preprocessor: reset executor error state for each event in serve

Per https://cuelang.org/issue/2826, currently if the serve mode of the
preprocessor sees an error, the preprocessor can never recover from that
error state. Even if an error in a page is fixed, the preprocessor
thinks that there is still an error and hence never transforms the fixed
page, hence the rendered HTML page (via Hugo) never updates.

This CL fixes a bug in the serve mode of the executor that did not reset
the error status after the handling of a watched event (or batch of
events).

Fixes cue-lang/cue#2826.

Preprocessor-No-Write-Cache: true
Signed-off-by: Paul Jolly <[email protected]>
Change-Id: I72231fd8df31945465eb81c48060c2612a0aa192
Dispatch-Trailer: {"type":"trybot","CL":1176998,"patchset":1,"ref":"refs/changes/98/1176998/1","targetBranch":"alpha"}

internal/cmd/preprocessor/cmd/buffered_error_context.go

@@ -42,6 +42,12 @@ type errorContextBuffer struct {
 
 var _ errorContext = (*errorContextBuffer)(nil)
 
+func (e *errorContextBuffer) reset() error {
+	e.inError = false
+	e.log.Reset()
+	return nil
+}
+
 func (e *errorContextBuffer) isInError() bool {
 	return e.inError
 }

internal/cmd/preprocessor/cmd/error_context.go

@@ -19,7 +19,7 @@ import (
 	"fmt"
 	"io"
 
-	"cuelang.org/go/cue/errors"
+	"errors"
 )
 
 // errorContext is an error-aware logging interface embedded by types.
@@ -42,6 +42,10 @@ type errorContext interface {
 
 	// isInError returns the error state.
 	isInError() bool
+
+	// reset resets the error state. It is up to the caller to ensure that it
+	// is safe to call this from a race condition perspective.
+	reset() error
 }
 
 // errorContextWriter is a base implementation of errorContext
@@ -53,6 +57,20 @@ type errorContextWriter struct {
 	log     io.Writer
 }
 
+func (e *errorContextWriter) reset() error {
+	// resetWriter is an io.Writer than also provides a Reset() method.
+	type resetWriter interface {
+		io.Writer
+		Reset() error
+	}
+
+	e.inError = false
+	if l, ok := e.log.(resetWriter); ok {
+		return l.Reset()
+	}
+	return nil
+}
+
 func (e *errorContextWriter) isInError() bool {
 	return e.inError
 }

internal/cmd/preprocessor/cmd/serve.go

@@ -208,6 +208,12 @@ func (sc *serveContext) watcherEventLoop() {
 		// TODO: see the TODO agains the signal handling in (*executor).serve for ideas on
 		// how we do better tidy up.
 
+		// Reset the exeutor... because we either finished the first run or
+		// handled a watcher event
+		if err := sc.e.reset(); err != nil {
+			sc.e.debugf(sc.e.debugGeneral, "failed to reset executor: %v", err)
+		}
+
 		select {
 		case evs := <-sc.w.Events():
 			// Ignore if we only got events for the index file