imageProgressWriter doesn't propagate docker errors under certain conditions

[mmilata]

The issue is that imageProgressWriter can be used in two ways, by calling ReadFrom or by calling Write (possibly multiple times). Both go-dockerclient and unit tests pass the writer to io.Copy which uses the ReadFrom method. However when I wrap imageProgressWriter in an io.MultiWriter the Write method is used which results in slightly different behavior. For example when the decoded JSON object contains error key, ReadFrom returns the error immediately but Write returns it on the next invocation which may never happen.

For openshift/origin#12407 I have an implementation of Write that does not use io.Pipe/goroutine but it's ugly and does a lot of unnecessary copying: https://github.com/mmilata/origin/blob/25a5c910d1afb87a0f8b6266954a72ee7eca4ec8/pkg/build/builder/dockerutil.go#L253

To reproduce:

diff --git a/imageprogress/progress_test.go b/imageprogress/progress_test.go
index 2438d80..d959f5f 100644
--- a/imageprogress/progress_test.go
+++ b/imageprogress/progress_test.go
@@ -79,7 +79,7 @@ func TestReports(t *testing.T) {
                        },
                )
                w.(*imageProgressWriter).stableThreshhold = 0
-               _, err := io.Copy(w, pipeIn)
+               _, err := io.Copy(io.MultiWriter(w), pipeIn)
                if err != nil {
                        if !test.errExpected {
                                t.Errorf("%s: unexpected: %v", test.name, err)
@@ -115,7 +115,7 @@ func TestErrorOnCopy(t *testing.T) {
        w := newWriter(func(r report) {}, func(a, b report) bool { return true })
 
        // Ensure that the error is propagated to the copy
-       _, err := io.Copy(w, genIn)
+       _, err := io.Copy(io.MultiWriter(w), genIn)
        if err == nil {
                t.Errorf("Did not get an error when copying to writer")
        }

[smarterclayton]

Since this was just a copy of @csrwng's original code, we should just port the changes back.

[openshift-bot]

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[openshift-ci-robot]

@openshift-bot: Closing this issue.

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