Calculate MaxInstance fix (#399)

* MaxInstance fist checks environment variable, then the app domain value, then the default value.
VerifyTests · Sep 23, 2023 · fc489ef · fc489ef
1 parent 3de00aa
commit fc489ef
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diff --git a/docs/mdsource/diff-tool.source.md b/docs/mdsource/diff-tool.source.md
@@ -39,7 +39,7 @@ include: diffToolCleanup
 
 By default a maximum of 5 tool instances will be launched. This prevents a change that breaks many tests from causing too much load on a machine.
 
-This value can be changed:
+This value can be changed using an environment variable or by explicitly specifying the value by code. When both are used, the environment variable value will be used.
 
 
 ### Using an environment variable

diff --git a/src/DiffEngine/MaxInstance.cs b/src/DiffEngine/MaxInstance.cs
@@ -1,16 +1,20 @@
-static class MaxInstance
+static class MaxInstance
 {
-    public static int MaxInstancesToLaunch { get; private set; } = GetMaxInstances();
+    public static int MaxInstancesToLaunch => capturedMaxInstancesToLaunch ??= GetMaxInstances();
+    static int? capturedMaxInstancesToLaunch;
+    static int? appDomainMaxInstancesToLaunch;
     static int launchedInstances;
     const int defaultMax = 5;
 
-    static int GetMaxInstances()
+    static int GetMaxInstances() => GetEnvironmentValue() ?? appDomainMaxInstancesToLaunch ?? defaultMax;
+
+    static int? GetEnvironmentValue()
     {
         var variable = Environment.GetEnvironmentVariable("DiffEngine_MaxInstances");
 
         if (string.IsNullOrEmpty(variable))
         {
-            return defaultMax;
+            return null;
         }
 
         if (ushort.TryParse(variable, out var result))
@@ -21,31 +25,20 @@ static int GetMaxInstances()
         throw new($"Could not parse the DiffEngine_MaxInstances environment variable: {variable}");
     }
 
+    static void ResetCapturedValue() => capturedMaxInstancesToLaunch = null;
+
     public static void SetForAppDomain(int value)
     {
         Guard.AgainstNegativeAndZero(value, nameof(value));
-        MaxInstancesToLaunch = value;
+        appDomainMaxInstancesToLaunch = value;
+        ResetCapturedValue();
     }
 
     public static void SetForUser(int value)
     {
-        if (MaxInstancesToLaunch == value)
-        {
-            return;
-        }
-
-        MaxInstancesToLaunch = value;
-        string? envVariable;
-        if (value == defaultMax)
-        {
-            envVariable = null;
-        }
-        else
-        {
-            envVariable = value.ToString();
-        }
-
-        Environment.SetEnvironmentVariable("DiffEngine_MaxInstances", envVariable, EnvironmentVariableTarget.User);
+        Guard.AgainstNegativeAndZero(value, nameof(value));
+        Environment.SetEnvironmentVariable("DiffEngine_MaxInstances", value.ToString(), EnvironmentVariableTarget.User);
+        ResetCapturedValue();
     }
 
     public static bool Reached()