[Projects] Fix concurrent access to FileNestingService

The providers are initialized on demand, so in solutions with several
projects, that load each project on a separate Task, we were getting race
conditions on the NestingRulesProvider's initialization, so make sure
they are initialized only once and not called until they are fully loaded.

2 files changed, 15 insertions, 12 deletions

diff --git a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.FileNesting/FileNestingService.cs b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.FileNesting/FileNestingService.cs
index ae7da7da54..13aecfa2ee 100644
--- a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.FileNesting/FileNestingService.cs
+++ b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.FileNesting/FileNestingService.cs
@@ -44,7 +44,7 @@ namespace MonoDevelop.Projects.FileNesting
 		{
 			var pr = args.ExtensionObject as NestingRulesProvider;
 			if (pr != null) {
-				if (args.Change == ExtensionChange.Add) {
+				if (args.Change == ExtensionChange.Add && !rulesProviders.Contains (pr)) {
 					rulesProviders = rulesProviders.Add (pr);
 				} else {
 					rulesProviders = rulesProviders.Remove (pr);
diff --git a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.FileNesting/NestingRulesProvider.cs b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.FileNesting/NestingRulesProvider.cs
index 307c11dafa..1222b193eb 100644
--- a/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.FileNesting/NestingRulesProvider.cs
+++ b/main/src/core/MonoDevelop.Core/MonoDevelop.Projects.FileNesting/NestingRulesProvider.cs
@@ -46,6 +46,7 @@ namespace MonoDevelop.Projects.FileNesting
 		const string TokenNameAdd = "add";
 
 		List<NestingRule> nestingRules;
+		readonly object loadingLock = new object ();
 
 		public NestingRulesProvider ()
 		{
@@ -122,18 +123,20 @@ namespace MonoDevelop.Projects.FileNesting
 
 		public FilePath GetParentFile (Project project, FilePath inputFile)
 		{
-			if (nestingRules == null) {
-				// Create the list here, so that we don't get this path called
-				// again and again for every file in the solution if something
-				// goes wrong when loading the file.
-				nestingRules = new List<NestingRule> ();
-
-				if (!File.Exists (SourceFile)) {
-					return null;
-				}
+			lock (loadingLock) {
+				if (nestingRules == null) {
+					// Create the list here, so that we don't get this path called
+					// again and again for every file in the solution if something
+					// goes wrong when loading the file.
+					nestingRules = new List<NestingRule> ();
+
+					if (!File.Exists (SourceFile)) {
+						return null;
+					}
 
-				if (!LoadFromFile (this)) {
-					return null;
+					if (!LoadFromFile (this)) {
+						return null;
+					}
 				}
 			}