Gbytes

gbytes: testing streaming buffers

gbytes provides *gbytes.Buffer, an in-memory io.Writer (and io.Reader/io.Closer) that captures everything written to it, plus the Say matcher for making ordered assertions against streaming data as it arrives. Pairs naturally with gomega:async (poll with Eventually) and gomega:gexec (a session's .Out/.Err are *gbytes.Buffers, and the session itself is a BufferProvider). Docs: https://onsi.github.io/gomega/#gbytes-testing-streaming-buffers.

import (
	. "github.com/onsi/gomega"
	"github.com/onsi/gomega/gbytes"
)

The buffer

gbytes.NewBuffer() returns an empty *gbytes.Buffer. Hand it to anything that wants an io.Writer and it accumulates the bytes:

buffer := gbytes.NewBuffer()
go client.AttachToDataStream(buffer) // client writes to buffer concurrently

• gbytes.BufferWithBytes(b []byte) — seed a buffer with already-captured bytes.
• buffer.Contents() []byte — all bytes ever written, regardless of cursor position. Use this for whole-buffer assertions (Expect(buffer.Contents()).To(ContainSubstring(...))).
• buffer.Close() / buffer.Closed() — mark a buffer done. Writes to a closed buffer error. A closed buffer tells Eventually to give up (see below).

The Say matcher

gbytes.Say(pattern, args...) takes a regular expression (optionally fmt.Sprintf-formatted with args) and matches against the buffer's unread portion:

Expect(buffer).To(gbytes.Say(`hello \w+`))

Say is cursor-based, not whole-buffer. Each buffer carries an opaque read cursor you cannot access. When Say matches, it fast-forwards the cursor to just past the match. The next Say only sees bytes after that point. This is what makes ordered assertions work — and the gotcha below.

The canonical streaming pattern: Eventually + Say

Streaming output arrives over time, so poll the buffer with Eventually (gomega:async). Each successful Say advances the cursor, so successive Says assert on successive output in order:

Eventually(buffer).Should(gbytes.Say(`Attached as client \d+`))

client.ReticulateSplines()
Eventually(buffer).Should(gbytes.Say(`reticulating splines`))

client.EncabulateRetros(7)
Eventually(buffer).Should(gbytes.Say(`encabulating 7 retros`))

Because the cursor only moves forward, this counts repeats correctly — these two assertions pass only if reticulating splines appears twice:

client.ReticulateSplines()
Eventually(buffer).Should(gbytes.Say(`reticulating splines`))
client.ReticulateSplines()
Eventually(buffer).Should(gbytes.Say(`reticulating splines`))

And consequently this (counterintuitively) passes — the first Say consumed the match:

Eventually(buffer).Should(gbytes.Say(`reticulating splines`))
Consistently(buffer).ShouldNot(gbytes.Say(`reticulating splines`))

Say works on a *gbytes.Buffer or any BufferProvider (anything with a Buffer() *gbytes.Buffer method, e.g. a gexec.Session), so Eventually(session).Should(gbytes.Say(...)) works directly.

Handling branches

When the test must react to whichever output arrives, use buffer.Detect(regexp, args...), which returns a channel that fires once on match (and fast-forwards the cursor). Always CancelDetects() to clean up the spawned goroutines:

client.Authorize()
select {
case <-buffer.Detect("You are not logged in"):
	client.Login()
case <-buffer.Detect("Success"):
	// carry on
case <-time.After(time.Second):
	Fail("timed out waiting for output")
}
buffer.CancelDetects()

Testing io.Reader/io.Writer/io.Closer

These interfaces are expected to block, so calling Read/Write/Close directly in a test risks hanging forever. Wrap them with timeouts; the wrappers return gbytes.ErrTimeout if the operation doesn't complete in time:

p := make([]byte, 5)
_, err := gbytes.TimeoutReader(reader, time.Second).Read(p)
Expect(err).NotTo(HaveOccurred())

gbytes.TimeoutReader, gbytes.TimeoutWriter, and gbytes.TimeoutCloser each wrap the matching interface. To use Say against an io.Reader, wrap it with gbytes.BufferReader(reader) — it launches an io.Copy goroutine into a fresh buffer (closed when the copy completes). Because the copy is async you must use Eventually:

Eventually(gbytes.BufferReader(reader)).Should(gbytes.Say("abcde"))

Gotchas

• Say takes a regexp — escape metacharacters. (, ), ., [, +, ?, \d, etc. are regex syntax. To match a literal splines (v2) write gbytes.Say(splines (v2)). The pattern is compiled with regexp.MustCompile, so a bad pattern panics.
• The cursor only moves forward. You cannot re-match output an earlier Say already consumed. If you need to assert on the whole buffer, use buffer.Contents() with a string matcher (gomega:matchers) instead of Say.
• Pair Say with Eventually for live streams; use plain Expect only on an already-complete buffer. A bare Expect(buffer).To(Say(...)) checks the buffer once, right now — it will flake if the data hasn't arrived yet.
• A closed buffer aborts Eventually early. Once Close() is called, a pending Say can never succeed on new data, so the matcher signals Eventually/Consistently to stop polling and fail immediately rather than wait out the timeout.