Try invalidating NFS cache by doing a write

Instead of calling `sync`, which doesn't seem to be invalidating the
correct NFS dir cache, we'll try to force the NFS cache to be sent to
the server by writing a file into the .hg folder, then trying to do an
`ls` on the other side.

backend/LexBoxApi/Services/HgService.cs

@@ -69,7 +69,9 @@ await Task.Run(() =>
         {
             InitRepoAt(new DirectoryInfo(PrefixRepoFilePath(code)));
         });
-        await Process.Start("sync").WaitForExitAsync();  // TODO: Put behind an OS check so we don't break Windows
+        // TODO 789: In theory this shouldn't need an invalidate call? Try with and without
+        await InvalidateDirCache(code);
+        // await Process.Start("sync").WaitForExitAsync();  // TODO: Put behind an OS check so we don't break Windows
         await WaitForRepoEmptyState(code, RepoEmptyState.Empty);
     }
 
@@ -108,8 +110,8 @@ public async Task ResetRepo(string code)
         await SoftDeleteRepo(code, $"{FileUtils.ToTimestamp(DateTimeOffset.UtcNow)}__reset");
         //we must init the repo as uploading a zip is optional
         tmpRepo.MoveTo(PrefixRepoFilePath(code));
-        await Process.Start("sync").WaitForExitAsync();  // TODO: Put behind an OS check so we don't break Windows
-        // await InvalidateDirCache(code); // TODO 789: Sometimes NFS hasn't finished the MoveTo above! So we need to find a way to wait until InvalidateDirCache sees an *empty* repo...
+        // await Process.Start("sync").WaitForExitAsync();  // TODO: Put behind an OS check so we don't break Windows
+        await InvalidateDirCache(code); // TODO 789: Does this work now?
         await WaitForRepoEmptyState(code, RepoEmptyState.Empty);
     }
 
@@ -144,8 +146,8 @@ await Task.Run(() =>
         // Now we're ready to move the new repo into place, replacing the old one
         await DeleteRepo(code);
         tempRepo.MoveTo(PrefixRepoFilePath(code));
-        // await InvalidateDirCache(code);
-        await Process.Start("sync").WaitForExitAsync();  // TODO: Put behind an OS check so we don't break Windows
+        await InvalidateDirCache(code);
+        // await Process.Start("sync").WaitForExitAsync();  // TODO: Put behind an OS check so we don't break Windows
         await WaitForRepoEmptyState(code, RepoEmptyState.NonEmpty); // TODO: Either catch the case where someone uploaded a .zip of an empty .hg repo, or set a timeout in WaitForRepoEmptyState
     }
 
@@ -274,7 +276,12 @@ public async Task<HttpContent> ExecuteHgRecover(string code, CancellationToken t
 
     public Task<HttpContent> InvalidateDirCache(string code)
     {
-        return ExecuteHgCommandServerCommand(code, "invalidatedircache", default);
+        // TODO: Ensure we don't accidentally re-create a repo we tried to delete by doing the line below
+        // I.e., check if PrefixRepoFilePath(code) exists and if it doesn't, create and delete it alone
+        var d = Directory.CreateDirectory(Path.Join(PrefixRepoFilePath(code), ".tmp-invalidate")); // Try to force NFS to invalidate
+        var result = ExecuteHgCommandServerCommand(code, "invalidatedircache", default);
+        try { if (d.Exists) d.Delete(); } catch(Exception) {} // Clean up no matter what
+        return result;
     }
 
     public async Task<string> GetTipHash(string code)