Do not try running mono when it's not available on the runner

Author: tamasvajk

Diff for: csharp/autobuilder/Semmle.Autobuild.CSharp.Tests/BuildScripts.cs

@@ -46,6 +46,11 @@ bool IBuildActions.FileExists(string file)
 
         public IList<string> RunProcessIn { get; } = new List<string>();
         public IDictionary<string, int> RunProcess { get; } = new Dictionary<string, int>();
+
+        /// <summary>
+        /// Process-exit code pairs for commands that are executed during the assembly of the autobuild script.
+        /// </summary>
+        public IDictionary<string, int> RunProcessExecuteDuring { get; } = new Dictionary<string, int>();
         public IDictionary<string, string> RunProcessOut { get; } = new Dictionary<string, string>();
         public IDictionary<string, string> RunProcessWorkingDirectory { get; } = new Dictionary<string, string>();
         public HashSet<string> CreateDirectories { get; } = new HashSet<string>();
@@ -66,7 +71,7 @@ int IBuildActions.RunProcess(string cmd, string args, string? workingDirectory,
             if (wd != workingDirectory)
                 throw new ArgumentException($"Unexpected RunProcessWorkingDirectory, got {wd ?? "null"} expected {workingDirectory ?? "null"} in {pattern}");
 
-            if (!RunProcess.TryGetValue(pattern, out var ret))
+            if (!RunProcess.TryGetValue(pattern, out var ret) && !RunProcessExecuteDuring.TryGetValue(pattern, out ret))
                 throw new ArgumentException("Missing RunProcess " + pattern);
 
             return ret;
@@ -81,7 +86,7 @@ int IBuildActions.RunProcess(string cmd, string args, string? workingDirectory,
             if (wd != workingDirectory)
                 throw new ArgumentException($"Unexpected RunProcessWorkingDirectory, got {wd ?? "null"} expected {workingDirectory ?? "null"} in {pattern}");
 
-            if (!RunProcess.TryGetValue(pattern, out var ret))
+            if (!RunProcess.TryGetValue(pattern, out var ret) && !RunProcessExecuteDuring.TryGetValue(pattern, out ret))
                 throw new ArgumentException("Missing RunProcess " + pattern);
 
             return ret;
@@ -797,11 +802,32 @@ public void TestDirsProjWindows()
         }
 
         [Fact]
-        public void TestDirsProjLinux()
+        public void TestDirsProjLinux_WithMono()
         {
+            actions.RunProcessExecuteDuring[@"mono --version"] = 0;
+
             actions.RunProcess[@"nuget restore C:\Project/dirs.proj -DisableParallelProcessing"] = 1;
             actions.RunProcess[@"mono scratch/.nuget/nuget.exe restore C:\Project/dirs.proj -DisableParallelProcessing"] = 0;
             actions.RunProcess[@"msbuild C:\Project/dirs.proj /t:rebuild"] = 0;
+
+            var autobuilder = TestDirsProjLinux();
+            TestAutobuilderScript(autobuilder, 0, 3);
+        }
+
+        [Fact]
+        public void TestDirsProjLinux_WithoutMono()
+        {
+            actions.RunProcessExecuteDuring[@"mono --version"] = 1;
+
+            actions.RunProcess[@"dotnet msbuild /t:restore C:\Project/dirs.proj"] = 0;
+            actions.RunProcess[@"dotnet msbuild C:\Project/dirs.proj /t:rebuild"] = 0;
+
+            var autobuilder = TestDirsProjLinux();
+            TestAutobuilderScript(autobuilder, 0, 2);
+        }
+
+        private CSharpAutobuilder TestDirsProjLinux()
+        {
             actions.FileExists["csharp.log"] = true;
             actions.FileExists[@"C:\Project/a/test.csproj"] = true;
             actions.FileExists[@"C:\Project/dirs.proj"] = true;
@@ -830,8 +856,7 @@ public void TestDirsProjLinux()
 </Project>");
             actions.LoadXml[@"C:\Project/dirs.proj"] = dirsproj;
 
-            var autobuilder = CreateAutoBuilder(false);
-            TestAutobuilderScript(autobuilder, 0, 3);
+            return CreateAutoBuilder(false);
         }
 
         [Fact]

Diff for: csharp/autobuilder/Semmle.Autobuild.Shared/MsBuildRule.cs

@@ -10,15 +10,13 @@ internal static class MsBuildCommandExtensions
         /// <summary>
         /// Appends a call to msbuild.
         /// </summary>
-        /// <param name="cmdBuilder"></param>
-        /// <param name="builder"></param>
         /// <returns></returns>
-        public static CommandBuilder MsBuildCommand(this CommandBuilder cmdBuilder, IAutobuilder<AutobuildOptionsShared> builder)
+        public static CommandBuilder MsBuildCommand(this CommandBuilder cmdBuilder, IAutobuilder<AutobuildOptionsShared> builder, bool preferDotnet)
         {
             // mono doesn't ship with `msbuild` on Arm-based Macs, but we can fall back to
             // msbuild that ships with `dotnet` which can be invoked with `dotnet msbuild`
             // perhaps we should do this on all platforms?
-            return builder.Actions.IsRunningOnAppleSilicon()
+            return builder.Actions.IsRunningOnAppleSilicon() || preferDotnet
                 ? cmdBuilder.RunCommand("dotnet").Argument("msbuild")
                 : cmdBuilder.RunCommand("msbuild");
         }
@@ -75,13 +73,21 @@ BuildScript GetNugetRestoreScript() =>
                         QuoteArgument(projectOrSolution.FullPath).
                         Argument("-DisableParallelProcessing").
                         Script;
+
+                BuildScript GetMonoVersionScript() => new CommandBuilder(builder.Actions).
+                    RunCommand("mono").
+                    Argument("--version").
+                    Script;
+
+                var preferDotnet = !builder.Actions.IsWindows() && GetMonoVersionScript().Run(builder.Actions, (_, _) => { }, (_, _, _) => { }) != 0;
+
                 var nugetRestore = GetNugetRestoreScript();
                 var msbuildRestoreCommand = new CommandBuilder(builder.Actions).
-                    MsBuildCommand(builder).
+                    MsBuildCommand(builder, preferDotnet).
                     Argument("/t:restore").
                     QuoteArgument(projectOrSolution.FullPath);
 
-                if (builder.Actions.IsRunningOnAppleSilicon())
+                if (builder.Actions.IsRunningOnAppleSilicon() || preferDotnet)
                 {
                     // On Apple Silicon, only try package restore with `dotnet msbuild /t:restore`
                     ret &= BuildScript.Try(msbuildRestoreCommand.Script);
@@ -119,7 +125,7 @@ BuildScript GetNugetRestoreScript() =>
                     command.RunCommand("set Platform=&& type NUL", quoteExe: false);
                 }
 
-                command.MsBuildCommand(builder);
+                command.MsBuildCommand(builder, preferDotnet);
                 command.QuoteArgument(projectOrSolution.FullPath);
 
                 var target = "rebuild";