Fix #4656: Improve diagnostics for invalid proto files

protoc-core/src/main/java/io/ballerina/protoc/core/BalFileGenerationUtils.java

@@ -79,11 +79,13 @@ public static ArrayList<String> generateDescriptor(String command) throws CodeGe
                     " running the protoc executor. " + e.getMessage(), e);
         }
         if (process.exitValue() != 0) {
-            StringBuilder errMsg = new StringBuilder();
-            for (String line : output) {
-                errMsg.append(line).append(System.lineSeparator());
+            if (output.isEmpty()) {
+                throw new CodeGeneratorException("Proto compilation failed with exit code: " + process.exitValue());
+            } else {
+                String rawError = String.join(System.lineSeparator(), output);
+                String formattedError = formatProtoError(rawError);
+                throw new CodeGeneratorException(formattedError);
             }
-            throw new CodeGeneratorException("Process exited with the following output: \n" + errMsg);
         }
         return output;
     }
@@ -100,21 +102,53 @@ public static Process runProcess(String command) throws IOException {
     }
 
     public static void handleProcessExecutionErrors(Process process) throws CodeGeneratorException {
-        if (process.exitValue() != 0) {
-            try (BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(process.getErrorStream(),
-                    StandardCharsets.UTF_8))) {
-                String err;
-                StringBuilder errMsg = new StringBuilder();
-                while ((err = bufferedReader.readLine()) != null) {
-                    errMsg.append(System.lineSeparator()).append(err);
+        // Capture error stream before checking exit value to avoid "Stream closed" errors
+        StringBuilder errMsg = new StringBuilder();
+        try (BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(process.getErrorStream(),
+                StandardCharsets.UTF_8))) {
+            String err;
+            while ((err = bufferedReader.readLine()) != null) {
+                if (!errMsg.isEmpty()) {
+                    errMsg.append(System.lineSeparator());
                 }
-                throw new CodeGeneratorException(errMsg.toString());
-            } catch (IOException e) {
-                throw new CodeGeneratorException("Invalid command syntax. " + e.getMessage(), e);
+                errMsg.append(err);
+            }
+        } catch (IOException e) {
+            throw new CodeGeneratorException("Failed to read error output from protoc command. " + e.getMessage(), e);
+        }
+
+        // Now check exit value after reading error stream
+        if (process.exitValue() != 0) {
+            if (errMsg.isEmpty()) {
+                throw new CodeGeneratorException("Proto compilation failed with exit code: " + process.exitValue());
+            } else {
+                // Format error message to be more user-friendly
+                String formattedError = formatProtoError(errMsg.toString());
+                throw new CodeGeneratorException(formattedError);
             }
         }
     }
 
+    /**
+     * Format protoc error messages to be more user-friendly.
+     *
+     * @param rawError The raw error output from protoc
+     * @return A formatted, user-friendly error message
+     */
+    private static String formatProtoError(String rawError) {
+    StringBuilder formatted = new StringBuilder();
+    formatted.append("Error: Failed to generate Ballerina files from proto definition.")
+        .append(System.lineSeparator());
+    formatted.append(System.lineSeparator());
+    formatted.append("Proto compilation failed with the following error:")
+        .append(System.lineSeparator());
+        formatted.append(rawError);
+        formatted.append(System.lineSeparator());
+        formatted.append(System.lineSeparator());
+        formatted.append("Please check your .proto file for syntax errors and undefined types.");
+        return formatted.toString();
+    }
+
     /**
      * Resolve proto folder path from Proto file path.
      *

protoc-core/src/main/java/io/ballerina/protoc/core/builder/BallerinaFileBuilder.java

@@ -69,9 +69,9 @@
 import static io.ballerina.protoc.core.builder.balgen.BalGenConstants.PACKAGE_SEPARATOR;
 import static io.ballerina.protoc.core.builder.syntaxtree.utils.ClientSampleSyntaxTreeUtils.generateSyntaxTreeForClientSample;
 import static io.ballerina.protoc.core.builder.syntaxtree.utils.CommonUtils.removeStandardImports;
-import static io.ballerina.protoc.core.utils.BalFileGenerationUtils.delete;
-import static io.ballerina.protoc.core.utils.BalFileGenerationUtils.handleProcessExecutionErrors;
-import static io.ballerina.protoc.core.utils.BalFileGenerationUtils.runProcess;
+import static io.ballerina.protoc.core.BalFileGenerationUtils.delete;
+import static io.ballerina.protoc.core.BalFileGenerationUtils.handleProcessExecutionErrors;
+import static io.ballerina.protoc.core.BalFileGenerationUtils.runProcess;
 
 /**
  * Class is responsible of generating the ballerina stub which is mapping proto definition.