Add example algorithmic contracts

- To start, I've added the contracts for iterating the thread list and gathering information about individual threads
- Note that this algorithmic description is paired with a simplification of the SList data structure
  - See PR dotnet#100107

docs/design/datacontracts/GCHandle.md

@@ -0,0 +1,23 @@
+# Contract GCHandle
+
+This contract allows decoding and reading of GCHandles. This will also include handle enumeration in the future
+
+## Data structures defined by contract
+``` csharp
+```
+
+## Apis of contract
+``` csharp
+TargetPointer GetObject(TargetPointer gcHandle);
+```
+
+## Version 1
+
+``` csharp
+TargetPointer GetObject(TargetPointer gcHandle)
+{
+    if (gcHandle == TargetPointer.Null)
+        return TargetPointer.Null;
+    return Target.ReadTargetPointer(gcHandle);
+}
+```

docs/design/datacontracts/SList.md

@@ -0,0 +1,78 @@
+# Contract SList
+
+This contract allows reading and iterating over an SList data structure.
+
+## Data structures defined by contract
+``` csharp
+class SListReader
+{
+    public abstract TargetPointer GetHead(TargetPointer slistPointer);
+    public abstract TargetPointer GetNext(TargetPointer entryInSList);
+    public IEnumerator<TargetPointer> EnumerateList(TargetPointer slistPointer)
+    {
+        TargetPointer current = GetHead(slistPointer);
+
+        while (current != TargetPointer.Null)
+        {
+            yield return current;
+            current = GetNext(current);
+        }
+    }
+    public IEnumerator<TargetPointer> EnumerateListFromEntry(TargetPointer entryInSList)
+    {
+        TargetPointer current = entryInSList;
+
+        while (current != TargetPointer.Null)
+        {
+            yield return current;
+            current = GetNext(current);
+        }
+    }
+}
+```
+
+## Apis of contract
+``` csharp
+SListReader GetReader(string typeOfDataStructure);
+```
+
+## Version 1
+
+``` csharp
+private class SListReaderV1 : SListReader
+{
+    uint _offsetToSLinkField;
+    Target Target;
+
+    SListReaderV1(Target target, string typeToEnumerate)
+    {
+        Target = target;
+        _offsetToSLinkField = Target.Contracts.GetFieldLayout(typeToEnumerate, "m_Link").Offset;
+    }
+    public override TargetPointer GetHead(TargetPointer slistPointer)
+    {
+        TargetPointer headPointer = new SListBase(Target, slistPointer).m_pHead;
+        TargetPointer slinkInHeadObject = new SLink(Target, headPointer).m_pNext;
+        if (slinkInHeadObject == TargetPointer.Null)
+            return TargetPointer.Null;
+        return slinkInHeadObject - _offsetToSLinkField;
+    }
+
+    public override TargetPointer GetNext(TargetPointer entryInSList)
+    {
+        if (entryInSList == TargetPointer.Null)
+            throw new ArgumentException();
+
+        TargetPointer slinkPointer = entryInSList + _offsetToSLinkField;
+        TargetPointer slinkInObject = new SLink(Target, slinkPointer).m_pNext;
+        if (slinkInObject == TargetPointer.Null)
+            return TargetPointer.Null;
+        return slinkInHeadObject - _offsetToSLinkField;
+    }
+}
+
+SListReader GetReader(string typeOfDataStructure)
+{
+    return new SListReaderV1(typeOfDataStructure);
+}
+```

docs/design/datacontracts/Thread.md

@@ -0,0 +1,174 @@
+# Contract Thread
+
+This contract is for reading and iterating the threads of the process.
+
+## Data structures defined by contract
+``` csharp
+record struct DacThreadStoreData (
+    int ThreadCount,
+    int UnstartedThreadCount,
+    int BackgroundThreadCount,
+    int PendingThreadCount,
+    int DeadThreadCount,
+    TargetPointer FirstThread,
+    TargetPointer FinalizerThread,
+    TargetPointer GcThread);
+
+enum ThreadState
+{
+    TS_Unknown                = 0x00000000,    // threads are initialized this way
+
+    TS_AbortRequested         = 0x00000001,    // Abort the thread
+
+    TS_GCSuspendPending       = 0x00000002,    // ThreadSuspend::SuspendRuntime watches this thread to leave coop mode.
+    TS_GCSuspendRedirected    = 0x00000004,    // ThreadSuspend::SuspendRuntime has redirected the thread to suspention routine.
+    TS_GCSuspendFlags         = TS_GCSuspendPending | TS_GCSuspendRedirected, // used to track suspension progress. Only SuspendRuntime writes/resets these.
+
+    TS_DebugSuspendPending    = 0x00000008,    // Is the debugger suspending threads?
+    TS_GCOnTransitions        = 0x00000010,    // Force a GC on stub transitions (GCStress only)
+
+    TS_LegalToJoin            = 0x00000020,    // Is it now legal to attempt a Join()
+
+    TS_ExecutingOnAltStack    = 0x00000040,    // Runtime is executing on an alternate stack located anywhere in the memory
+
+    TS_Hijacked               = 0x00000080,    // Return address has been hijacked
+
+    // unused                 = 0x00000100,
+    TS_Background             = 0x00000200,    // Thread is a background thread
+    TS_Unstarted              = 0x00000400,    // Thread has never been started
+    TS_Dead                   = 0x00000800,    // Thread is dead
+
+    TS_WeOwn                  = 0x00001000,    // Exposed object initiated this thread
+    TS_CoInitialized          = 0x00002000,    // CoInitialize has been called for this thread
+
+    TS_InSTA                  = 0x00004000,    // Thread hosts an STA
+    TS_InMTA                  = 0x00008000,    // Thread is part of the MTA
+
+    // Some bits that only have meaning for reporting the state to clients.
+    TS_ReportDead             = 0x00010000,    // in WaitForOtherThreads()
+    TS_FullyInitialized       = 0x00020000,    // Thread is fully initialized and we are ready to broadcast its existence to external clients
+
+    TS_TaskReset              = 0x00040000,    // The task is reset
+
+    TS_SyncSuspended          = 0x00080000,    // Suspended via WaitSuspendEvent
+    TS_DebugWillSync          = 0x00100000,    // Debugger will wait for this thread to sync
+
+    TS_StackCrawlNeeded       = 0x00200000,    // A stackcrawl is needed on this thread, such as for thread abort
+                                                // See comment for s_pWaitForStackCrawlEvent for reason.
+
+    // unused                 = 0x00400000,
+
+    // unused                 = 0x00800000,
+    TS_TPWorkerThread         = 0x01000000,    // is this a threadpool worker thread?
+
+    TS_Interruptible          = 0x02000000,    // sitting in a Sleep(), Wait(), Join()
+    TS_Interrupted            = 0x04000000,    // was awakened by an interrupt APC. !!! This can be moved to TSNC
+
+    TS_CompletionPortThread   = 0x08000000,    // Completion port thread
+
+    TS_AbortInitiated         = 0x10000000,    // set when abort is begun
+
+    TS_Finalized              = 0x20000000,    // The associated managed Thread object has been finalized.
+                                                // We can clean up the unmanaged part now.
+
+    TS_FailStarted            = 0x40000000,    // The thread fails during startup.
+    TS_Detached               = 0x80000000,    // Thread was detached by DllMain
+}
+
+record struct DacThreadData (
+    uint ThreadId;
+    uint OsThreadId;
+    ThreadState State;
+    bool PreemptiveGCDisabled
+    TargetPointer AllocContextPointer;
+    TargetPointer AllocContextLimit;
+    TargetPointer Frame;
+    TargetPointer FirstNestedException;
+    TargetPointer TEB;
+    TargetPointer LastThrownObjectHandle;
+    TargetPointer NextThread;
+);
+```
+
+## Apis of contract
+``` csharp
+DacThreadStoreData GetThreadStoreData();
+```
+
+## Version 1
+
+
+
+``` csharp
+SListReader ThreadListReader = Contracts.SList.GetReader("Thread");
+
+DacThreadStoreData GetThreadStoreData()
+{
+    TargetPointer threadStore = Target.ReadGlobalTargetPointer("s_pThreadStore");
+    var runtimeThreadStore = new ThreadStore(Target, threadStore);
+
+    TargetPointer firstThread = ThreadListReader.GetHead(runtimeThreadStore.SList.Pointer);
+
+    return new DacThreadStoreData(
+        ThreadCount : runtimeThreadStore.m_ThreadCount,
+        UnstartedThreadCount : runtimeThreadStore.m_UnstartedThreadCount,
+        BackgroundThreadCount : runtimeThreadStore.m_BackgroundThreadCount,
+        PendingThreadCount : runtimeThreadStore.m_PendingThreadCount,
+        DeadThreadCount: runtimeThreadStore.m_DeadThreadCount,
+        FirstThread: firstThread,
+        FinalizerThread: Target.ReadGlobalTargetPointer("g_pFinalizerThread"),
+        GcThread: Target.ReadGlobalTargetPointer("g_pSuspensionThread"));
+}
+
+DacThreadData GetThreadData(TargetPointer threadPointer)
+{
+    var runtimeThread = new Thread(Target, threadPointer);
+
+    TargetPointer firstNestedException = TargetPointer.Null;
+    if (Target.ReadGlobalInt32("FEATURE_EH_FUNCLETS"))
+    {
+        if (runtimeThread.m_ExceptionState.m_pCurrentTracker != TargetPointer.Null)
+        {
+            firstNestedException = new ExceptionTrackerBase(Target, runtimeThread.m_ExceptionState.m_pCurrentTracker).m_pPrevNestedInfo;
+        }
+    }
+    else
+    {
+        firstNestedException = runtimeThread.m_ExceptionState.m_currentExInfo.m_pPrevNestedInfo;
+    }
+
+    return new DacThread(
+        ThreadId : runtimeThread.m_ThreadId,
+        OsThreadId : runtimeThread.m_OSThreadId,
+        State : (ThreadState)runtimeThread.m_State,
+        PreemptiveGCDisabled : thread.m_fPreemptiveGCDisabled != 0,
+        AllocContextPointer : thread.m_alloc_context.alloc_ptr,
+        AllocContextLimit : thread.m_alloc_context.alloc_limit,
+        Frame : thread.m_pFrame,
+        TEB : thread.Has_m_pTEB ? thread.m_pTEB : TargetPointer.Null,
+        LastThreadObjectHandle : thread.m_LastThrownObjectHandle,
+        FirstNestedException : firstNestedException,
+        NextThread : ThreadListReader.GetHead.GetNext(threadPointer)
+    );
+}
+
+TargetPointer GetNestedExceptionInfo(TargetPointer nestedExceptionPointer, out TargetPointer nextNestedException)
+{
+    if (nestedExceptionPointer == TargetPointer.Null)
+    {
+        throw new InvalidArgumentException();
+    }
+    if (Target.ReadGlobalInt32("FEATURE_EH_FUNCLETS"))
+    {
+        var exData = new ExceptionTrackerBase(Target, nestedExceptionPointer);
+        nextNestedException = exData.m_pPrevNestedInfo;
+        return Contracts.GCHandle.GetObject(exData.m_hThrowable);
+    }
+    else
+    {
+        var exData = new ExInfo(Target, nestedExceptionPointer);
+        nextNestedException = exData.m_pPrevNestedInfo;
+        return Contracts.GCHandle.GetObject(exData.m_hThrowable);
+    }
+}
+```

docs/design/datacontracts/contract_csharp_api_design.cs

@@ -33,6 +33,7 @@ class DataContractAlgorithmAttribute : System.Attribute
     struct TargetPointer
     {
         public ulong Value;
+        public static TargetPointer Null = new TargetPointer(0);
         // Add a full set of operators to support pointer arithmetic
     }