fix: Sketch preprocessing errors were displayed on stdout instead of stderr

internal/arduino/builder/preprocess_sketch.go

@@ -32,7 +32,7 @@ func (b *Builder) preprocessSketch(includes paths.PathList) error {
 		if b.logger.Verbose() {
 			b.logger.WriteStdout(result.Stdout())
 		}
-		b.logger.WriteStdout(result.Stderr())
+		b.logger.WriteStderr(result.Stderr())
 		b.diagnosticStore.Parse(result.Args(), result.Stderr())
 	}
 

internal/integrationtest/compile_3/compile_test.go

@@ -107,7 +107,7 @@ func TestCompilerErrOutput(t *testing.T) {
 	_, _, err := cli.Run("core", "install", "arduino:[email protected]")
 	require.NoError(t, err)
 
-	{
+	t.Run("Diagnostics", func(t *testing.T) {
 		// prepare sketch
 		sketch, err := paths.New("testdata", "blink_with_wrong_cpp").Abs()
 		require.NoError(t, err)
@@ -126,10 +126,11 @@ func TestCompilerErrOutput(t *testing.T) {
 			  "context": [ { "message": "In function 'void wrong()':" } ]
 			}
 		]`)
-	}
+	})
+
+	t.Run("PreprocessorDiagnostics", func(t *testing.T) {
+		// Test the preprocessor errors are present in the diagnostics
 
-	// Test the preprocessor errors are present in the diagnostics
-	{
 		// prepare sketch
 		sketch, err := paths.New("testdata", "blink_with_wrong_include").Abs()
 		require.NoError(t, err)
@@ -148,14 +149,15 @@ func TestCompilerErrOutput(t *testing.T) {
 			  "message": "invalid preprocessing directive #wrong\n #wrong\n  ^~~~~",
 			}
 		]`)
-	}
+	})
+
+	t.Run("PreprocessorDiagnosticsWithLibErrors", func(t *testing.T) {
+		// Test the preprocessor errors are present in the diagnostics.
+		// In case we have 2 libraries:
+		// 1. one is missing
+		// 2. the other one is missing only from the first GCC run
+		// The diagnostics should report only 1 missing library.
 
-	// Test the preprocessor errors are present in the diagnostics.
-	// In case we have 2 libraries:
-	// 1. one is missing
-	// 2. the other one is missing only from the first GCC run
-	// The diagnostics should report only 1 missing library.
-	{
 		// prepare sketch
 		sketch, err := paths.New("testdata", "using_Wire_with_missing_lib").Abs()
 		require.NoError(t, err)
@@ -175,11 +177,12 @@ func TestCompilerErrOutput(t *testing.T) {
 			  "column": 10,
 			}
 		]`)
-	}
+	})
+
+	t.Run("LibraryDiscoverFalseErrors", func(t *testing.T) {
+		// Check that library discover do not generate false errors
+		// https://github.com/arduino/arduino-cli/issues/2263
 
-	// Check that library discover do not generate false errors
-	// https://github.com/arduino/arduino-cli/issues/2263
-	{
 		// prepare sketch
 		sketch, err := paths.New("testdata", "using_Wire").Abs()
 		require.NoError(t, err)
@@ -191,7 +194,32 @@ func TestCompilerErrOutput(t *testing.T) {
 		jsonOut.Query(".compiler_out").MustNotContain(`"fatal error"`)
 		jsonOut.Query(".compiler_err").MustNotContain(`"fatal error"`)
 		jsonOut.MustNotContain(`{ "diagnostics" : [] }`)
-	}
+	})
+
+	t.Run("PreprocessorErrorsOnStderr", func(t *testing.T) {
+		// Test the preprocessor errors are present in the diagnostics
+		// when they are printed on stderr
+
+		// prepare sketch
+		sketch, err := paths.New("testdata", "blink_with_error_directive").Abs()
+		require.NoError(t, err)
+
+		// Run compile and catch err stream
+		out, _, err := cli.Run("compile", "-b", "arduino:avr:uno", "-v", "--json", sketch.String())
+		require.Error(t, err)
+		jsonOut := requirejson.Parse(t, out)
+		jsonOut.Query(".compiler_out").MustNotContain(`"error:"`)
+		jsonOut.Query(".compiler_err").MustContain(`"error:"`)
+		jsonOut.Query(`.builder_result.diagnostics`).MustContain(`
+		[
+			{
+				"severity": "ERROR",
+				"message": "#error void setup(){} void loop(){}\n #error void setup(){} void loop(){}\n  ^~~~~",
+				"line": 1,
+				"column": 2
+			}
+		]`)
+	})
 }
 
 func TestCompileRelativeLibraryPath(t *testing.T) {

internal/integrationtest/compile_3/testdata/blink_with_error_directive/blink_with_error_directive.ino

@@ -0,0 +1 @@
+#error void setup(){} void loop(){}