events endpoint: fix panic and race condition

Fix a potential panic in the events endpoint when parsing the filters
parameter.  Values of the filters map might be empty, so we need to
account for that instead of uncondtitionally accessing the first item.

Also apply a similar for race conditions as done in commit f4a2d25c0fca:

	Fix a race that could cause read errors to be masked.  Masking
	such errors is likely to report red herrings since users don't
	see that reading failed for some reasons but that a given event
	could not be found.

Another race was the handler closing event channel, which could lead to
two kinds of panics: double close, send to close channel.  The backend
takes care of that.  However, make sure that the backend stops working
in case the context has been cancelled.

Fixes: #6899
Signed-off-by: Valentin Rothberg <rothberg@redhat.com>

2 files changed, 72 insertions, 49 deletions

diff --git a/pkg/api/handlers/compat/events.go b/pkg/api/handlers/compat/events.go
index 5acc94153..3fc8248d6 100644
--- a/pkg/api/handlers/compat/events.go
+++ b/pkg/api/handlers/compat/events.go
@@ -1,9 +1,9 @@
 package compat
 
 import (
-	"context"
 	"fmt"
 	"net/http"
+	"sync"
 
 	"github.com/containers/libpod/v2/libpod"
 	"github.com/containers/libpod/v2/libpod/events"
@@ -17,10 +17,10 @@ import (
 
 func GetEvents(w http.ResponseWriter, r *http.Request) {
 	var (
-		fromStart   bool
-		eventsError error
-		decoder     = r.Context().Value("decoder").(*schema.Decoder)
-		runtime     = r.Context().Value("runtime").(*libpod.Runtime)
+		fromStart bool
+		decoder   = r.Context().Value("decoder").(*schema.Decoder)
+		runtime   = r.Context().Value("runtime").(*libpod.Runtime)
+		json      = jsoniter.ConfigCompatibleWithStandardLibrary // FIXME: this should happen on the package level
 	)
 
 	query := struct {
@@ -33,11 +33,16 @@ func GetEvents(w http.ResponseWriter, r *http.Request) {
 	}
 	if err := decoder.Decode(&query, r.URL.Query()); err != nil {
 		utils.Error(w, "Failed to parse parameters", http.StatusBadRequest, errors.Wrapf(err, "Failed to parse parameters for %s", r.URL.String()))
+		return
 	}
 
 	var libpodFilters = []string{}
 	if _, found := r.URL.Query()["filters"]; found {
 		for k, v := range query.Filters {
+			if len(v) == 0 {
+				utils.Error(w, "Failed to parse parameters", http.StatusBadRequest, errors.Errorf("empty value for filter %q", k))
+				return
+			}
 			libpodFilters = append(libpodFilters, fmt.Sprintf("%s=%s", k, v[0]))
 		}
 	}
@@ -46,46 +51,57 @@ func GetEvents(w http.ResponseWriter, r *http.Request) {
 		fromStart = true
 	}
 
-	eventCtx, eventCancel := context.WithCancel(r.Context())
 	eventChannel := make(chan *events.Event)
+	errorChannel := make(chan error)
+
+	// Start reading events.
 	go func() {
-		readOpts := events.ReadOptions{FromStart: fromStart, Stream: query.Stream, Filters: libpodFilters, EventChannel: eventChannel, Since: query.Since, Until: query.Until}
-		eventsError = runtime.Events(eventCtx, readOpts)
+		readOpts := events.ReadOptions{
+			FromStart:    fromStart,
+			Stream:       query.Stream,
+			Filters:      libpodFilters,
+			EventChannel: eventChannel,
+			Since:        query.Since,
+			Until:        query.Until,
+		}
+		errorChannel <- runtime.Events(r.Context(), readOpts)
 	}()
-	if eventsError != nil {
-		utils.InternalServerError(w, eventsError)
-		eventCancel()
-		close(eventChannel)
-		return
-	}
 
-	// If client disappears we need to stop listening for events
-	go func(done <-chan struct{}) {
-		<-done
-		eventCancel()
-		if _, ok := <-eventChannel; ok {
-			close(eventChannel)
-		}
-	}(r.Context().Done())
+	var coder *jsoniter.Encoder
+	var writeHeader sync.Once
 
-	// Headers need to be written out before turning Writer() over to json encoder
-	w.Header().Set("Content-Type", "application/json")
-	w.WriteHeader(http.StatusOK)
-	if flusher, ok := w.(http.Flusher); ok {
-		flusher.Flush()
-	}
+	for stream := true; stream; stream = query.Stream {
+		select {
+		case err := <-errorChannel:
+			if err != nil {
+				utils.InternalServerError(w, err)
+				return
+			}
+		case evt := <-eventChannel:
+			writeHeader.Do(func() {
+				// Use a sync.Once so that we write the header
+				// only once.
+				w.Header().Set("Content-Type", "application/json")
+				w.WriteHeader(http.StatusOK)
+				if flusher, ok := w.(http.Flusher); ok {
+					flusher.Flush()
+				}
+				coder = json.NewEncoder(w)
+				coder.SetEscapeHTML(true)
+			})
 
-	json := jsoniter.ConfigCompatibleWithStandardLibrary
-	coder := json.NewEncoder(w)
-	coder.SetEscapeHTML(true)
+			if evt == nil {
+				continue
+			}
 
-	for event := range eventChannel {
-		e := entities.ConvertToEntitiesEvent(*event)
-		if err := coder.Encode(e); err != nil {
-			logrus.Errorf("unable to write json: %q", err)
-		}
-		if flusher, ok := w.(http.Flusher); ok {
-			flusher.Flush()
+			e := entities.ConvertToEntitiesEvent(*evt)
+			if err := coder.Encode(e); err != nil {
+				logrus.Errorf("unable to write json: %q", err)
+			}
+			if flusher, ok := w.(http.Flusher); ok {
+				flusher.Flush()
+			}
 		}
+
 	}
 }
diff --git a/pkg/bindings/test/system_test.go b/pkg/bindings/test/system_test.go
index 93141400b..430184f4a 100644
--- a/pkg/bindings/test/system_test.go
+++ b/pkg/bindings/test/system_test.go
@@ -1,6 +1,7 @@
 package test_bindings
 
 import (
+	"sync"
 	"time"
 
 	"github.com/containers/libpod/v2/pkg/bindings"
@@ -38,22 +39,28 @@ var _ = Describe("Podman system", func() {
 	})
 
 	It("podman events", func() {
-		eChan := make(chan entities.Event, 1)
-		var messages []entities.Event
-		cancelChan := make(chan bool, 1)
+		var name = "top"
+		_, err := bt.RunTopContainer(&name, bindings.PFalse, nil)
+		Expect(err).To(BeNil())
+
+		filters := make(map[string][]string)
+		filters["container"] = []string{name}
+
+		binChan := make(chan entities.Event)
+		done := sync.Mutex{}
+		done.Lock()
+		eventCounter := 0
 		go func() {
-			for e := range eChan {
-				messages = append(messages, e)
+			defer done.Unlock()
+			for range binChan {
+				eventCounter++
 			}
 		}()
-		go func() {
-			system.Events(bt.conn, eChan, cancelChan, nil, nil, nil, bindings.PFalse)
-		}()
 
-		_, err := bt.RunTopContainer(nil, nil, nil)
+		err = system.Events(bt.conn, binChan, nil, nil, nil, filters, bindings.PFalse)
 		Expect(err).To(BeNil())
-		cancelChan <- true
-		Expect(len(messages)).To(BeNumerically("==", 5))
+		done.Lock()
+		Expect(eventCounter).To(BeNumerically(">", 0))
 	})
 
 	It("podman system prune - pod,container stopped", func() {