fix: use concurrency-safe bytes buffer to avoid race (#15142)

Fixes https://github.com/coder/internal/issues/93

`bytes.Buffer` is not concurrency-safe.

`cmd` could write to the buffer concurrently while we're reading the
buffer in

```
require.Eventually(t, func() bool {
	return bytes.Contains(output.Bytes(), []byte("ERROR: Downloaded agent binary returned unexpected version output"))
}, testutil.WaitShort, testutil.IntervalSlow)
```

Not sure about the `os: process already finished` flake, though.

---------

Signed-off-by: Danny Kopping <danny@coder.com>

provisionersdk/agent_test.go

@@ -8,7 +8,6 @@ package provisionersdk_test
 
 import (
 	"bytes"
-	"context"
 	"errors"
 	"fmt"
 	"net/http"
@@ -47,12 +46,10 @@ func TestAgentScript(t *testing.T) {
 	t.Run("Valid", func(t *testing.T) {
 		t.Parallel()
 
+		ctx := testutil.Context(t, testutil.WaitShort)
 		script := serveScript(t, bashEcho)
 
-		ctx, cancel := context.WithTimeout(context.Background(), testutil.WaitShort)
-		t.Cleanup(cancel)
-
-		var output bytes.Buffer
+		var output safeBuffer
 		// This is intentionally ran in single quotes to mimic how a customer may
 		// embed our script. Our scripts should not include any single quotes.
 		// nolint:gosec
@@ -84,12 +81,10 @@ func TestAgentScript(t *testing.T) {
 	t.Run("Invalid", func(t *testing.T) {
 		t.Parallel()
 
+		ctx := testutil.Context(t, testutil.WaitShort)
 		script := serveScript(t, unexpectedEcho)
 
-		ctx, cancel := context.WithTimeout(context.Background(), testutil.WaitShort)
-		t.Cleanup(cancel)
-
-		var output bytes.Buffer
+		var output safeBuffer
 		// This is intentionally ran in single quotes to mimic how a customer may
 		// embed our script. Our scripts should not include any single quotes.
 		// nolint:gosec
@@ -159,3 +154,33 @@ func serveScript(t *testing.T, in string) string {
 	script = strings.ReplaceAll(script, "${AUTH_TYPE}", "token")
 	return script
 }
+
+// safeBuffer is a concurrency-safe bytes.Buffer
+type safeBuffer struct {
+	mu  sync.Mutex
+	buf bytes.Buffer
+}
+
+func (sb *safeBuffer) Write(p []byte) (n int, err error) {
+	sb.mu.Lock()
+	defer sb.mu.Unlock()
+	return sb.buf.Write(p)
+}
+
+func (sb *safeBuffer) Read(p []byte) (n int, err error) {
+	sb.mu.Lock()
+	defer sb.mu.Unlock()
+	return sb.buf.Read(p)
+}
+
+func (sb *safeBuffer) Bytes() []byte {
+	sb.mu.Lock()
+	defer sb.mu.Unlock()
+	return sb.buf.Bytes()
+}
+
+func (sb *safeBuffer) String() string {
+	sb.mu.Lock()
+	defer sb.mu.Unlock()
+	return sb.buf.String()
+}