotNull(environment, nameof(environment));
@@ -334,6 +336,30 @@ public IEnumerable<string> GetSettingValues(string envarName, string section, st
             {
                 IGitConfiguration config = _git.GetConfiguration();
 
+                //
+                // Enumerate all configuration entries for all sections and property names and make a
+                // local copy of them here to avoid needing to call `TryGetValue` on the IGitConfiguration
+                // object multiple times in a loop below.
+                //
+                // This is a performance optimisation to avoid calling `TryGet` on the IGitConfiguration
+                // object multiple times in a loop below, or each time this entire method is called.
+                // The assumption is that the configuration entries will not change during a single invocation
+                // of Git Credential Manager, which is reasonable given process lifetime is typically less
+                // than a few seconds. For some entries (type=path), we still need to ask Git in order to
+                // expand the path correctly.
+                //
+                if (_configEntries is null)
+                {
+                    _configEntries = new Dictionary<string, string>(GitConfigurationKeyComparer.Instance);
+                    config.Enumerate(entry =>
+                    {
+                        _configEntries[entry.Key] = entry.Value;
+
+                        // Continue the enumeration
+                        return true;
+                    });
+                }
+
                 if (RemoteUri != null)
                 {
                     /*
@@ -379,26 +405,14 @@ public IEnumerable<string> GetSettingValues(string envarName, string section, st
                      *
                      */
 
-                    // Enumerate all configuration entries with the correct section and property name
-                    // and make a local copy of them here to avoid needing to call `TryGetValue` on the
-                    // IGitConfiguration object multiple times in a loop below.
-                    var configEntries = new Dictionary<string, string>(GitConfigurationKeyComparer.Instance);
-                    config.Enumerate(section, property, entry =>
-                    {
-                        configEntries[entry.Key] = entry.Value;
-
-                        // Continue the enumeration
-                        return true;
-                    });
-
                     foreach (string scope in RemoteUri.GetGitConfigurationScopes())
                     {
                         string queryName = $"{section}.{scope}.{property}";
                         // Look for a scoped entry that includes the scheme "protocol://example.com" first as
                         // this is more specific. If `isPath` is true, then re-get the value from the
                         // `GitConfiguration` with `isPath` specified.
                         if ((isPath && config.TryGet(queryName, isPath, out value)) ||
-                            configEntries.TryGetValue(queryName, out value))
+                            _configEntries.TryGetValue(queryName, out value))
                         {
                             yield return value;
                         }
@@ -409,7 +423,7 @@ public IEnumerable<string> GetSettingValues(string envarName, string section, st
                         string scopeWithoutScheme = scope.TrimUntilIndexOf(Uri.SchemeDelimiter);
                         string queryWithSchemeName = $"{section}.{scopeWithoutScheme}.{property}";
                         if ((isPath && config.TryGet(queryWithSchemeName, isPath, out value)) ||
-                            configEntries.TryGetValue(queryWithSchemeName, out value))
+                            _configEntries.TryGetValue(queryWithSchemeName, out value))
                         {
                             yield return value;
                         }
@@ -431,7 +445,7 @@ public IEnumerable<string> GetSettingValues(string envarName, string section, st
                  */
                 string name = $"{section}.{property}";
                 if ((isPath && config.TryGet(name, isPath, out value)) ||
-                    configEntries.TryGetValue(name, out value))
+                    _configEntries.TryGetValue(name, out value))
                 {
                     yield return value;
                 }