cmServerProtocol: fix test of empty values

If a required value is in the cache, it is not necessary to set it
explicitly.

Fixes: #16948, #16988

Help/manual/cmake-server.7.rst

@@ -277,7 +277,9 @@ Giving the "major" version of the requested protocol version will make the serve
 use the latest minor version of that protocol. Use this if you do not explicitly
 need to depend on a specific minor version.
 
-Each protocol version may request additional attributes to be present.
+If the build directory already contains a CMake cache, it is sufficient to set
+the "buildDirectory" attribute. To create a fresh build directory, additional
+attributes are required depending on the protocol version.
 
 Protocol version 1.0 requires the following attributes to be set:
 

Source/cmServerProtocol.cxx

@@ -269,6 +269,10 @@ static bool testHomeDirectory(cmState* state, std::string& value,
 {
   const std::string cachedValue =
     std::string(state->GetCacheEntryValue("CMAKE_HOME_DIRECTORY"));
+  if (value.empty()) {
+    value = cachedValue;
+    return true;
+  }
   const std::string suffix = "/CMakeLists.txt";
   const std::string cachedValueCML = cachedValue + suffix;
   const std::string valueCML = value + suffix;
@@ -279,9 +283,6 @@ static bool testHomeDirectory(cmState* state, std::string& value,
                                 "source directory value."));
     return false;
   }
-  if (value.empty()) {
-    value = cachedValue;
-  }
   return true;
 }
 
@@ -292,15 +293,15 @@ static bool testValue(cmState* state, const std::string& key,
   const char* entry = state->GetCacheEntryValue(key);
   const std::string cachedValue =
     entry == nullptr ? std::string() : std::string(entry);
-  if (!cachedValue.empty() && !value.empty() && cachedValue != value) {
+  if (value.empty()) {
+    value = cachedValue;
+  }
+  if (!cachedValue.empty() && cachedValue != value) {
     setErrorMessage(errorMessage, std::string("\"") + key +
                       "\" is set but incompatible with configured " +
                       keyDescription + " value.");
     return false;
   }
-  if (value.empty()) {
-    value = cachedValue;
-  }
   return true;
 }