Mutually exclude mainSourceFile of different configurations

Unless explicitly added to sourceFiles, a configuration will exclude
other configurations' mainSourceFile's.

source/dub/package_.d

@@ -118,6 +118,7 @@ class Package {
 
 		checkDubRequirements();
 		fillWithDefaults();
+		mutuallyExcludeMainFiles();
 	}
 
 	/** Searches the given directory for package recipe files.
@@ -740,6 +741,30 @@ class Package {
 			cnames[c.name] = true;
 		}
 	}
+
+	private void mutuallyExcludeMainFiles()
+	{
+		import std.stdio;
+		string[] allMainFiles;
+		foreach (ref config; m_info.configurations)
+			if (!config.buildSettings.mainSourceFile.empty())
+				allMainFiles ~= config.buildSettings.mainSourceFile;
+
+		if (allMainFiles.length == 0)
+			return;
+
+		foreach (ref config; m_info.configurations) {
+			import std.algorithm.searching : canFind;
+			auto bs = &config.buildSettings;
+			auto otherMainFiles = allMainFiles.filter!(elem => (elem != bs.mainSourceFile)).array;
+
+			if (bs.sourceFiles.length == 0)
+				bs.excludedSourceFiles[""] ~= otherMainFiles;
+			else
+				foreach (suffix, arr; bs.sourceFiles)
+					bs.excludedSourceFiles[suffix] ~= otherMainFiles.filter!(elem => !canFind(arr, elem)).array;
+		}
+	}
 }
 
 private string determineVersionFromSCM(NativePath path)

test/mutex-main-1/dub.json

@@ -0,0 +1,24 @@
+{
+	"description": "A minimal D application.",
+	"name": "mutex-main",
+	"targetType": "executable",
+
+	"configurations": [
+		{
+            "name": "app",
+            "targetName": "app",
+			"mainSourceFile": "source/app.d"
+		},
+		{
+            "name": "app2",
+            "targetName": "app2",
+            "mainSourceFile": "source/app2.d"
+		},
+		{
+            "name": "failapp",
+            "targetName": "failapp",
+			"mainSourceFile": "source/app.d",
+			"sourceFiles": ["source/app2.d"]
+		}
+	]
+}

test/mutex-main-1/source/app.d

@@ -0,0 +1,8 @@
+module app;
+
+import std.stdio;
+
+void main()
+{
+	writeln("Edit source/app.d to start your project.");
+}

test/mutex-main-1/source/app2.d

@@ -0,0 +1,8 @@
+module app2;
+
+import std.stdio;
+
+void main()
+{
+	writeln("Edit source/app2.d to start your project.");
+}

test/mutex-main-2/dub.json

@@ -0,0 +1,24 @@
+{
+	"description": "A minimal D application.",
+	"name": "mutex-main",
+	"targetType": "executable",
+
+	"configurations": [
+		{
+            "name": "app2",
+            "targetName": "app2",
+            "mainSourceFile": "source/app2.d"
+		},
+		{
+            "name": "app",
+            "targetName": "app",
+			"mainSourceFile": "source/app.d"
+		},
+		{
+            "name": "failapp",
+            "targetName": "failapp",
+			"mainSourceFile": "source/app.d",
+			"sourceFiles": ["source/app2.d"]
+		}
+	]
+}

test/mutex-main-2/source/app.d

@@ -0,0 +1,8 @@
+module app;
+
+import std.stdio;
+
+void main()
+{
+	writeln("Edit source/app.d to start your project.");
+}

test/mutex-main-2/source/app2.d

@@ -0,0 +1,8 @@
+module app2;
+
+import std.stdio;
+
+void main()
+{
+	writeln("Edit source/app2.d to start your project.");
+}

test/mutex-main-3/dub.json

@@ -0,0 +1,24 @@
+{
+	"description": "A minimal D application.",
+	"name": "mutex-main",
+	"targetType": "executable",
+
+	"configurations": [
+		{
+            "name": "failapp",
+            "targetName": "failapp",
+			"mainSourceFile": "source/app.d",
+			"sourceFiles": ["source/app2.d"]
+		},
+		{
+            "name": "app",
+            "targetName": "app",
+			"mainSourceFile": "source/app.d",
+		},
+		{
+            "name": "app2",
+            "targetName": "app2",
+            "mainSourceFile": "source/app2.d"
+		}
+	]
+}

test/mutex-main-3/source/app.d

@@ -0,0 +1,8 @@
+module app;
+
+import std.stdio;
+
+void main()
+{
+	writeln("Edit source/app.d to start your project.");
+}

test/mutex-main-3/source/app2.d

@@ -0,0 +1,8 @@
+module app2;
+
+import std.stdio;
+
+void main()
+{
+	writeln("Edit source/app2.d to start your project.");
+}