Probe a musl RID and nothing else for the head

src/BuildMonitor.Tests/Launcher/HeadLocatorTests.cs

@@ -35,4 +35,29 @@ public async Task RidsStartWithTheExactOne()
         await Assert.That(rids[0]).IsEqualTo(RuntimeInformation.RuntimeIdentifier);
         await Assert.That(rids.Count).IsGreaterThanOrEqualTo(1);
     }
+
+    /// <summary>
+    /// The probe for a musl RID is that RID alone. The linux-{arch} candidate after it names the
+    /// glibc head, which cannot start on musl: the launcher would wait out its timeout for a tray
+    /// that was never going to answer, rather than say there is no head for the machine.
+    /// </summary>
+    [Test]
+    [Arguments("linux-musl-x64")]
+    [Arguments("linux-musl-arm64")]
+    public async Task MuslProbesItsOwnRidAndNothingElse(string runtimeIdentifier)
+    {
+        var rids = HeadLocator.Rids(runtimeIdentifier).ToList();
+        await Assert.That(rids).IsEquivalentTo([runtimeIdentifier]);
+    }
+
+    /// <summary>
+    /// Everywhere else the synthesised RID still follows the framework's own.
+    /// </summary>
+    [Test]
+    public async Task OtherRidsStillFallBackToTheSynthesisedRid()
+    {
+        var rids = HeadLocator.Rids("some-rid").ToList();
+        await Assert.That(rids.Count).IsGreaterThan(1);
+        await Assert.That(rids[0]).IsEqualTo("some-rid");
+    }
 }

src/BuildMonitor/HeadLocator.cs

@@ -1,7 +1,11 @@
 /// <summary>
 /// Finds the tray executable for this machine under heads/{rid}. The exact RID is tried first,
-/// then the plain os-arch one, the same order DiffEngine resolves its native renderers in, so
-/// a linux-musl-x64 machine misses rather than starting a glibc build.
+/// then the plain os-arch one, the same order DiffEngine resolves its native renderers in.
+/// <para>
+/// A musl RID is the exception and is tried alone. No head is built for musl, and the os-arch
+/// one after it is linux-{arch}, the glibc build: found, it would be started and never answer,
+/// so a linux-musl-x64 machine has to miss here to be told there is no head for it.
+/// </para>
 /// </summary>
 static class HeadLocator
 {
@@ -31,9 +35,24 @@ static class HeadLocator
         return null;
     }
 
-    public static IEnumerable<string> Rids()
+    public static IEnumerable<string> Rids() =>
+        Rids(RuntimeInformation.RuntimeIdentifier);
+
+    public static IEnumerable<string> Rids(string runtimeIdentifier)
     {
-        yield return RuntimeInformation.RuntimeIdentifier;
+        // The exact RID first. On Alpine that is linux-musl-x64, which we do not ship, so the
+        // probe misses and the launcher says there is no head for this machine.
+        yield return runtimeIdentifier;
+
+        // And nothing else, or the synthesised RID below undoes that: linux-{arch} is the glibc
+        // build, an apphost that cannot start on musl. The launcher would start it detached, wait
+        // out its timeout and report a tray that did not answer, when the truth is that no head
+        // is built for this machine.
+        if (runtimeIdentifier.Contains("-musl-", StringComparison.Ordinal))
+        {
+            yield break;
+        }
+
         var architecture = RuntimeInformation.OSArchitecture switch
         {
             Architecture.X64 => "x64",