Add a lighweight size-computation method on DecodableList. (#10801)

Just computes an optimistic size, without checking whether the
individual items decode properly.

examples/chip-tool/commands/tests/TestCommand.h

@@ -162,6 +162,8 @@ class TestCommand : public CHIPCommand
     template <typename T>
     bool CheckValueAsListLength(const char * itemName, chip::app::DataModel::DecodableList<T> list, uint64_t expectedLength)
     {
+        // We don't just use list.ComputeSize(), because we want to check that
+        // all the values in the list correctly decode to our type too.
         auto iter      = list.begin();
         uint64_t count = 0;
         while (iter.Next())

examples/chip-tool/templates/commands.zapt

@@ -139,16 +139,12 @@ static void On{{asUpperCamelCase parent.name}}{{asUpperCamelCase name}}ListAttri
     ModelCommand * command = static_cast<ModelCommand *>(context);
 
     size_t count = 0;
-    {
-      auto iter = list.begin();
-      while (iter.Next()) {
-        ++count;
-      }
-      if (iter.GetStatus() != CHIP_NO_ERROR) {
-        command->SetCommandExitStatus(iter.GetStatus());
-        return;
-      }
+    CHIP_ERROR err = list.ComputeSize(&count);
+    if (err != CHIP_NO_ERROR) {
+      command->SetCommandExitStatus(err);
+      return;
     }
+
     ChipLogProgress(chipTool, "On{{asUpperCamelCase parent.name}}{{asUpperCamelCase name}}ListAttributeResponse: %zu entries", count);
 
     auto iter = list.begin();
@@ -180,8 +176,7 @@ static void On{{asUpperCamelCase parent.name}}{{asUpperCamelCase name}}ListAttri
         {{/if}}
         {{/if}}
     }
-
-    command->SetCommandExitStatus(CHIP_NO_ERROR);
+    command->SetCommandExitStatus(iter.GetStatus());
 }
 
 {{/if}}

src/app/clusters/test-cluster-server/test-cluster-server.cpp

@@ -394,15 +394,9 @@ bool emberAfTestClusterClusterTestListInt8UReverseRequestCallback(
     CommandHandler * commandObj, ConcreteCommandPath const & commandPath,
     Commands::TestListInt8UReverseRequest::DecodableType const & commandData)
 {
-    size_t count = 0;
-    {
-        auto iter = commandData.arg1.begin();
-        while (iter.Next())
-        {
-            ++count;
-        }
-        VerifyOrExit(iter.GetStatus() == CHIP_NO_ERROR, );
-    }
+    size_t count   = 0;
+    CHIP_ERROR err = commandData.arg1.ComputeSize(&count);
+    VerifyOrExit(err == CHIP_NO_ERROR, );
 
     {
         auto iter = commandData.arg1.begin();

src/app/data-model/DecodableList.h

@@ -127,6 +127,14 @@ class DecodableList
 
     Iterator begin() const { return Iterator(mReader); }
 
+    /*
+     * Compute the size of the list.  This can fail if the TLV is malformed.  If
+     * this succeeds, that does not guarantee that the individual items can be
+     * successfully decoded; consumers should check Iterator::GetStatus() when
+     * actually decoding them.
+     */
+    CHIP_ERROR ComputeSize(size_t * size) const { return mReader.CountRemainingInContainer(size); }
+
 private:
     TLV::TLVReader mReader;
 };